Is the Relationship between Body Size and Trophic Niche Position Time-Invariant in a Predatory Fish? First Stable Isotope Evidence

Characterizing relationships between individual body size and trophic niche position is essential for understanding how population and food-web dynamics are mediated by size-dependent trophic interactions. However, whether (and how) intraspecific size-trophic relationships (i.e., trophic ontogeny pattern at the population level) vary with time remains poorly understood. Using archival specimens of a freshwater predatory fish Gymnogobius isaza (Tanaka 1916) from Lake Biwa, Japan, we assembled a long-term (>40 years) time-series of the size-dependence of trophic niche position by examining nitrogen stable isotope ratios (δ ¹⁵N) of the fish specimens. The size-dependence of trophic niche position was defined as the slope of the relationship between δ ¹⁵N and log body size. Our analyses showed that the slope was significantly positive in about 60% of years and null in other years, changing through time. This is the first quantitative (i.e., stable isotope) evidence of long-term variability in the size-trophic relationship in a predatory fish. This finding had implications for the fish trophic dynamics, despite that about 60% of the yearly values were not statistically different from the long-term average. We proposed hypotheses for the underlying mechanism of the time-varying size-trophic relationship.     

Phosphorus and nitrogen co-limitation of forest ground vegetation under elevated anthropogenic nitrogen deposition

Amino acid nitrogen isotopic analysis is a relatively new method for estimating trophic position. It uses the isotopic difference between an individual’s ‘trophic’ and ‘source’ amino acids to determine its trophic position. So far, there is no accepted explanation for the mechanism by which the isotopic signals in ‘trophic’ and ‘source’ amino acids arise. Yet without a metabolic understanding, the utility of nitrogen isotopic analyses as a method for probing trophic relations, at either bulk tissue or amino acid level, is limited. I draw on isotopic tracer studies of protein metabolism, together with a consideration of amino acid metabolic pathways, to suggest that the ‘trophic’/‘source’ groupings have a fundamental metabolic origin, to do with the cycling of amino-nitrogen between amino acids. ‘Trophic’ amino acids are those whose amino-nitrogens are interchangeable, part of a metabolic amino-nitrogen pool, and ‘source’ amino acids are those whose amino-nitrogens are not interchangeable with the metabolic pool. Nitrogen isotopic values of ‘trophic’ amino acids will reflect an averaged isotopic signal of all such dietary amino acids, offset by the integrated effect of isotopic fractionation from nitrogen cycling, and modulated by metabolic and physiological effects. Isotopic values of ‘source’ amino acids will be more closely linked to those of equivalent dietary amino acids, but also modulated by metabolism and physiology. The complexity of nitrogen cycling suggests that a single identifiable value for ‘trophic discrimination factors’ is unlikely to exist. Greater consideration of physiology and metabolism should help in better understanding observed patterns in nitrogen isotopic values.     

<Superscript>15</Superscript>N/<Superscript>14</Superscript>N ratios of amino acids as a tool for studying terrestrial food webs: a case study of terrestrial insects (bees,wasps, and hornets)

Compound-specific stable isotope analysis (CSIA) of amino acids is a new method that enables estimates of trophic position for consumers in food webs. We examined the nitrogen isotopic composition (δ¹⁵N) of amino acids of Japanese social insects (three bee, three wasp, and four hornet species) to evaluate the potential of CSIA of amino acids in studies of terrestrial food webs. For wasps, we also examined samples at different growth stages (ranging from egg to adult) to assess the effect of metamorphosis on CSIA estimates of trophic position. The δ¹⁵N values of bulk tissues for Japanese social insects are only weakly correlated with the biologically expected trophic positions. In contrast, the trophic positions estimated from the δ¹⁵N values of amino acids (yielding values of between 2.0 and 2.3 for bees, between 2.8 and 3.3 for wasps, and between 3.5 and 4.1 for hornets) are consistent with the biologically expected trophic positions for these insects (i.e., 2.0 for bees, 3.0 for wasps, and 3.0–4.0 for hornets). Although large variability is observed among the δ¹⁵N values of individual amino acids (e.g., ranging from 3.0 to 14.9‰ for phenylalanine), no significant change is observed in the trophic position during wasp metamorphosis. Thus, the CSIA of amino acids is a powerful tool for investigating not only aquatic food webs but also terrestrial food webs with predatory insects.     

Are you what you eat? Effects of trophic discrimination factors on estimates of food assimilation and trophic position with a new estimation method

A key factor for estimates of assimilation of resources and trophic position based on stable isotope data is the trophic discrimination factor (TDF). TDFs are assumed based on literature reviews, but may vary depending on a variety of factors, including the type of diet. We analyzed effects of alternative TDFs on estimates of assimilated resources and trophic positions for an omnivorous fish, Jenynsia multidentata, that reveals dietary variation among locations across a salinity gradient of a coastal lagoon in southern Brazil. We also compared estimates of foods ingested vs. foods assimilated. Food assimilation was estimated using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios of food sources and consumer muscle tissue and an isotopic mixing model (SIAR); consumer trophic position (TP) was estimated from consumer and production source δ¹⁵N values. Diet was estimated using an index of relative importance based on frequency of occurrence and volumetric and numeric proportions of food items from stomach contents. The effect of variation in TDF on food assimilation and TP was tested using three alternative TDFs reported in review papers. We then created a new method that used food source-specific TDFs (reported separately for herbivores and carnivores) weighted in proportion to estimated assimilation of resources according to mixing model estimates to estimate TP (hereafter TP_WAR). We found that plant material was not assimilated in a proportion similar to its importance in the diet of fish at a freshwater site, and the new method yielded best assimilation estimates. Animal material made greatest contributions to fish biomass irrespective of TDFs used in the mixing model. The new method produced TP estimates consistent with differences in estimated food assimilation along the salinity gradient. Our findings support the idea that food source-specific TDFs should be used in trophic studies of omnivores, since the method improved our ability to estimate trophic position and resource assimilation, two important ecological indicators.     

Turnover rates of nitrogen stable isotopes in the salt marsh mummichog, Fundulus heteroclitus, following a laboratory diet switch

Nitrogen stable isotopes are frequently used in ecological studies to estimate trophic position and determine movement patterns. Knowledge of tissue-specific turnover and nitrogen discrimination for the study organisms is important for accurate interpretation of isotopic data. We measured δ¹⁵ N turnover in liver and muscle tissue in juvenile mummichogs, Fundulus heteroclitus, following a laboratory diet switch. Liver tissue turned over significantly faster than muscle tissue suggesting the potential for a multiple tissue stable isotope approach to study movement and trophic position over different time scales; metabolism contributed significantly to isotopic turnover for both liver and muscle. Nitrogen diet-tissue discrimination was estimated at between 0.0 and 1.2‰ for liver and –1.0 and 0.2‰ for muscle. This is the first experiment to demonstrate a significant variation in δ¹⁵ N turnover between liver and muscle tissues in a fish species.     

Global Trophic Position Comparison of Two Dominant Mesopelagic Fish Families (Myctophidae,Stomiidae) Using Amino Acid Nitrogen Isotopic Analyses

The δ¹⁵N values of organisms are commonly used across diverse ecosystems to estimate trophic position and infer trophic connectivity. We undertook a novel cross-basin comparison of trophic position in two ecologically well-characterized and different groups of dominant mid-water fish consumers using amino acid nitrogen isotope compositions. We found that trophic positions estimated from the δ¹⁵N values of individual amino acids are nearly uniform within both families of these fishes across five global regions despite great variability in bulk tissue δ¹⁵N values. Regional differences in the δ¹⁵N values of phenylalanine confirmed that bulk tissue δ¹⁵N values reflect region-specific water mass biogeochemistry controlling δ¹⁵N values at the base of the food web. Trophic positions calculated from amino acid isotopic analyses (AA-TP) for lanternfishes (family Myctophidae) (AA-TP ∼2.9) largely align with expectations from stomach content studies (TP ∼3.2), while AA-TPs for dragonfishes (family Stomiidae) (AA-TP ∼3.2) were lower than TPs derived from stomach content studies (TP∼4.1). We demonstrate that amino acid nitrogen isotope analysis can overcome shortcomings of bulk tissue isotope analysis across biogeochemically distinct systems to provide globally comparative information regarding marine food web structure.     

Fluid‐preserved fishes are one solution for assessing historical change in fish trophic level

1. There are few resources available for assessing historical change in fish trophic dynamics, but specimens held in natural history collections could serve as this resource. In contemporary trophic ecology studies, trophic and source information can be obtained from compound‐specific stable isotope analysis of amino acids of nitrogen (CSIA‐AA‐N).
2. We subjected whole Sebastes ruberrimus and Clupea pallasii to formalin fixation and 70% ethanol preservation. We extracted tissue samples from each fish pre‐fixation, after each chemical change, and then in doubling time for 32–64 days once placed in the final preservative. All samples were subjected to CSIA‐AA‐N, and their glutamic acid and phenylalanine profiles and associated trophic position were examined for differences over time by species.
3. Glutamic acid and phenylalanine values were inconsistent in direction and magnitude, particularly during formalin fixation, but stabilized similarly (in 70% ethanol) among conspecifics. In some cases, the amino acid values of our final samples were significantly different than our initial pre‐preservation samples. Nonetheless, significant differences in glutamic acid, phenylalanine, and estimated trophic position were not detected among samples that were in 70% ethanol for >24 hr.
4. Our results suggest that the relative trophic position of fluid‐preserved specimens can be estimated using CSIA‐AA‐N, and CSIA‐AA‐N estimates for fluid‐preserved specimens should only be reported as relative differences. Timelines of trophic position change can be developed by comparing specimens collected at different points in time, revealing trophic information of the past and cryptic ecosystem responses.

     

Determination of dissolved and particulate thiols in Lake Biwa water and extracted fulvic acids by solid phase extraction followed by HPLC with fluorescence detection

Thiols are important antioxidants that can modulate the bioavailability and biogeochemistry of many soft metals, although their detection remains challenging in both their reduced (R–S) and oxidized (R–S–S–R) forms. Here, a modified biochemical method was applied to determine the levels of dissolved and particulate thiols in Lake Biwa water and extracted Lake Biwa fulvic acids obtained at various depths. This method involves the use of the reducing agent tris(2-carboxyethyl)phosphine and the fluorescent label 7-fluorobenzofurazan-4-sulfonic acid ammonium salt (SBD-F), followed by solid-phase extraction and HPLC with fluorescence detection. Dissolved cysteine (Cys) (2.0–6.0 nM), glutathione (GSH) (2.8–5.1 nM), and N-acetyl-l-cysteine (NAC) (1.6–4.2 nM) were detected throughout the water column but were broadly consistent at depths of 5–20 m. In contrast, abundant levels of particulate cysteine (1.3–3.5?×?10² nM) and glutathione (1.6–3.1?×?10² nM) were detected down to depths of 15 m. The particulate cysteine and glutathione were significantly covariant, and the ratios between them reflected the differences in the plankton community composition and availability of these compounds. This work also studied the concentrations of Cys, GSH and NAC in Lake Biwa fulvic acids (LBFAs) for the first time (at 0 m: cysteine, 0.8 nM; glutathione, 1.6 nM; NAC, 2.5 nM; at 10 m: cysteine, 1.4 nM; glutathione, 0.6 nM; NAC, 1.6 nM). The nanomolar to sub-nanomolar concentrations of the particulate and dissolved Cys, GSH and NAC in the lake indicates that these are an important class of ligands for chalcophile metals and may influence the distribution of plankton communities from the epilimnion to the hypolimnion of the lake.     

Impacts of zebra mussels (Dreissena polymorpha) on isotopic niche size and niche overlap among fish species in a mesotrophic lake

Zebra mussels (Dreissena polymorpha) filter feed phytoplankton and reduce available pelagic energy, potentially driving fish to use littoral energy sources in lakes. However, changes in food webs and energy flow in complex fish communities after zebra mussel establishment are poorly known. We assessed impacts of zebra mussels on fish littoral carbon use, trophic position, isotopic niche size, and isotopic niche overlap among individual fish species using δ¹³C and δ¹⁵N data collected before (2014) and after (2019) zebra mussel establishment in Lake Ida, MN. Isotope data were collected from 11 fish species, and from zooplankton and littoral invertebrates to estimate baseline isotope values. Mixing models were used to convert fish δ¹³C and δ¹⁵N into estimates of littoral carbon and trophic position, respectively. We tested whether trophic position, littoral carbon use, isotopic niche size, and isotopic niche overlap changed from 2014 to 2019 for each fish species. We found few effects on fish trophic position, but 10 out of 11 fish species increased littoral carbon use after zebra mussel establishment, with mean littoral carbon increasing from 43% before to 67% after establishment. Average isotopic niche size of individual species increased significantly (2.1-fold) post zebra mussels, and pairwise-niche overlap between species increased significantly (1.2-fold). These results indicate zebra mussels increase littoral energy dependence in the fish community, resulting in larger individual isotopic niches and increased isotopic niche overlap. These effects may increase interspecific competition among fish species and could ultimately result in reduced abundance of species less able to utilize littoral energy sources.

     

Diet quality influences isotopic discrimination among amino acids in an aquatic vertebrate

Stable nitrogen isotopic composition of amino acids (δ¹⁵N_AA) has recently been employed as a powerful tool in ecological food web studies, particularly for estimating the trophic position (TP) of animal species in food webs. However, the validity of these estimates depends on the consistency of the trophic discrimination factor (TDF; = Δδ¹⁵N_AA at each shift of trophic level) among a suite of amino acids within the tissues of consumer species. In this study, we determined the TDF values of amino acids in tadpoles (the Japanese toad, Bufo japonicus) reared exclusively on one of three diets that differed in nutritional quality. The diets were commercial fish‐food pellets (plant and animal biomass), bloodworms (animal biomass), and boiled white rice (plant carbohydrate), representing a balanced, protein‐rich, and protein‐poor diet, respectively. The TDF values of two “source amino acids” (Src‐AAs), methionine and phenylalanine, were close to zero (0.3–0.5‰) among the three diets, typifying the values reported in the literature (~0.5‰ and ~0.4‰, respectively). However, TDF values of “trophic amino acids” (Tr‐AAs) including alanine, valine, leucine, isoleucine, and glutamic acid varied by diet: for example, the glutamic acid TDF was similar to the standard value (~8.0‰) when tadpoles were fed either the commercial pellets (8.0‰) or bloodworms (7.9‰), but when they were fed boiled rice, the TDF was significantly reduced (0.6‰). These results suggest that a profound lack of dietary protein may alter the TDF values of glutamic acid (and other Tr‐AAs and glycine) within consumer species, but not the two Src‐AAs (i.e., methionine and phenylalanine). Knowledge of how a nutritionally poor diet can influence the TDF of Tr‐ and Src‐AAs will allow amino acid isotopic analyses to better estimate TP among free‐roaming animals.     

High‐resolution food webs based on nitrogen isotopic composition of amino acids

Food webs are known to have myriad trophic links between resource and consumer species. While herbivores have well‐understood trophic tendencies, the difficulties associated with characterizing the trophic positions of higher‐order consumers have remained a major problem in food web ecology. To better understand trophic linkages in food webs, analysis of the stable nitrogen isotopic composition of amino acids has been introduced as a potential means of providing accurate trophic position estimates. In the present study, we employ this method to estimate the trophic positions of 200 free‐roaming organisms, representing 39 species in coastal marine (a stony shore) and 38 species in terrestrial (a fruit farm) environments. Based on the trophic positions from the isotopic composition of amino acids, we are able to resolve the trophic structure of these complex food webs. Our approach reveals a high degree of trophic omnivory (i.e., noninteger trophic positions) among carnivorous species such as marine fish and terrestrial hornets.This information not only clarifies the trophic tendencies of species within their respective communities, but also suggests that trophic omnivory may be common in these webs.     

Morphological and Phylogenetic Characterizations of Freshwater Thioploca Species from Lake Biwa, Japan, and Lake Constance, Germany

Filamentous, gliding, sulfide-oxidizing bacteria of the genus Thioploca were found on sediments in profundal areas of Lake Biwa, a Japanese freshwater mesotrophic lake, and were characterized morphologically and phylogenetically. The Lake Biwa Thioploca resembled morphologically Thioploca ingrica, a brackish water species from a Danish fjord. The diameters of individual trichomes were 3 to 5.6 μm; the diameters of complete Thioploca filaments ranged from 18 to 75 μm. The cell lengths ranged from 1.2 to 3.8 μm. In transmission electron microscope specimens stained with uranyl acetate, dense intracellular particles were found, which did not show any positive signals for phosphorus and sulfur in an X-ray analysis. The 16S rRNA gene of the Thioploca from Lake Biwa was amplified by using newly designed Thioploca-specific primers (706-Thioploca, Biwa160F, and Biwa829R) in combination with general bacterial primers in order to avoid nonspecific amplification of contaminating bacterial DNA. Denaturing gradient gel electrophoresis (DGGE) analysis of the three overlapping PCR products resulted in single DGGE bands, indicating that a single 16S rRNA gene had been amplified. With the same method, the Thioploca from Lake Constance was examined. The 16S rRNA sequence was verified by performing fluorescence in situ hybridization targeted at specific motifs of the Lake Biwa Thioploca. Positive signals were obtained with the bacterial probe EUB-338, the γ-proteobacterial probe GAM42a, and probe Biwa829 targeting the Lake Biwa Thioploca. Based on the nearly complete 16S rRNA sequence and on morphological similarities, the Thioploca from Lake Biwa and the Thioploca from Lake Constance are closely related to T. ingrica and to each other.     

Hidden mitochondrial DNA divergence in the Lake Biwa endemic goby Gymnogobius isaza: implications for its evolutionary history

The gobiid Gymnogobius isaza is an endemic species that has adapted remarkably to the pelagic environment in Lake Biwa, Japan, a representative ancient lake in East Asia. To obtain clues that would reveal the origin and evolution of this species, we conducted phylogenetic and population genetic analyses based on partial sequences of the mitochondrial cytochrome b gene. Consistent with previous studies, our Bayesian phylogenetic analysis with a relaxed molecular clock model reconfirmed a sister relationship between G. isaza and Gymnogobius urotaenia?+?Gymnogobius petschiliensis, with a divergence time of about 2.9 million years (Myr), and provided an evolutionary rate of 3.0 %/Myr (pairwise) for their clade. Population genetic analysis revealed two distinct mtDNA groups in G. isaza, which were estimated to have diverged 0.66 million years ago (Mya) at the Lake Katata stage of Paleo-Lake Biwa, preceding the origin of the present Lake Biwa environment with its extensive deep pelagic area (0.3–0.4 Mya). A dumbbell-like haplotype network and bimodal mismatch distribution suggested that the population of G. isaza experienced secondary contact between two genetically differentiated populations. Demographic parameters from mismatch distribution analysis and Bayesian skyline plot analysis suggested that both of the mtDNA groups of G. isaza exhibited a signal of sudden population expansion at approximately the same time (80–90 thousand years ago) during the last glacial period after the development of the present Lake Biwa. These results imply the complex population history of G. isaza, including genetic isolation and secondary contact following differentiation from its relatives in the Pliocene.     

Past and present trophic position and decadal changes in diet of Yellow-legged Gull in the Azores Archipelago, NE Atlantic

This study evaluates the trophic position of adult Yellow-legged Gulls Larus michahellis atlantis resident in the Azores archipelago in the past (1921–1928) and in the present (2009–2010), and analyses the decadal variation in the diet of breeding birds from the 1990s to the 2000s for three main colonies (Topo Islet, Baixo Islet and Mistério da Prainha). Using mixing models, we compared stable isotope signatures of nitrogen and carbon in adult breast feathers between birds from 1921 to 1928 (held in museum collections) and 2009 to 2010, jointly with both isotopic signatures of their main prey groups (fish, goose barnacles (Lepas anatifera), seabirds, mammals and refuse). The diet of breeding birds was analysed using pellets collected in 1989, 1995, 1996, 2004, 2009 and 2010. Stable isotopes analysis (SIA) results were in accordance with the results provided from the analysis of pellets, showing a relatively recent and significant change in the diet of adult gulls. In particular, SIA revealed a significant decrease in the trophic position of Yellow-legged Gulls in the Azores, over the last 89 years in response to the decrease in the consumption of seabirds and fish and, an increase in the consumption of marine invertebrates (goose barnacles) and refuse. The analysis of pellets confirmed the significant decrease in the fish ingested, whereas the ingestion of lower trophic level prey (i.e. goose barnacles, mammals and refuse) increased. Both methods reflect the feeding plasticity and opportunistic foraging behavior of this species, and are in accordance with patterns described for continental Europe.     

Trophic response to ecological conditions of habitats: Evidence from trophic variability of freshwater fish

1. To adapt to ecological and environmental conditions, species can change their ecological niche (e.g., interactions among species) and function (e.g., prey‐predation, diet competition, and habitat segregation) at the species and guild levels. Stable isotope analysis of bulk carbon and nitrogen of organisms has conventionally been used to evaluate such adaptabilities in the scenopoetic and bionomic views as the isotopic niche width.
2. Compound‐specific stable isotope analysis (CSIA) of nitrogen within amino acids provides trophic information without any disruption of scenopoetic views in the isotope ratios, unlike conventional bulk isotope analysis provides both information and therefore frequently hinders its usefulness for trophic information.
3. We performed CSIA of amino acids to understand the trophic variability of the pike gudgeon Pseudogobio esocinus and largemouth bass Micropterus salmoides as representative specialist and generalist fish species, respectively, from 16 ecologically variable habitats in the four major rivers of Korea.
4. There was little variation (1σ) in the trophic position (TP) among habitats for P. esocinus (± 0.2); however, there was considerably large variation for M. salmoides (± 0.6). The TP of M. salmoides was negatively correlated with the benthic invertebrate indices of the habitats, whereas the TP of P. esocinus showed no significant correlation with any indices. Thus, these two representative fish species have different trophic responses to ecological conditions, which is related to known differences in the trophic niche between specialists (i.e., small niche width) and generalists (i.e., large niche width).
5. Over the past four decades, the conventional bulk isotope analysis has not been capable of deconvoluting “scenopoetic” and “bionomic” information. However, in the present study, we demonstrated that the CSIA of amino acids could isolate trophic niches from the traditional ecological niche composed of trophic and habitat information and evaluated how biological and ecological indices influence the trophic response of specialists and generalists.

     

Resilience to changes in lake trophic state: Nutrient allocation into Daphnia resting eggs

During past decades, many lakes underwent drastic human‐caused changes in trophic state with strong implications for population dynamics and food web processes. We investigated the influence of trophic state on nutrient allocation into Daphnia resting eggs. The production of resting eggs is an important survival strategy, allowing Daphnia to cope with unfavorable environmental conditions. Allocation of essential nutrients into resting eggs may crucially influence embryonic development and offspring survival and thus is of great ecological and evolutionary interest. The capacity of Daphnia to adjust the allocation of nutrients into resting eggs may depend on the dietary nutrient supply, which may vary with trophic state‐related changes in the phytoplankton community composition. Resting eggs were isolated from sediment cores taken from Lake Constance, a large prealpine lake with a distinct eutrophication and reoligotrophication history, and analyzed for elemental (carbon, nitrogen, and phosphorus) and biochemical (sterols and fatty acids) nutrients. Carbon allocation into Daphnia resting eggs continuously decreased over time, irrespective of changes in trophic state. The allocation of nitrogen into Daphnia resting eggs followed the changes in trophic state, that is, nitrogen concentrations in resting eggs increased with eutrophication and decreased again with reoligotrophication. The allocation of phosphorus, sterols and long‐chain polyunsaturated fatty acids, such as eicosapentaenoic acid, into Daphnia resting eggs did not change significantly over time. Changes in trophic state strikingly influenced all trophic levels in Lake Constance. However, nutrient allocation into Daphnia resting eggs was mostly resilient to changes in lake trophic state.     

Fatty acid composition of fish species with different feeding habits from an Arctic Lake

We compared the composition and content of fatty acids (FAs) in fish with different feeding habits (sardine (least) cisco Coregonus sardinella, goggle-eyed charr (pucheglazka) form of Salvelinus alpinus complex, humpback whitefish Coregonus pidschian, broad whitefish Coregonus nasus, boganid charr Salvelinus boganidae, and northern pike Esox lucius from an Arctic Lake. Feeding habits of the studied fish (planktivore, benthivore, or piscivore) significantly affected the composition of biomarker fatty acids and the ratio of stable isotopes of carbon and nitrogen in their biomass. The hypothesis on a higher content of eicosapentaenoic and docosahexaenoic acids in the fish of higher trophic level (piscivores) when compared within the same taxonomic group (order Salmoniformes) was confirmed.     

Role of phytoplankton size distribution in lake ecosystems revealed by a comparison of whole plankton community structure between Lake Baikal and Lake Biwa

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 (>20μm) in mesotrophic Lake Biwa, and picophytoplankton (<2μm) or nanophytoplankton (2–20μm) 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.     

How important are trophic state,macrophyte and fish population effects on cladoceran community? A study in Lake Erhai

Environmental controls on cladoceran community structure in lake ecosystems are complex and may involve many environmental parameters including trophic state and fish populations. In Lake Erhai, a plateau lake located in southwest China, it was hypothesized that a combination of lake eutrophication and planktivorous fish introduction would increase the abundance of cladoceran, while also decreasing cladoceran size. To test this hypothesis, we examined temporal changes in cladoceran microfossils in the sediments of Lake Erhai over the past century. The influence of changing macrophyte coverage within the littoral region of the lake was also considered. Results demonstrated that cladoceran abundance (measured as flux of cladoceran fossils in the sediments) increased markedly accompanying eutrophication of the lake. In addition, there was a shift in the dominant cladoceran species from those species that prefer oligotrophic conditions to those that prefer mesotrophic and eutrophic conditions. A reduction in the ephippium length of Daphnia spp. was observed and attributed to the introduction of the planktivorous fish Neosalanx taihuensis. Our findings indicated that eutrophication and fish introduction were the main controls affecting cladoceran community structure during the recent decades, and predation by planktivorous fish had an important impact on Daphnia body size.