Abstract: Following the Great East Japan Earthquake in 2011, a large tsunami developed and struck the Pacific coast of eastern Japan. To assess the immediate impacts of the tsunami on coastal communities, changes in taxon composition and richness of macrobenthic animals before and after the tsunami were examined at nine intertidal flats in Sendai Bay and the Sanriku Ria coast. The results showed that 30-80% of taxa indigenously inhabiting intertidal flats disappeared after the tsunami. Among animal types, endobenthic and sessile epibenthic animals were more vulnerable to the tsunami than mobile epibenthic animals like shore crabs and snails. For all the intertidal flats examined, animals that were originally dwellers in lower tidal zones and not recorded before the tsunami were also found right after the tsunami, indicating that the tsunami not only took away many benthic taxa from the intertidal flats but also brought in some taxa from elsewhere. However, overall changes in taxon community composition were greater for intertidal flats that experienced larger inundation heights. These results showed that the ecological impacts of the tsunami were proportional to the physical impacts as gauged by wave height and that mobile epibenthic animals were less vulnerable to the tsunami.
Abstract: Concentrations of 18 elements, including Sb, In, Sn, and Bi, were measured in sediment cores from two pristine alpine lakes on Mount Hachimantai, northern Japan, representing the past 250 years. Vertical variations in concentrations are better explained by atmospheric metal deposition than by diagenetic redistribution of Fe and Mn hydroxide and organic matter. Anthropogenic metal fluxes were estimated from (210)Pb-derived accumulation rates and metal concentrations in excess of the Al-normalized mean background concentration before 1850. Anthropogenic fluxes of Sb and In showed gradual increases starting around 1900 in both lakes, and marked increases after 1980. Comparison of Sb/Pb and Pb stable isotope ratios in sediments with those in aerosols of China or northern Japan and Japanese source materials (recent traffic- and incinerator-derived dust) suggest that the markedly elevated Sb flux after 1980 resulted primarily from enhanced long-range transport in aerosols containing Sb and Pb from coal combustion on the Asian continent. The fluxes of In, Sn, and Bi which are present in Chinese coal showed increasing trends similar to Sb for both study lakes. This suggests that the same source although incinerators in Japan may not be ruled out as sources of In. The sedimentary records for the last 250 years indicate that atmospheric pollution of Sb and In in East Asia have intensified during recent decades.
Abstract: Relationships between environmental factors and bacterial communities were investigated in 41 freshwater lakes located in mountainous regions of eastern Japan. Bacterioplankton community composition (BCC) was determined by polymerase chain reaction-denaturing gradient gel electrophoresis of the 16S rRNA gene and then evaluated on the basis of physicochemical and biological variables of the lakes. Canonical correspondence analysis revealed that BCC of oligotrophic lakes was significantly influenced by dissolved organic carbon (DOC) content, but its effect was not apparent in the analysis covering all lakes including mesotrophic and eutrophic ones. The generalized linear model showed the negative association of DOC on the taxon richness of bacterioplankton communities. DOC was positively correlated with the catchment area per lake volume, suggesting that a large fraction of DOC supplied to the lake was derived from terrestrial sources. These results suggest that allochthonous DOC has a significant effect on bacterioplankton communities especially in oligotrophic lakes. The genus Polynucleobacter was detected most frequently. The occurrence of Polynucleobacter species was positively associated with DOC and negatively associated with total phosphorus (TP) levels. In addition, TP had a stronger effect than DOC, suggesting that oligotrophy is the most important factor on the occurrence of this genus.
Abstract: Ecological specializations such as antipredator defense can reinforce morphological and distributional divergence within hybridizing species. Two hybridizing species of Daphnia (D. galeata and D. dentifera) are distributed in both Japan and North America; however, these populations have a longer history in Japan than in North America due to the differing impact of the last glaciation on these two regions. We tested the hypothesis that this longer coexistence in Japan would lead to extensive genetic admixture in nuclear and mitochondrial DNA whilst the distinct morphological traits and distributional patterns would be maintained.
Abstract: Recent investigations of molecular diversity in the plankton of lakes and coastal lagoons have detected an unexpected diversity of fungi including chytrids. Microscopic observations have provided evidence for the presence of two main forms. The sporangia are implied in algal parasitism. The propagules, i.e. uniflagellated zoospores, may constitute an alternate resource for consumers. These results suggest a need to reconsider the concept of plankton food web functioning. In order to describe the potential role of fungi in food web functioning, we revisit the model of carbon flows in the photic zone of the North basin of Lake Biwa in summer, established using the inverse analysis method for estimating missing flow values. In the absence of quantification of the flows induced by fungal activity, simulations are realised of their potential role in the plankton food web. Different rates of parasitism of micro-phytoplankton are tested, with a return of this carbon to the consumer via the consumption of zoospores by mesozooplankton and, at a lower rate, microzooplankton. The presence of this indirect pathway channelling micro-phytoplankton production to the consumers via the fungi, leads to the following trends: (i) an enhancement of the trophic efficiency index, (ii) a decrease of the ratio detritivory/herbivory, (iii) a decrease of the percentage of carbon flowing in cyclic pathways, and (iv) an increase in the relative ascendency of the system. Relative ascendency, which indicates pathways more specialised and less redundant, is related to theories linking food web patterns and stability. A high ascendency in the plankton food web (low trophic level), if connected to a food web of high redundancy at higher trophic levels (e.g. nekton food web) would fit well to the stabilising pattern called structural asymmetry, considered a stability criterion. More precise models, taking into account the species diversity of fungi and the high specificity of their parasitism on the micro-phytoplankton, would further accentuate this observation.
Abstract: To identify lake properties and watershed environments regulating partial pressure of carbon dioxide (pCO(2)) at the surface water, a field survey was performed in summer for 77 lakes. These lakes were located at latitudes between 35 and 43 degrees N with altitudes from 5 to 2700 m in Japanese islands and differed largely with respect to trophic conditions, basin morphometries, and land use and land cover in the watersheds. Among the lakes, pCO(2) in the surface water varied more than three orders of magnitude (1.4-4749.7 Pa) and was higher than that at atmospheric levels in 73% of the lakes, suggesting that the majority of the lakes were net heterotrophic. Among the within-lake variables, biomass of both zooplankton and phytoplankton as well as the sediment area of mixed layer were important predictors of pCO(2) at the surface. Thus, lake CO(2) concentrations are regulated by the balance of autotrophic and heterotrophic activities, and sediment respiration is a crucial source of CO(2) supersaturation, especially in shallow isothermal lakes. The best model for pCO(2) at the lake surface water included relative size of deciduous forests, grasslands, and urban areas in the watershed. The balance of autotrophic and heterotrophic activities in lakes depends highly on land use and cover in the watershed. Changes in terrestrial vegetation can affect carbon metabolism, even if local anthropogenic activities in the watersheds are unchanged.
Abstract: We consider a singular Cauchy problem for the Euler-Poisson-Darboux equation of Fuchsian type in the time variable with ramified Cauchy data. In this paper we establish an expansion of the solutions in a series of hypergeometric functions and then investigate the nature of the singularities of the solutions. (C) 2009 Elsevier Masson SAS. All rights reserved.
Abstract: We examined algal remains and fossil pigments in (210)Pb-dated sediment cores from Lake Biwa to explore historical changes in the phytoplankton community of the lake over the past 100 years and to identify environmental factors that caused those changes. Fluxes of fossil pigments and algal remains were very low before the 1960s, but increased through the 1960s and 1970s, indicating that the lake had eutrophied in the 20 years since 1960. After 1980, however, fluxes of all fossil pigments and algal remains decreased or stabilized. Redundancy analysis with meteorological and limnological variables explained more than 70% of the variation of these fluxes and showed that the decrease in fluxes of most algal taxa that occurred in the 1980s was related to changes in meteorological variables such as wind velocity, rather than changes in the lake's trophic state. Sedimentary records of algal remains also revealed that Aulacoseira nipponica, an endemic diatom species that grows in winter, decreased dramatically after 1980, while Fragilaria crotonensis, a cosmopolitan spring diatom species, became dominant. Replacement of one dominant diatom species by another could not be explained simply by changes in the lake trophic state, but was reasonably strongly related with an increase in winter water temperature. These results suggest that the phytoplankton community in Lake Biwa was influenced by changes in local environmental conditions (nutrient loading) through the 1960s and 1970s, but more so by regional (meteorological) and global (climate warming) factors since 1980.
Abstract: Contrary to an expectation from the size-efficiency hypothesis, small herbivore zooplankton such as Ceriodaphnia often competitively predominate against large species such as Daphnia. However, little is known about critical feeding conditions favoring Ceriodaphnia over Daphnia. To elucidate these conditions, a series of growth experiments was performed with various types of foods in terms of phosphorus (P) contents and composition (algae and bacteria). An experiment with P-rich algae showed that the threshold food level, at which an individual's growth rate equals zero, was not significantly different between the two species. However, the food P:C ratio, at which the growth rate becomes zero, was lower for Daphnia than for Ceriodaphnia, suggesting that the latter species is rather disfavored by P-poor algae. Ceriodaphnia showed a higher growth rate than Daphnia only when a substantial amount of bacteria was supplied together with a low amount of P-poor algae as food. These results suggest that an abundance of bacteria relative to algae plays a crucial role in favoring Ceriodaphnia over Daphnia because these are an important food resource for the former species but not for the latter.
Abstract: Interfaces between terrestrial and stream ecosystems often enhance species diversity and population abundance of ecological communities beyond levels that would be expected separately from both the ecosystems. Nevertheless, no study has examined how stream configuration within a watershed influences the population of terrestrial predators at the drainage-basin scale. We examined the habitat and abundance relationships of forest insectivorous birds in eight drainage basins in a cool temperate forest of Japan during spring and summer. Each basin has different drainage-basin geomorphology, such as the density and frequency of stream channels. In spring, when terrestrial arthropod prey biomass is limited, insectivorous birds aggregated in habitats closer to streams, where emerging aquatic prey was abundant. Nevertheless, birds ceased to aggregate around streams in summer because terrestrial prey became plentiful. Watershed-scale analyses showed that drainage basins with longer stream channels per unit area sustained higher densities of insectivorous birds. Moreover, such effects of streams on birds continued from spring through summer, even though birds dispersed out of riparian areas in the summer. Although our data are from only a single year, our findings imply that physical modifications of stream channels may reduce populations of forest birds; thus, they emphasize the importance of landscape-based management approaches that consider both stream and forest ecosystems for watershed biodiversity conservation.
Abstract: Putative future increase in atmospheric CO(2) is expected to adversely affect herbivore growth due to decrease in contents of key nutrients such as nitrogen and phosphorus (P) relative to carbon in primary producers including plant and algal species. However, as many herbivores are polyphagous and as the response of primary producers to elevated CO(2) is highly species-specific, effects of elevated CO(2) on herbivore growth may differ between feeding conditions with monospecific and multiproducer diets. To examine this possibility, we performed CO(2) manipulation experiments under a P-limited condition with a planktonic herbivore, Daphnia, and three algal species, Scenedesmus obliquus (green algae), Cyclotella sp. (diatoms) and Synechococcus sp. (cyanobacteria). Semibatch cultures with single algal species (monocultures) and multiple algal species (mixed cultures) were grown at ambient (360 ppm) and high CO(2) levels (2000 ppm) that were within the natural range in lakes. Both in the mono- and mixed cultures, algal steady state abundance increased but algal P : C and N : C ratios decreased when they were grown at high CO(2). As expected, Daphnia fed monospecific algae cultured at high CO(2) had decreased growth rates despite increased algal abundance. However, when fed mixed algae cultured at high CO(2), especially consisting of diatoms and cyanobacteria or the three algal species, Daphnia maintained high growth rates despite lowered P and N contents relative to C in the algal diets. These results imply that algal diets composed of multiple species can mitigate the adverse effects of elevated CO(2) on herbivore performance, although the magnitude of this mitigation depends on the composition of algal species involved in the diets.
Abstract: To clarify long-term variations in the resting egg production of Daphnia galeata in Lake Biwa during the 20th century, we examined an abundance of plankton remains and ephippia in a 26-cm sediment core with a time resolution of approximately 2-6 years. Historical changes shown by these plankton remains indicated that the Daphnia population has not produced resting eggs since the 1980s, but it has remained the most abundant zooplankton species in the lake. Plankton monitoring data collected from 1966 to 2000 revealed that the overwintering individuals (January-March) of D. galeata plankters showed a significant increasing trend in recent years, such increase being negatively correlated with ephippial abundance in the sediment samples. Further analyses showed that the dominant phytoplankton in winter has changed from large inedible diatoms to edible flagellated algae, probably due to changes in vertical mixing regimes resulting from winter warming. These changes that occurred in the last several decades suggest that global warming has improved winter food conditions and thus enabled the Daphnia plankton to maintain its population without producing resting eggs in Lake Biwa.
Abstract: The classical Redfield ratio of carbon(106) : nitrogen(16) : phosphorus(1) is a cornerstone of biogeochemistry. With the use of > 2,000 observations of the chemistry of particulate matter from small and large lakes, as well as near-and off-shore marine environments, we found that the best model to describe seston stoichiometry depended on the scale of analysis. We also found that there were better estimates for seston chemistry than the classical ratio for all habitats, whether freshwater or marine. Across the entire data set, a constant proportionality of C-166: N-20 : P-1 ( +/- error) described the data, which implies higher C sequestration per unit of N and P in surface waters than given in the classical ratio. At a regional scale, however, C: P and C: N often declined with increasing seston abundance, rejecting a constant ratio model. Within both freshwater and marine habitats, higher seston abundance is often associated with lower C: P and C: N ratios ( higher nutrient content). The difference in appropriateness of the constant ratio model with respect to the entire data compared with subsets of the data indicates a scale dependence in stoichiometric relationships in seston C: N: P ratios. Given these consistent shifts in seston chemistry with particle abundance, the narrower variation in seston chemistry associated with marine seston chemistry could occur because of a reduced range of particulate nutrient concentration. For all but the largest scales, the classical Redfield model of biogeochemical cycling should be replaced with a more general power function model.
Abstract: Paralytic shellfish toxins produced by dinoflagellates are known to deter copepod grazing. Dinoflagellate species, including Protoceratium reticulatum, also produce disulfated polyether yessotoxins that were previously referred to as diarrheic shellfish toxins. However, the role of yessotoxins in predator-prey relationships is not yet clear. In the present study, the effects of purified yessotoxin (YTX) on feeding activities of Acartia hudsonica (Copepoda, Calanoida) were experimentally investigated. Polystyrene fluorescent microspheres (10 mu m in diameter) colored bright blue or yellow-green were coated with cell extracts of P. reticulatum that do not produce yessotoxins. The bright blue microspheres were further coated with YTX, and the yellow-green microspheres were used as the reference. The microspheres were then given to the copepods separately or in combination to measure clearance rates and feeding selectivity. A. hudsonica was found to feed on the yellow-green microspheres without YTX at twice the rate of the bright blue microspheres with YTX. We also confirmed that microsphere color per se did not affect the feeding rates. The bright blue microspheres adsorbed 1.8-43.3 pg of YTX per microsphere, which is similar to the cell-specific yessotoxin content of toxic P. reticulatum found in natural environments. These results suggest that production of yessotoxin is advantageous for P. reticulatum by deterring predation by copepods. (c) 2008 Elsevier B.V. All rights reserved.
Abstract: We measured stable nitrogen (N) and carbon (C) isotope ratios, lignin-derived phenols, and fossil pigments in sediments of known ages to elucidate the historical changes in the ecosystem status of Lake Biwa, Japan, over the last 100 years. Stable N isotope ratios and algal pigments in the sediments increased rapidly from the early 1960s to the 1980s, and then remained relatively constant, indicating that eutrophication occurred in the early 1960s but ceased in the 1980s. Stable C isotope ratios of the sediment increased from the 1960s, but decreased after the 1980s to the present. This decrease in stable C isotope ratios after the 1980s could not be explained by annual changes in either terrestrial input or algal production. However, when the C isotope ratios were corrected for the Suess effect, the shift to more negative isotopic value in atmospheric CO(2) by fossil fuel burning, the isotopic value showed a trend, which is consistent with the other biomarkers and the monitoring data. The trend was also mirrored by the relative abundance of lignin-derived phenols, a unique organic tracer of material that originated from terrestrial plants, which decreased in the early 1960s and recovered to some degree in the 1980s. We detected no notable difference in the composition of lignin phenols, suggesting that the terrestrial plant composition did not change markedly. However, we found that lignin accumulation rate increased around the 1980s. These results suggest that although eutrophication has stabilized since the 1980s, allochthonous organic matter input has changed in Lake Biwa over the past 25 years.
Abstract: Initial theories of ecological stoichiometry were based on the assumption that the mass-specific content of key nutrient elements (such as P), changes little within a consumer species. However, evidence has shown that this content changes substantially according to feeding conditions. To clarify how the specific P content (S (P)) of a consumer species depends on food conditions and relates to the growth rate, we constructed a multiple mass-balance model incorporating feeding and metabolic costs and stoichiometrically regulated releases for C and P. The validity of the model was then tested experimentally by examining the growth rates and S (P) of Daphnia pulicaria under various food conditions. The experimental observation agreed qualitatively well with the model, showing that the S (P) of consumers relates positively to growth rate at high food C:P ratios but negatively at low food C:P ratios. Thus, within a consumer species, individuals with high S (P) do not necessarily grow at high rates. The concordance in results between the model and our observation suggests that maintenance costs for both P and C are substantial regardless of food conditions and play crucial roles in determining the relationship between the S (P) and growth rate of consumers.
Abstract: The influence of the size distribution of phytoplankton on changes in the planktonic food web structures with eutrophication was examined using natural planktonic communities in two world-famous lakes: Lake Baikal and Lake Biwa. The size distribution of phytoplankton and the ratio of heterotrophic to autotrophic biomass (H/A ratio), indicating the balance between primary production and its consumption, were investigated in the lakes of different trophic status. The results revealed that microphytoplankton (>20 mu m) in mesotrophic Lake Biwa, and picophytoplankton (<2 mu m) or nanophytoplankton (2-20 mu m) in oligotrophic Lake Baikal, comprised the highest proportion of the total phytoplankton biomass. The H/A ratio was lower in Lake Biwa (<1.) than in Lake Baikal (>1). The low H/A ratio in Lake Biwa appeared to be the consequence of the lack of consumption of the more abundant microphytoplankton, which were inferior competitors in nutrient uptake under oligotrophic conditions but less vulnerable to grazing. As a result, unconsumed microphytoplankton accumulated in the water column, decreasing the H/A ratio in Lake Biwa. Our results showed that food web structure and energy flow in planktonic communities were greatly influenced by the size distribution of phytoplankton, in conjunction with bottom-up (nutrient uptake) and top-down (grazing) effects at the trophic level of primary producers.
Abstract: A spatially explicit plant-herbivore model composed of planktonic herbivores, algal preys and nutrients was constructed to examine the effects of consumer-driven nutrient recycling (CNR) on the algal species richness with and without spatial structure. The model assumed that either of two essential nutrients (N and P) limited growth of algal populations and that consumer individuals moved randomly in the lattice and grazed all the algal species with the same efficiency. The results showed that when there was no CNR, the number of persistent algal species was affected by neither supply rates of external nutrients nor spatial structure and was consistently low. When consumers recycled nutrients according to their stoichiometry, the algal species richness changed with supply rates of external nutrients depending on spatial structure: the algal species richness decreased with increasing nutrient loadings when there were no spatial structure because CNR increased the probability of stochastic extinction of algal species by amplifying the oscillation of algae-consumer dynamics. However, when spatial structures were created by the migration of consumers, CNR increased the algal species richness in a range of nutrient loadings because spatial variation of grazing pressure functioned to stabilize the algal-consumer dynamics. The present study suggests that through grazing and nutrient recycling, consumer individuals can create ephemeral heterogeneity in growth environments for algal species and that this ephemerality is one of the keys to understanding algal species in nature.
Abstract: We estimated the contributions of sedimentation and cell lysis to the loss of two large phytoplankton species (Fragilaria crotonensis and Staurastrum dorsidentiferum) in Lake Biwa. Major loss process differed between species, and large phytoplankton did not necessarily function as a nutrient sink. The spring bloom of F crotonensis was terminated by nutrient depletion and a subsequent increase in sedimentation rate. Although this species could be occasionally grazed by zooplankton, sedimentation eliminated nutrients from the surface. In contrast, the summer bloom of Staurastrum dorsidentiferum crashed mainly as a result of cell lysis caused by fungal infection within the surface mixing layer, which accounted for more than 75% of the mortality rate of this species. Cell lysis of S. dorsidentiferum may lead to nutrients within the surface mixing layer, supporting the production of bacteria and zooplankton. The different loss process of these two species implies that the function of phytoplankton in material flows cannot be assessed by cell size alone. Fungal parasitism can result in a different fate, which may play an important role in material cycling in lakes.
Abstract: 1. A steady-state model of carbon flows was developed to describe the summer planktonic food web in the surface mixed-layer of the North Basin in Lake Biwa, Japan. This model synthesised results from numerous studies on the plankton of Lake Biwa. 2. An inverse analysis procedure was used to estimate missing flow values in a manner consistent with known information. Network analysis was applied to characterise emergent properties of the resulting food web. 3. The system strongly relied on flows related to detrital particles. Whereas primary production was mainly by phytoplankton > 20 mu m, microzooplankton were active and mainly ingested detritus and bacteria. 4. The main emergent property of the system was strong recycling, through either direct ingestion of non-living material by zooplankton, or ingestion of bacteria after degradation of detritus to release dissolved organic carbon.
Abstract: Almost all digital equipment has electromagnetic in terference (EMI) suppression ferrite cores to encircle cables, such as mains and peripheral cables. However, their EMI suppression capability is hardly evaluated from the impedance characteristics provided by the manufacturers. Hence, this paper proposes to characterize the ferrite cores in terms of the insertion loss and the reflection coefficient. Theoretical and experimental investigations are carried out to develop a measurement jig for these parameters in the frequency range from 30 to 1000 MHz. It consists of a cylindrical metal rod placed above a metal ground plane, yielding a characteristic impedance of 270 ohm. In addition, a simple method is proposed for measuring the insertion loss and the reflection coefficient of a ferrite core under test. It is found that the proposed method can provide insertion loss data with an error of less than +/- 1.2 dB by using commercially available matching networks (baluns). The reflection coefficient of a ferrite core can be measured with an error less than +/- 1.0 dB. The matching characteristics of the adapters are critical factors that deteriorate measurement accuracy.
Abstract: The role of stoichiometric food quality in influencing genotype coexistence and competitive interactions between clones of the freshwater microcrustacean, Daphnia pulex, was examined in controlled laboratory microcosm experiments. Two genetically distinct clones of D. pulex, which show variation in their ribosomal rDNA structure, as well as differences in a number of previously characterized growth-rate-related features (i.e., life-history features), were allowed to compete in two different arenas: (1) batch cultures differing in algal food quality (i.e., high vs. low carbon:phosphorus (C:P ratio) in the green alga, Scenedesmus acutus); (2) continuous flow microcosms receiving different light levels (i.e., photosynthetically active radiation) that affected algal C:P ratios. In experiment 1, a clear genotype x environment interaction was determined with clone 1 out-competing clone 2 under high nutrient (i.e., low food C:P) conditions, while the exact opposite pattern was observed under low nutrient (i.e., high C:P) conditions. In experiment 2, clone 1 dominated over clone 2 under high light (higher C:P) conditions, but clonal coexistence was observed under low light (low C:P) conditions. These results indicate that food (nutrient) quality effects (hitherto an often overlooked factor) may play a role in microevolutionary (genotypic) responses to changing stoichiometric conditions in natural populations.
Abstract: Animals encountering nutritionally imbalanced foods should release elements in excess of requirements in order to maintain overall homeostasis. Quantifying these excesses and predicting their fate is, however, problematic. A new model of the stoichiometry of consumers is formulated that incorporates the separate terms in the metabolic budget, namely, assimilation of ingested substrates and associated costs, protein turnover, other basal costs, such as osmoregulation, and the use of remaining substrates for production. The model indicates that release of excess C and nonlimiting nutrients may often be a significant fraction of the total metabolic budget of animals consuming the nutrient-deficient forages that are common in terrestrial and aquatic systems. The cost of maintenance, in terms of not just C but also N and P, is considerable, such that food quality is important even when intake is low. Many generalist consumers experience short-term and unpredictable fluctuations in their diets. Comparison of model output with data for one such consumer, Daphnia, indicates that mechanisms operating postabsorption in the gut are likely the primary means of regulating excess C, N, and P in these organisms, notably respiration decoupled from biochemical or mechanical work and excretion of carbon and nutrients. This stoichiometrically regulated release may often be in organic rather than inorganic form, with important consequences for the balance of autotrophic and heterotrophic processes in ecosystems.
Abstract: Measurement instruments are discussed for the common-mode current and the differential-mode current of signals and interference at frequencies in the range of I to 30 MHz propagating on power lines. An instrument is fabricated for trial. The present instrument basically consists of a pair of current probes and a 180 degrees hybrid circuit. Voltage outputs proportional to the currents of both modes are obtained. In this paper, first the basic properties are theoretically discussed and a design guideline is provided for improvement of the sensitivity and the mode separation characteristic. In particular, it is necessary to assure a common-mode separation characteristic of -40 dB in order to measure the common-mode current that is a cause of electromagnetic interference in power line communications. It is found that for this purpose the errors in the dimensions of two probes and in the spacing of the probe and the power supply line must be kept below 1 to 2%. Further, the disturbance currents in a microwave oven and a notebook PC are actually measured. It is shown that the commonmode current and the differential-mode current can be measured simultaneously. Thus, it is demonstrated that the present instrument is effective for investigation of interference measurement methods and for setting allowable values, and is extremely useful for prediction and reduction of electromagnetic disturbances due to interference waves propagating on power lines. (c) 2005 Wiley Periodicals, Inc.
Abstract: We examined the seasonal variation in vertical distribution, gut fullness and diet of Jesogammarus annandalei, an endemic amphipod in Lake Biwa. The vertical distribution changed ontogenetically throughout the year and was restricted to depths with a water temperature < 12 degrees C. Newborn amphipods remained in the pelagic zone throughout the day while larger amphipods exhibited marked diurnal vertical migrations: individuals stayed on the lake bottom during the day and ascended to the pelagic zone at night. Gut fullness and contents of the migrating amphipods indicated that they fed upon plankton in the pelagic zone at night. Our results indicate that these migration patterns probably reduce the amphipod's predation risk from fish. As a result, J. annandalei may play an important role in the vertical translocation of material and energy in the Lake Biwa ecosystem.
Abstract: Production-to-respiration (P:R) ratio was estimated at an offshore site of Lake Biwa in order to examine whether the plankton and benthic community is subsidized with allochthonous organic carbon, and to clarify the role of this lake as potential source or sink of carbon dioxide. The respiration rate of protozoan and metazoan plankton was calculated from their biomass and empirical equations of oxygen consumption rates, and that of bacterioplankton was derived from their production rate and growth efficiency. In addition, the carbon mineralization rate in the lake sediments was estimated from the accumulation rate of organic carbon, which was determined using a Pb-210 dating technique. On an annual basis, the sum of respiration rates of heterotrophic plankton was comparable to net primary production rate measured by the C-13 method. However, when the mineralization rate in the lake sediments was included, the areal P:R ratio was 0.89, suggesting that Lake Biwa is net heterotrophic at the offshore site with the community being subsidized with allochthonous organic carbon. Such a view was supported by the surface water pCO(2) that was on average higher than that of the atmosphere. However, the estimate of net CO2 release rate was close to that of carbon burial rate in the sediments. The result suggests that the role of Lake Biwa in relation to atmospheric carbon is almost null at the offshore site, although the community is supported partially by organic carbon released from the surrounding areas.
Abstract: Changes in the abundance of an endemic amphipod, Jesogammarus annandalei, that inhabits the profundal zone of Lake Biwa were examined from 1966 to 1999. The abundance of J. annandalei increased suddenly during the mid 1980s and thereafter stayed at a level that was sevenfold higher than that before 1980. Compared with the amphipod in the 1960s, both the clutch and body size of the matured individuals decreased during the 1990s, which suggests that per capita food supply decreased. Thus, the increase in the abundance of J. annandalei appeared to be due largely to an increase in their survival rate and not their growth or reproduction rates. In accord with the increase in amphipod abundance, the fishery catch of an endemic gobiid fish species that is dominant in the profundal zone and preys on this amphipod as a major food, decreased dramatically during the mid 1980s. These results suggest that long-term changes in abundance of J. annandalei in Lake Biwa are mainly regulated by fish predation rather than by food supply.
Abstract: A highly sensitive method to analyze the intact lipids in a single zooplankter individual at the level of a few tenths of a microgram was developed by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) combined with a direct sampling technique. The sampling procedure involved (1) putting a zooplankter individual sample onto the MALDI sample plate, (2) cutting the sample into a few pieces by means of tweezers, (3) depositing aliquots of matrix and cationization reagent solutions on the zooplankter sample, and (4) irradiating with a N2 laser to cause MALDI. By using this technique, the mass spectra of the single zooplankter samples showed a series of ions generated from phospholipids with 34 or 36 carbons in the acyl groups and neutral lipids such as triglycerides and diacylglyceryl ethers with 50-54 carbons in their acyl and alkenyl groups. Accordingly, this method enabled us to estimate the relative quantity between "structured lipids" (phospholipids) and "storage lipids" (neutral lipids) in an individual zooplankter, which should give us a good clue to elucidate the roles of each class of lipids in its growth.
Abstract: The 'light:nutrient hypothesis' (LNH) states that the importance of the microbial food web relative to grazing impacts by macrozooplankton and the nature of the relationship between algae and bacteria (competition or commensalism) are affected by the balance of light and nutrients experienced by phytoplankton. We tested this hypothesis in field enclosures by manipulating irradiance and nutrient supply in a P-limited lake in Ontario, Canada. Shading and P-enrichment had little effect on standing biomass of small suspended particles (<1 μm) but both decreased C:P ratio in this size fraction. P-fertilization had no effect on algal biomass or bacterial abundance but shading significantly lowered algal biomass and the ratio of algal biomass to bacterial abundance. Shading had no effect on heterotrophic nanoflagellates (HNF) but HNF abundance declined strongly with P-enrichment under both shaded and unshaded conditions, coincident with large increases in macrozooplankton biomass that accompanied P-fertilization. Shading and nutrient enrichment also affected resource limitation as indicated by dilution bioassays. Increased algal light limitation in the enclosures was associated with lower seston C:P ratio, as expected under the LNH. In addition, the C:P ratio of new seston produced in bioassay bottles was also affected by enclosure treatment; unshaded enclosures generally produced seston with moderately high C:P ratio while shaded enclosures produced new seston at low C:P under low ambient light but at very high C:P ratio when algae were incubated at higher light. Our data provide support for some aspects of the LNH, including predicted impacts on the microbial loop and on resource limitations and C:P stoichiometry, However, some responses were not consistent with the LNH, indicating that the hypothesis needs to be modified to incorporate potential indirect effects.
Abstract: Thermally assisted hydrolysis and methylation-gas chromatography (THM-GC) in the presence of trimethylsulfonium. hydroxide (TMSH) was applied to the analysis of the lipid contents and their fatty acid compositions in individual zooplankters, especially focusing on polyunsaturated fatty acid (PUFA) components. On the resulting chromatograms, fatty acid components including PUFAs such as eicosapentaenoic acid (EPA; C20:5) and docosahexaenoic acid (DHA; C22:6) in single zooplankters were clearly observed as their methyl esters without doing any tedious and time-consuming pretreatment. By using this technique, the lipid contents and fatty acid compositions in a powdered plankter sample prepared by cryomilling homogenization were estimated on the basis of the observed peak intensities with relative standard deviations of less than 6%. The fatty acid compositions thus determined were in good agreement with those obtained by the conventional technique involving solvent extraction of the samples for every fatty acid component including PUFAs. The differences among individual zooplankter samples were then evaluated by THM-GC method. As a result, the chemical compositions of EPA and DHA showed relatively large inter-individual fluctuations even for the samples cultured under the same conditions. This suggests that lipid biosynthesis of the highly unsaturated fatty acid components and their consumption proceeded differently depending on the life history for each individual zooplankter. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: Predation and food supply are generally perceived as important determinants of spatial and temporal variations of populations. The population dynamics of freshwater rotifers have been well researched in this aspect. However, their spatial variations have attracted less attention and have not been studied by simultaneously considering both predation and food supply. We studied spatial variations of rotifer abundance among 34 Canadian boreal lakes. A large part of the variance of rotifer abundance was associated with variables related to trophic status including chlorophyll a and total phosphorus. However, abundances of mesozooplankton such as potential predators and competitors did not correlate with rotifer abundance and did not explain the residual of the regression between rotifer abundance and chlorophyll a. The results of the present study indicated that variation in rotifer abundance among lakes was caused by 'bottom-up' forces related to food supply and not by 'top-down' predatory interactions. This provides a contrast to previous empiric and experimental studies that reported that temporal variations of rotifer abundance were mainly regulated by 'top-down' interactions. This discrepancy suggests that overall differences in rotifer abundance among lakes are mainly determined by 'bottom-up' forces while temporal changes in single lakes are shaped by 'top-down' forces. Meanwhile, the composition of rotifer species was correlated with mesozooplankton abundance as well as trophic status. Rotifer species with long spines or rigid loricae were found in the lakes where mesozooplankton were abundant, which suggests that defensive morphology could have affected the rotifer species distribution among the study lakes.
Abstract: Daphnia galeata has been the sole Daphnia species and one of the dominant zooplankton species in Lake Biwa for the past 30 years. In 1999, however, another Daphnia species suddenly appeared in the lake. Based on morphological characteristics, the species was initially identified as Daphnia pulicaria. This identification was supported by mitochondrial (mt)DNA sequences of the 12S rRNA and NADH-5 (ND5) genes. In addition, comparison of the latter sequences showed that D. pulicaria in Lake Biwa is genetically very similar to populations distributed across the western regions of North America rather than those in other regions, including Europe. Thus, it is likely that D. pulicaria in Lake Biwa is a zooplankton invader from North America. Through routine sampling, we first detected D. pulicaria in winter 1999. This species gradually increased in abundance and reached a maximum density of 10000 individuals m(-2) in mid-May 1999. Thereafter, it decreased in abundance and almost disappeared from the water column after August 1999. D. pulicaria are much larger than D. galeata and are distributed in deeper water. They showed a marked diel vertical migration, although substantial numbers of individuals stayed above and within the thermocline, both during the day and at night. Based on seasonal and vertical distributions, we discuss the ecological implications of the sudden appearance of D. pulicaria in Lake Biwa.
Abstract: Detailed zooplankton records from a 26-cm sediment core with a time resolution of approximately 3-10 years were obtained from Lake Biwa, Japan, to examine the historical variations in the zooplankton community during the 20th century. In the sediments, selected zooplankton remains have fluctuated over the years. Daphnia - large zooplankton herbivores - did not occur from 1900 to 1920, and formed a very minor component of the zooplankton community in the following 30 years, while Bosmina - small zooplankton herbivores - were common during this period. In the mid-1960s, however, when eutrophication was noticeable in this lake, Daphnia numbers increased dramatically and became the dominant zooplankton thereafter. In contrast, Difflugia brevicolla and D. biwae, two amoeboid protozoans that live in connection with the lake bottom environment, occurred abundantly until the late 1950s. but gradually decreased after the mid-1960s. In particular, D. biwae, a species peculiar to this lake, was not found in sediment dated after 1980, suggesting its extinction. These results indicate that the zooplankton community structure changed greatly in the 1960s, and suggest that the eutrophication occurring at this time altered the relative strength of top-down and bottom-up forces on the zooplankton community in Lake Biwa.
Abstract: The partial pressure of carbon dioxide (pCO(2)) in lake ecosystems varies over four orders of magnitude and is affected by local and global environmental perturbations associated with both natural and anthropogenic processes. Little is known, however, about how changes in pCO(2) extend into the function and structure of food webs in freshwater ecosystems. To fill this gap, we performed laboratory experiments using the ecologically important planktonic herbivore Daphnia and its algal prey under a natural range of pCO(2) with low light and phosphorus supplies. The experiment showed that increased pCO(2) stimulated algal growth but reduced algal P:C ratio. When feeding on algae grown under high pCO(2), herbivore growth decreased regardless of algal abundance. Thus, high CO2-raised algae were poor food for Daphnia. Short-term experimental supplementation of PO4 raised the P content of the high CO2-raised algae and improved Daphnia growth, indicating that low Daphnia growth rates under high pCO(2) conditions were due to lowered P content in the algal food. These results suggest that, in freshwater ecosystems with low nutrient supplies, natural 'processes as well as anthropogenic perturbations resulting in increased pCO(2) enhance algal production but reduce energy and mass transfer efficiency to herbivores by decreasing algal nutritional quality.
Abstract: We performed bag experiments in a Canadian Shield lake with generally high Seston (suspended food particles mainly composed of algae) carbon (C):phosphorus (P) ratios, and investigated the responses of individual and population growth of herbivorous Daphnia dentifera on their abundance with ( + P) and without ( - P) a phosphorus enrichment to lake water. In both treatments. increased abundance of D. dentifera reduced seston C concentration and was accompanied by decreases in population and individual growth rates. However, P-enrichment increased seston P concentration and then reduced seston C:P ratio from 400-700 to ca 100 (by atoms). As a result. both individual and Population growth rates were significantly higher in the + P treatment at all animal abundances even though seston C concentrations were similar between the treatments. The magnitude of the growth enhancement by the P-enrichment was independent of animal abundance, Stepwise regression analyses revealed that 71 and 90% of the variance in the population and individual growth rates, respectively, were explained by seston C and P concentrations, and that the contribution of the seston P concentration was roughly the same as that of seston C. Such joint effects of seston C and P indicate that food quality (P content) as well as food quantity (C concentration) can influence Daphnia not only at the level of individual growth but also at the level of population dynamics in P-limited lakes. Our results thus strongly corroborate the hypothesis that the population development of a key herbivore Daphnia in P-limited Canadian Shield lakes is inhibited b the direct effects of P-limited food on individual growth, which weaken the strength of trophic cascading interactions starting from piscivorous fish through planktivorous fish and zooplankton to algae.
Abstract: Recent evidence shows that high supply ratios of light and nutrients Emit planktonic herbivore growth by lowering the nutritional quality of algae. Over longer time scales, however, grazers may ameliorate this effect by their impact on nutrient cycling. We examine this possibility using two species of the herbivorous zooplankter Daphnia, and its algal prey under different light intensities and low phosphorus supply in laboratory microcosms. At high light, Daphnia biomass was limited for a substantial period because of low P content of algal cells. However, a gradual increase in Daphnia density eventually improved food quality through grazing and nutrient cycling and via a novel process involving positive density dependence. Competitive exclusion of one of the two Daphnia species occurred under low light but not under high light when algae were nutritionally unsuitable. Such stoichiometrically mediated interactions among herbivorous animals may represent important mechanisms that affect community structure and material flows in ecosystems.
Abstract: A field enclosure experiment was performed over 12 weeks in a P-limited lake to test the hypothesis that light: nutrient balance affects pelagic communities by altering the C: P stoichiometry of seston and by influencing exudation of labile DOC by algae. Three levels of light intensity (ambient, 50% of ambient, 25% of ambient) were cross-classified with three levels of nutrients in a factorial design (n=2). Dissolved nutrient concentrations, seston C concentration and C: P ratios in small (< 1 μm) and larger (1-85 μm) size fractions were monitored, along with chlorophyll a concentration, abundance of bacteria and protozoa, and biomass and P-content of macrozooplankton. Algal exudation of recently-fixed C into the dissolved pool was also measured at the end of the experiment in selected enclosures. Treatments had no effect on seston C concentration but reduction of light intensity significantly decreased whole seston and large (1-85 μm) seston C: P ratios. However, the magnitude of these effects was modest and not likely to be ecologically significant. There were no effects of nutrient addition or light x nutrient interaction on seston stoichiometry. Algae tended to release a higher percentage of fixed C as DOC in high light enclosures but this difference was not statistically significant. There were no effects of treatments on the abundance of bacteria or protozoa but nutrient enrichment led to a statistically significant but generally modest increase in macrozooplankton biomass. No effects on zooplankton community composition or P-content were observed. Comparison of effect sizes and treatment variances indicated a high probability of type II error and thus our confidence in failing to reject the null hypothesis in most of the above cases was low. Thus, our data provide support for only some aspects of the light: nutrient hypothesis but more appropriate tests of the hypothesis should involve stronger treatments and/or increased replication in order to be better able to evaluate its validity.
Abstract: To examine both direct and indirect effects of macrozooplankton on phytoplankton species in Lake Biwa, we conducted in situ grazer-gradient experiments under different nutrient levels in summer, when Daphnia galeata dominated, and in autumn, when Eodiaptomus japonicus dominated. The experiments revealed that grazing pressure on phytoplankton was highly dependent on zooplankton species composition. Smaller phytoplankton species such as Stephanodiscus carconensis were more grazed when D. galeata was abundant, whereas large colonial diatom species such as Aulacoseira granulata were preferentially grazed when E. japonicus dominated. In addition, indirect effect of macrozooplankton through nutrient regeneration was suggested, although the magnitude of nutrient regeneration effects seemed to differ between D. galeata and E. japonicus. Specifically, growth rates of Sphaerocystis schroeteri were stimulated more by E. japonicus than by D. galeata. Macrozooplankton also enhanced the growth rates of colonial cyanobacteria such as Microcystis incerta, probably through decreasing the density of microzooplankton grazers (ciliates and rotifers). The results suggest that the effects of large zooplankton on phytoplankton populations are species-specific and cannot be understood without consideration of changes in abundance of other components of plankton communities.
Abstract: 1. Population dynamics and production of Jesogammarus annandalei, an endemic amphipod in Lake Biwa, were examined from April 1997 to June 1998. The life cycle of this species was 1 year with the new generation beginning in early autumn. They preferred low temperature (<12 degreesC) and their spatial distribution varied seasonally and accordingly. 2. In deep water, the abundance of J. annandalei ranged from 200 to 63 000 m(-2) and decreased towards summer and the biomass (0.01similar to3.6 g C m(-2)) was on average comparable that of zooplankton. The density was much higher than that recorded by a study conducted 35 years ago. 3. Individual growth rate of this amphipod was high in winter and spring but decreased in summer. Annual production of J. annandalei (6.2 g C m(-2) year(-1)) was only 2% of primary production but was at the higher end of the range reported for amphipods in oligo- and mesotrophic lakes. 4. These results are consistent with the view that Lake Biwa is becoming more eutrophic, with a consequent decrease in the abundance of predatory fish in the profundal zone.
Abstract: The influence of fluctuating light intensities on phytoplankton composition and diversity was investigated for 49 days under semi-continuous culture conditions with sufficient nutrient supply, using phytoplankton assemblages from Lake Biwa, Japan. Light conditions were either periodically changed from high intensity (100 mumol photons m(-2) s(-1)) to low intensity (20 mumol photons m(-2) s(-1)) at intervals of 1, 3, 6 and 12 days, or fixed to constant intensities (permanent high and low light levels). All treatments additionally experienced a day:night cycle of 16:8 h. Phytoplankton abundance increased and reached a saturation level on day 19 of the treatment with permanent high light, but increased continuously until the end of the experiment (day 49) in the treatment with permanent low light intensity. In treatments with periodically changing light intensities, the phytoplankton abundance reached saturation levels between these dates. Under phytoplankton abundance saturation, chlorophytes predominated in the treatment with permanent high light, while either cyanophytes or diatoms were abundant under permanent low light intensity. Treatments with changing light supply had chlorophyte- and cyanobacteria-dominated replicates as well as replicates with balanced proportions of both. Furthermore, species diversity, measured by the Shannon index, was low in cultures under permanent light intensity, while slow fluctuating light at the scale of 3-12 days resulted in an increased diversity index. These results indicate that species composition and diversity of the phytoplankton were affected by the periodically changing light regime in the order of days, and suggest that temporal changes in weather conditions are a major impediment to competitive exclusion of phytoplankton species in nature.
Abstract: Ecological common sense says that decreased solar energy should reduce herbivore production because of reduced energy flow through primary producers. However, a field experiment in a phosphorus-limited lake showed that production of zooplankton herbivores was increased by shading. This paradoxical outcome was caused by a decoupling of producer carbon fixation and nutrient uptake under high light that reduced food quality for herbivores. At low nutrient supplies, shading increased nutrient contents relative to carbon within algal food, outweighing effects of decreased primary production. Thus, light/ nutrient balance affects the degree of mismatch between primary producers and herbivores in nature, which in turn influences mass-transfer efficiencies along food chains. To predict how energy transfer efficiency and biological interactions will respond to perturbations, it is essential to take into account changes in light/nutrient balance and its effects on the stoichiometry of autotroph-herbivore interactions.
Abstract: Laboratory studies are increasingly indicating that the quality of nutrient-limited algae is suboptimal for zooplankton production. However, little is known about how quality is affected by nutrient limitation of phytoplankton in more natural situations. To test for phosphorus (P) limitation of zooplankton growth under realistic food conditions, we performed a set of 5-d experiments using Daphnia dentifera and suspended particulate matter (seston) from three lakes at the Experimental Lakes Area (Ontario, Canada). Neonate Daphnia fed for 6 h per day on freshly collected seston enriched or unenriched with PO, and spent the rest of the day feeding on unaltered natural seston. PO, enrichment did not affect food abundance or concentrations and composition of essential fatty acids but dramatically lowered seston C:P ratio and significantly stimulated Daphnia growth. These results demonstrate that, even with field-collected seston, the effects of algal phosphorus limitation can extend to herbivores through reduced food quality.
Abstract: Macrozooplankton may affect algal and microbial plankton directly through grazing or predation and indirectly through nutrient regeneration. They may also affect potential prey positively by removing alternative predators. Here, we examined the effects of a cladoceran (Daphnia) and a calanoid copepod (Eodiaptomus) on algal and microbial plankton in a Japanese lake using in situ experiments in which we manipulated the nutrient supply and biomass of these macrozooplankton. The response of algal and microbial plankton to macrozooplankton was diverse and varied depending on the level of nutrient supply. Eodiaptomus seemed to feed mainly on large algae (> 20 mum) and microzooplankton, while direct grazing by Daphnia on algae, bacteria, heterotrophic nanoflagellates (HNF), and microzooplankton (ciliates, heliozoa, and rotifers) was pronounced. Trophic linkages within these microbial plankton was also suggested; bacteria were grazed by HNF and these in turn were grazed by microzooplankton. When the nutrient supply was high, both HNF and microzooplankton were exposed to higher amounts of algae and lower bacterial abundance. Moreover, nutrient regeneration by daphnids and Eodiaptomus copepods seemed to differentially stimulate the growth of algae and bacteria. The results suggest that the relationship between macrozooplankton and microbial plankton cannot be fully understood without taking into consideration not only the feeding characteristics of the macrozooplankton, but also the food web structure. the subsidized algal resource, and nutrient regeneration from the macrozooplankton.
Abstract: 1, Life-history traits of animals are known to change according to food quantity, but there is another type of resource depression, namely depletion of food quality. To examine life-history responses of herbivores to changes in resource quality, Daphnia obtusa Kurz, a common planktonic herbivore, were reared individually under several algal food regimes with different quantity and quality in terms of chemical composition. 2. Compared with Daphnia fed on a small quantity of nitrogen (N) and phosphorus (P) sufficient food, Daphnia fed on a large amount of P deficient food grew more slowly at young ages but continually grew until older ages. This trend was also found for Daphnia fed on a large amount of N deficient food. 3, However, the size-specific reproductive investment (measured as biomass accrued per unit time) of Daphnia fed on a large amount of N or P deficient algae was similar to those fed on a small amount of N and P sufficient food, indicating complex life-history responses to food quality and suggesting that nutritional requirements for reproduction differ from somatic growth. 4, Compared with individuals fed high-quality and high-quantity food, Daphnia fed on a large amount of P deficient food produced smaller eggs, while individuals fed on a small amount of P and N sufficient food produced larger eggs. Furthermore, 15-30% of eggs produced by individuals fed on P or N deficient food ceased to develop and were aborted. 5, These results demonstrate that life-history responses to food quality differ from responses to food quantity. Since the chemical composition of algae is highly variable in nature, herbivorous plankton demographics may not be predictable from resource quantity alone.
Abstract: Diel changes in bacterial ingestion by a mixotrophic flagellate, Cryptomonas sp., and heterotrophic nanoflagellates (HNF) were examined in situ at 4-h intervals for 2 d in the epilimnion and metalimnion of Lake Biwa using bacteria-sized fluorescent microspheres as a tracer food. Clearance rates of HNF for the microspheres ranged between 1.3 and 4.5 nl cell(-1) h(-1), but the average rate did not differ between day and night. In contrast, clear diel changes were observed in the clearance rate of Cryptomonas sp. in the epilimnion from <0.5 nl cell(-1) h-l at midnight to >3 nl cell(-1) h(-1) at noon. in the metalimnion where light intensity was lower, however, the clearance rate of Cryptomonas sp. was always <0.5 nt cell(-1) h(-1) through the study period. Thus, bacterial ingestion of Cryptomonas sp. is not to acquire supplementary energy or carbon at low phototrophic activities. During the study period, both inorganic phosphorus and nitrogen concentrations were less than or close to the detection limits (10 nM P and 1 <mu>M N) in the epilimnion, but much higher in the metalimnion. The results strongly support the idea that Cryptomonas sp. utilizes N and P from bacteria as substitutable nutrients when photosynthesis takes place under conditions of nutrient depletion. To assess the grazing effect of mixotrophic algae on bacterial populations, it is essential to consider diel changes in their phagotrophic mode of nutrition that are induced by light regime and nutrient concentrations in ambient water.
Abstract: Grazers affect prey abundance negatively through grazing and positively through releasing nutrients and substrates. In the present study, the relative importance of these positive and negative effects of grazers on bacterial abundance was examined seasonally at 2.5 m and 30 m depths in Lake Biwa. We used a method by Elser and Goldman to estimate in situ growth rate, grazing rate, and the fraction of growth rate that was stimulated by grazers smaller than 100 mum. At 2.5 m, the bacterial grazing rate showed clear seasonal changes with higher values in summer. At 30 m, bacterial grazing was significant but consistently low throughout the year. The fraction of the growth rate stimulated by grazers was changed with season and depth. At 2.5 m, bacteria compensated 30-40% of grazing loss by using deficient nutrients released by the grazers from May to October. During that period, bacterial growth rate was severely limited by phosphorus, and it is most likely that phosphorus release by grazers was stimulating bacteria. At 30 m, however, the bacterial growth rate was not stimulated by the grazers' in the majority of the cases. Bacterial growth rate was limited by low temperature at this depth, so nutrient return from grazers does not have a beneficial. effect on bacterial growth. These results indicate that the relative importance of positive and negative effects of grazers on bacterial abundance differs seasonally and vertically; and that positive effect becomes important when bacterial growth is limited by nutrients. In a given habitat, bacterial abundance changes less than one order of magnitude in spite of high growth rates. The present study suggests that grazers contribute to numerical stability of bacterial assemblages not only through grazing but also by regenerating deficient nutrients.
Abstract: In this paper we consider the structure, in particular the singularities, of solutions of singular Cauchy problem for the following operator L with holomorphic coefficients in the neighbourhood of the origin of CZ under some conditions L = D-t(2) - (x + bt(2))D-x(2) - a(t, x)D-t - c(t, x)D-x - d(t, x). We construct its solution by so-called asymptotic expansion method and study its structure by the monodromy theory of the hypergeometric function.
Abstract: Simple correlation and multiple regression analyses were performed to examine the relationship between primary productivity and environmental factors in the north basin of Lake Biwa. The primary production rates used in the analyses were estimated monthly or bimonthly during the growing season (April-November) in 1992, 1996 and 1997 with the C-13 method. Elemental(C, N and P) contents of seston were used to assess nutrient conditions. Analyses revealed that 86% of variance in depth-integrated primary production rates (areal PP) can be explained by changes in light intensity, and sestonic C, N and P concentrations. Water temperature had no effect on areal PP. To assess relative effects of light and nutrients on PP, the P:B ratio was estimated by normalizing PP with sestonic C. The areal P:B ratio correlated most significantly with the sestonic N:P ratio, followed by light intensity. When regression analyses were made at each depth, however, the P:B ratio correlated significantly only with the sestonic N:P ratio at 0 and 1 m depths, while light intensity was also incorporated into the regressions at deeper than 2.5 m. In these regressions, the P:B ratio was negatively correlated with sestonic N:P ratio but positively with light intensity. The results suggest that the primary production rate in this lake was mainly limited by P relative to N supply rates, but was not free from light limitation in a large part of the epilimnion. In Lake Biwa, the vertical water mixing regime as well as the nutrient supply seem to be important in determining the growth and composition of primary producers, since the surface mixing layer extends into 10-15 m depths during most of the growing season.
Abstract: Dilution bioassays were performed to examine the seasonal and vertical difference in the relative importance of factors limiting growth of heterotrophic bacteria in Lake Biwa. The lake water diluted by 0.2 mu m lake filtrate (1:6.6) was enriched either with glucose (C), inorganic phosphorus (P), ammonium nitrogen (N), amino acids (AA), or a combination of these, and incubated for 2 days at the depths where lake water was collected (2.5, 20 and 30 m depths). Experiments showed that at 2.5 m, P was the most deficient resource for bacterial growth, but the magnitude of P limitation depended on water temperature. Among others, amino acids showed a slight but significant stimulation of bacterial growth rates during the fall. At 20 and 30 m, however, growth stimulation by resource addition was rarely detected. Vertically reciprocal translocation experiments revealed that the growth rate was limited by low temperature rather than resource supply at the greater depths. The results support a simple view that bacterial growth rate is basically regulated by water temperature, but high growth rate is not realized in summer because of resource depletion. The present study suggests that both temperature and P supply play a crucial role in biogeochemical cycling of organic matter in Lake Biwa through the bacterial growth rate.
Abstract: The mixotrophic alga Uroglena americana forms a dense 'bloom' in Lake Biwa, Japan, in spring when dissolved inorganic phosphorus is in short supply relative to the requirements for algal growth. To assess the possibility that this alga uses bacteria as a substitutable P source via phagotrophy, feeding experiments were conducted using bacteria-sized fluorescent microspheres as tracer particles. The experiments revealed that the ingestion rate of U. americana on the food particles (bacteria + microspheres) is significantly affected by the concentrations of phosphate as well as food particles in the lake water. The response of ingestion rate to phosphate concentration indicated that P levels greater than or equal to 0.4 mu M P were enough to reduce bacterivory by this alga. However, phosphate concentration had no effect on ingestion rate in heterotrophic nanoflagellates. These results suggest that U, americana can utilize bacteria as a substitutable P source. Such an ability favors the development and maintenance of dense blooms by this alga under P-limited conditions.
Abstract: Culture experiments were conducted with the alga Scenedesmus acutus and heterotrophic bacteria to examine if the nature of their relationship changes according to the balance of light and nutrient supplies. Mixtures of algae and bacteria were grown in various combinations of 6 light intensities and 4 phosphorus (P) concentrations at high N:P ratio (80:1). We used an artificial medium composed of inorganic nutrients so that bacteria relied on organic matter released by algae as carbon (C) source. Every 2 d, 25 % of the culture suspension was replaced by fresh medium. At the end of incubation when both bacterial and algal densities were stabilized, bacteria were separated from algae. Bioassays with glucose and/or inorganic P enrichment were then performed to assess the extent to which bacterial growth rate was limited by organic C or inorganic P. The algal density in the semibatch culture was low under the light intensity <55 mu E m(-2) s(-1) regardless of P concentrations, while it was higher at higher light and P supply rate above that light intensity. The bacterial density was higher in the cultures where algal density was higher. The bioassay revealed that bacteria were C limited at the light intensity <55 mu E m(-2) s(-1), indicating a commensal relationship between algae and bacteria. Above that light intensity, bacteria suffered from deficiency of organic carbon rather than P at lowest P supply rate, because of low algal biomass due to a shortage in P supply. At moderate P supply rates and Light intensities greater than or equal to 55 mu E m(-2) s(-1), however, bacterial growth was limited by P rather than organic C, because supply of organic C from algae exceeded P supply relative to bacterial demand. Further increase in P supply released both algae and bacteria from P limitation. Thus, competitive interaction for P was most intense at a moderate P supply rate. These results demonstrate that there is a shift between commensalism for C and competition for P depending on light intensity and nutrient supply rate.
Abstract: 1. The relationships between body length and dry weight were examined for eight species of freshwater planktonic crustacea common in Japan: Eodiaptomus japonicus, Acanthodiaptomus pacificus, Daphnia galeata, D. similis, D. magna, Scapholeberis mucronata, Simocephalus exspinosus and Bosmina longirostris. 2. The relationships of two diaptomid species were similar and the approximate equation was lnW = 2.7 +/- 2.6*lnL, where L is prosome length (mm) and W is dry weight (mu g). The carapace length-dry weight relationships in branchiopods were more variable, with the slope ranging from 1.9 to 2.9 and the intercept from 2.0 to 3.7. 3. The effects of food conditions on the relationship were examined and the seasonal changes in the field were also studied. 4. Practical advice is presented for predicting crustacean weight from body length.
Abstract: The possibility of P Limitation for zooplankton growth has many implications for understanding changes in production efficiency and feedback dynamics between consumers and resources. However, there have been no direct tests to determine whether the putative P limitation is real. To answer this question, we directly supplied inorganic P to Daphnia magna apart from food algae, Scenedesmus acutus, and then examined changes in body mass. During the period from birth to age 6 d, D. magna were fed on Live algae for 19 h and placed in water of high inorganic P (4 mM: P treatment) for 5 h each day. We used P-free water as a control treatment. Growth rate estimated from initial and final body mass during the 6-d incubation was significantly larger in the P treatment than in the control treatment when Daphnia fed on P-deficient algae, whereas a significant difference was not detected between the treatments for Daphnia fed on P-sufficient algae (C:P atomic < 300). The results clearly demonstrate that Daphnia growth is in fact limited by P itself when they feed on P-deficient algae.
Abstract: We examined whether or not the production of autotrophic picoplankton and heterotrophic bacteria (collectively, picoplankton) is funnelled up food web via a picoplankton-nanoflagellate-copepod food chain during a picocyanobacterial bloom in Lake Biwa. The experiment was carried out in late summer when the abundance of picocyanobacteria was of its seasonal maximum (7 x 10(5) cells ml(-1)) and copepods (Eodiaptomus japonicus) dominated the mesozooplankton assemblage. The grazing rates of a mesozooplankton assemblage on nanoflagellates and picoplankton were determined from changes in apparent growth rates of nanoflagellates and picoplankton with increasing mesozooplankton biomass in incubation bottles. Results indicated that the mesozooplankton assemblage consumed nanoflagellates but not picoplankton directly. Dilution culture experiments revealed that the production of picoplankton was mostly consumed by protozoan grazers in the cell-size fraction of <20 mu m (nanoflagellates). Carbon flux estimates indicated that of total carbon channelled from picoplankton to nanoflagellates (630 mg C m(-2) day(-1)), only 2 % was passed to mesozooplankton. The carbon from nanoflagellates accounted for <5 % of the total carbon consumption by mesozooplankton. Thus, the picoplankton-nanoflagellate-mesozooplankton trophic link was extremely inefficient and contributed little to the production of mesozooplankton dominated by copepods during a picocyanobacterial bloom in Lake Biwa.
Abstract: Experiments using planktonic organisms revealed that the balance of radiant energy and available nutrients regulated herbivore growth rates through their effects on abundance and chemical composition of primary producers. Both algae and herbivores were energy limited at low light/nutrient ratios, but both were nutrient limited at high light/nutrient ratios. Herbivore growth increased with increasing light intensity at low values of the light/nutrient ratio due to increases in algal biomass, but growth decreased with increasing light at a high light/nutrient ratio due to decreases in algal quality. Herbivore production therefore was maximal at intermediate levels of the light/nutrient ratio. The results contribute to an understanding of mass transfer mechanisms in ecosystems and illustrate the importance of integration of energy-based and material-based currencies in ecology.
Abstract: To clarify the contribution of metazoan plankton to N and P fluxes in Lake Biwa, we examined particle elimination, net production, and nutrient release rates simultaneously with primary production from June to November 1992. Throughout the study period, the zooplankton eliminated 70% of the elements fixed as particulate forms by primary producers. However, because the N:P ratio of the seston was higher than that of the zooplankton on most dates, the zooplankton selectively accumulated P from seston with higher efficiency, while a larger fraction of N was released to maintain elemental balance. As a result, the mass ratio of N to P released by the zooplankton changed markedly from 19 to 123 according to the magnitude of deviation in the N:P ratio of seston from that of the zooplankton. On average, the zooplankton released 50% of N but only 15% of P fixed by primary production during the study period. The results demonstrate that whether zooplankton act as source or sink for a bioelement depends on the relative abundance of that element in both the food and the zooplankton. In Lake Biwa, the zooplankton can promote P limitation for phytoplankton by fixing a large fraction of the P in the system into their own mass and recycling a substantial amount of the N to a dissolved form.
Abstract: Sestonic particles composed of phytoplankton, bacteria, and detritus are a potential food source for most zoo-plankton. In the present study, the effect of zooplankton on the stoichiometry of seston was examined in a small eutrophic pond from April to August. because the relative elemental contents of seston relates with phytoplankton growth rate and food quality. In the pond, most of the sestonic particles were edible size (< 30 mu m), and sestonic C:P and N:P ratios were high at low zooplankton biomass but low at high zooplankton biomass, suggesting that the zooplankton activity is one of the important factors inducing changes in seston stoichiometry. This possibility was substantiated by grazer gradient experiments that showed a significant increase in the relative P content of the seston with increasing zooplankton biomass. However, slopes of the regression line of the elemental ratios against the zooplankton biomass varied between seasons, and were greater at lower nutrient concentration and higher seston abundance. The relationship implies that the magnitude of the mass-specific effect of zooplankton depends on the relative nutrient concentration available for phytoplankton. In parallel with the grazer gradient experiments, zooplankton grazing was simulated by artificially eliminating some fraction of seston in order to separate the direct (eliminating particles) and indirect (nutrient release) effects on the seston stoichiometry. The response of sestonic elemental ratios to simulated grazing revealed that the zooplankton can affect seston stoichiometry by increasing the per capita availability of a deficient nutrient for phytoplankton through grazing as well as nutrient release. These results suggest that the effect of zooplankton on seston panicles is feedback to their growth rate by changing not only food abundance but also food quality.
Abstract: In order to clarify the role of zooplankton in the cycling of elements and its subsequent effect on algal populations, food elimination and nutrient release rates by zooplankton in a small pond dominated by daphnids were estimated for nitrogen and phosphorus from April to September. Comparison of the elemental contents of food and zooplankton showed that N was the element in shortest supply in the food sources utilized by zooplankton. The net clearance rate of the zooplankton was higher for N than for P and C throughout the study period, indicating that these organisms can eliminate the scarcest element from the food pool with a higher efficiency. Soluble N and P released by the zooplankton consisted mainly of ammonium nitrogen and soluble reactive phosphorus, and on a dry mass basis N release ranged from 0.27 to 1.05 mug.mg-1.h-1 and P release from 0.045 to 0.30 mug.mg-1.h-1. Statistical analysis revealed that the N release rate was most affected by food abundance, whereas the P release rate and N:P release ratio were affected by the N:P ratio of the food. The net production rate of the zooplankton, estimated from the difference between elimination and release rates, showed a quite similar N:P ratio to that of the body tissue, indicating that zooplankton can keep their elemental composition roughly constant even if the N:P ratio of their food supply varies. Thus, the N and P release rates of zooplankton are linked through processes maintaining the elemental composition of body tissues. The present results indicate that the cycling rate of an element in pelagic systems depends highly on the grazer's requirements as well as the relative abundances of other elements. Zooplankton can modify the nutritional environment for algae through accumulation and regeneration processes, as suggested by recent theoretical models.
Abstract: To examine the relationship between seston stoichiometry (C:N:P) and nutrient supply rate in a shallow eutrophic pond, nutrient enrichment experiments were carried out biweekly from May to September. During the study period, particulate carbon showed an eight-fold change, and the C:N ratio usually, and the C:P ratio on most dates, were higher than the Redfield ratio. However, signs of N limitation were detected in the experiment only in September. The experiments also revealed that phosphorus supply rate affects not only the C:P ratio but also the C:N ratio, and that the growth rate of seston is related linearly with the C:P ratio, although coefficients of the regression line differed among the experiments. The results indicate that seston stoichiometry per se is not useful for detecting limiting element and for evaluating the intensity of nutrient limitation of the phytoplankton. However, it was possible to estimate the intensity of nutrient limitation based on the straightforward relationship between the growth rate and sestonic elemental ratio established in each experiment. The estimated value indicates that the phytoplankton suffered from severe P limitation when seston was abundant in this pond.
Abstract: Threshold food N:C or P:C ratios, below which net production is N or P limited, were calculated for two cladocerans, Daphnia galeata and Bosmina longirostris, based on the N and P contents of body tissue and the C balance between ingestion and net production rates under experimental conditions with various food concentrations from 0.05 to 2.50 mg C liter-1. With changing food concentration, the threshold N:C and P:C ratios for D. galeata changed in weight ratio from 0.039 to 0.077 and 0.0067 to 0.0120, respectively, with the lowest value at the high food concentration. Changes in threshold ratios for B. longirostris, however, were small and mean values were 0.085 and 0.0080 for the N:C and P:C ratios. As a result, the threshold N:C ratio for D. galeata was lower than for B. longirostris at high food concentrations, while the P:C ratio for B. longirostris was lower than for D. galeata at lower food concentrations. Although the threshold elemental ratios calculated here are theoretically minimum estimates, they were higher than the lower end of the sestonic elemental ratios in some lakes, suggesting that it is not unusual for these cladocerans to be N or P limited at certain periods of the year. Sestonic elemental ratios may have potential importance in determining the community structure of zooplankton.
Abstract: Temporal changes in the density of rotifer plankton were examined in a shallow eutrophic pond during July to September 1990, when abiotic environmental variables were relatively stable. In early July when Daphnia similis was abundant, rotifer populations remained at a low density. This is probably due to interference by the large cladoceran, because possible food was abundant and the rotifers showed high egg ratio. After late July, when the large cladoceran disappeared, a striking succession was found in the rotifer plankton. In late July, when Filinia longiseta and Conochilus dossuarius dominated, and in mid- to late September when F.opoliensis and Brachionus falcatus dominated, there was a causal relationship between the density and egg ratio, suggesting that temporal changes in rotifer density were regulated mainly by food abundance. However, in August, changes in the density of most rotifers were not necessarily related with those in the egg ratio. Brachionus angularis, B.forficula and Keratella spp. decreased or remained at a low density regardless of the egg ratio, when the population of Asplanchna brightwelli was developed. However, Polyarthra vulgaris, B.calyciflorus and H.intermedia increased or maintained a high density at this time. Stomach contents of A.brightwelli revealed that B.angularis, B.forficula and Keratella spp. were the preferred prey, whereas P. vulgaris, B.calyciflorus and H.intermedia were not. These results provide strong evidence that the structure of rotifer plankton can change strikingly within a season due to species-specific differences not only in diet, but also in the ability to escape predation, even if abiotic environmental variables are stable.
Abstract: Feeding selectivities of Daphnia galeata were measured using various density combinations of C-14-labeled Chlamydomonas and H-3-labeled natural bacteria. In all food combinations, D. galeata preferred algae. Selectivity coefficient (bacterial clearance rate : algal clearance rate) was 0.35 at the level of incipient food limitation (1.5 x 10(6) mu-m3 ml-1). However, it increased up to 0.9 with increasing total food concentration. A simple mechanical sieving model alone cannot explain such a change in the selectivity coefficient. A mechanism of 'clogging' suggestively explain the increase of bacterial ingestion efficiency by D. galeata, especially at high food concentrations. The change of selectivity coefficient related to food condition suggests that bacterial feeding by the Daphnia is incidental event associated with the algal feeding.
Abstract: Measurements of growth, respiration and ingestion were used to examine the influence of food concentration on the assimilation and individual production efficiencies of Daphnia galeata G. O. Sars at food concentrations of 2.50, 0.25 and 0.05 mg C l-1. A plot of assimilation rate against body size showed an S-shaped curve. In the first adult instar, the rates at 2.50 mg C l-1 were two and five times higher than those at 0.25 and 0.05 mg C l-1, respectively. Assimilation efficiency, gross growth efficiency (K1) and net growth efficiency (K2) showed binomial curves with maxima at the last juvenile or early adult instars. Assimilation efficiency increased with decreasing food concentration; an opposite trend was found for K2. Except for early juvenile instars, K1 was highest at the intermediate food concentration, suggesting that there is an optimal resource concentration for energy transfer efficiency. The lower value of K1 at the higher food concentration was due to lower assimilation efficiency, while that at the lower food concentration was due to high respiration rate relative to the amount of food assimilated. These results demonstrate that food concentration affects the carbon balance not only quantitatively (rates), but also qualitatively (efficiencies). The change in carbon balance with changing food concentration is interpreted as a strategy for D. galeata to maximize its assimilation rate at given food concentrations.
Abstract: 1. The carbon balance of the small cladoceran, Bosmina longirostris, was examined at four food concentrations (0.05, 0.10, 0.25, 2.50 mg C l-1) based on long-term growth experiments. 2. At birth, B. longirostris allocated about 60% of assimilation to body growth at all food concentrations. However, allocation to body growth decreased with age and was less than 5% after the fourth instar at the lowest food concentration. The proportion allocated to reproduction increased with increasing food concentration, but was relatively constant among different adult instars. 3. Assimilation rate increased with increasing food concentration and increasing body size: the mean rate from birth to 15 days was 6.6 times higher at the highest food concentration than at the lowest food concentration. However, with decreasing food concentration and increasing body size, a higher proportion of the assimilated carbon was lost as respiration. As a result, the net production rate reached a plateau at intermediate body size and was 9.1 times higher at the highest food concentration than at the lowest food concentration. 4. Comparison with previous studies revealed that B. longirostris is more efficient than Daphnia with regard to carbon production, especially at low food conditions, if the death rate is negligible.
Abstract: 1. Body growth, reproduction and survivorship of Bosmina longirostris were monitored at four food concentrations (0.05, 0.10, 0.25 and 2.50 mg C l-1) to examine the extent to which food concentration affects these life-history parameters. 2. Food concentration had a significant effect on growth, and most of the reproductive parameters (size and age at maturation, brood size, instar duration and egg development time). More than 60% of animals died before maturation at the lowest food concentration, although most animals survived until maturation at other food concentrations. 3. Change in life-history parameters resulted in a decrease in rate of population growth (r) from 0.310 to 0.020 day-1 with decreasing food concentration, showing that this cladoceran is highly food-limited in many lakes. 4. Importance of the effect of food concentration on egg development time was emphasized in relation to analysis of natural populations.
Abstract: The zooplankton community of the Ogochi Reservoir was investigated with brief field experiments to examine the mechanisms that maintain a stable horizontal distribution of species in the warm-water season. Daphnia galeata predominated near the dam site, and its abundance declined toward the headwaters. The abundance of other species increased away from the dam, and Bosmina longirostris was found abundantly at the site near the headwaters. Demographic analysis and field experiments demonstrated that D. galeata had the potential to depress the abundance of other species through exploitative competition and to predominate at sites near the headwaters as well as at the dam site. This potential was never realized, however, at a site near the headwater because of a high mortality rate that could not be explained by food abundance or physical and chemical conditions in the water. The invertebrate predator Leptodora kindtii was abundant at sites nearer the headwaters, but its occurrence was limited temporally. Trammel gill-net samples revealed that planktivorous fish, which prey selectively on the superior competitor D. galeata and the invertebrate predator L. kindtii, were distributed toward the headwaters in the warm-water season. The high mortality rate of D. galeata at the site near the headwaters apparently was attributable to predation by fish. The data presented demonstrate that the pelagic environment in the reservoir is functionally heterogeneous and that species interactions play an important role in maintaining the stable horizontal pattern in the community structure.
