Diet segregation between two sympatric 'small'Barbus spp: an experimental study of mechanisms

Gut contents of two co‐occurring species of 'small' diploid barbs (<10  L _F cm) in Lake Tana revealed that zooplankton is the major diet component for B. tanapelagius (75% based on volume), but less prominent in B. humilis (40%). Functional response experiments in the laboratory were conducted to elucidate the mechanisms causing this difference. The type of functional response by the two 'small' barbs under different microcrustacean zooplankton densities (10, 20, 40, 60 and 80 ind.l⁻¹) was examined. The functional response of B. tanapelagius to increasing prey densities corroborates with Holling Type II model, whereas B. humilis exhibits a Type III functional response. Predation rate is higher for B. tanapelagius at low zooplankton density (<40 ind.l⁻¹) and equals the level of B. humilis at higher densities (>40 ind.l⁻¹). This suggests that at lower zooplankton densities B. humilis is a less efficient forager on zooplankton prey items than B. tanapelagius . In Lake Tana average zooplankton density is relatively low (<35 ind.l⁻¹). Under these food conditions, B. humilis is forced to feed on other food items (e.g. benthic invertebrates), whereas B. tanapelagius primarily feeds on zooplankton. The feeding potentials of the two 'small' barbs, as deduced from their morphology explain their different performances and their segregation in space and food resources.     

Food sources and lipid retention of zooplankton in subarctic ponds

1. Subarctic ponds are seasonal aquatic habitats subject to short summers but often have surprisingly numerous planktonic consumers relative to phytoplankton productivity. Because subarctic ponds have low pelagic productivity but a high biomass of benthic algae, we hypothesised that benthic mats provide a complementary and important food source for the zooplankton. To test this, we used a combination of fatty acid and stable isotope analyses to evaluate the nutritional content of benthic and pelagic food and their contributions to the diets of crustacean zooplankton in 10 Finnish subarctic ponds. 2. Benthic mats and seston differed significantly in total lipids, with seston (62.5 μg mg^?1) having approximately eight times higher total lipid concentrations than benthic mats (7.0 μg mg^?1). Moreover, the two potential food sources differed in their lipid quality, with benthic organic matter completely lacking some nutritionally important polyunsaturated fatty acids (PUFA), most notably docosahexaenoic acid and arachidonic acid. 3. Zooplankton had higher PUFA concentrations (27–67 μg mg^?1) than either of the food sources (mean benthic mats: 1.2 μg mg^?1; mean seston: 9.9 μg mg^?1), indicating that zooplankton metabolically regulate their accumulation of PUFA. In addition, when each pond was evaluated independently, the zooplankton was consistently more ¹³C‐depleted (δ¹³C ?20 to ?33‰) than seston (?23 to ?29‰) or benthic (?15 to ?27‰) food sources. In three ponds, a subset of the zooplankton (Eudiaptomus graciloides, Bosmina sp., Daphnia sp. and Branchinecta paludosa) showed evidence of feeding on both benthic and planktonic resources, whereas in most (seven out of 10) ponds the zooplankton appeared to feed primarily on plankton. 4. Our results indicate that pelagic primary production was consistently the principal food resource of most metazoans. While benthic mats were highly productive, they did not appear to be a major food source for zooplankton. The pond zooplankton, faced by strong seasonal food limitation, acquires particular dietary elements selectively.     

Food web complexity affects stoichiometric and trophic interactions

The stoichiometry of trophic interactions has mainly been studied in simple consumer–prey systems, whereas natural systems often harbour complex food webs with abundant indirect effects. We manipulated the complexity of trophic interactions by using simple laboratory food webs and complex field food webs in enclosures in Lake Erken. In the simple food web, one producer assemblage (periphyton) and its consumers (benthic snails) were amended by perch, which was externally fed by fish food. In the complex food web, two producer assemblages (periphyton and phytoplankton), their consumers (benthic invertebrates and zooplankton) and perch feeding on zooplankton were included. In the simple food web perch affected the stoichiometry of periphyton and increased periphyton biomass and the concentration of dissolved inorganic nitrogen. Grazers reduced periphyton biomass but increased its nutrient content. In the complex food web, in contrast to the simple food web, perch affected periphyton biomass negatively but increased phytoplankton abundance. Perch had no influence on benthic invertebrate density, zooplankton biomass or periphyton stoichiometry. Benthic grazers reduced periphyton biomass and nutrient content. The difference between the simple and the complex food web was presumably due to the increase of pelagic cyanobacteria ( Gloeotrichia sp.) with fish presence in the complex food web, thus fish had indirect negative effects on periphyton biomass through nutrient competition and shading by cyanobacteria. We conclude that the higher food web complexity through the presence of pelagic primary producers (in this case Gloeotrichia sp.) influences the direction and strength of trophic and stoichiometric interactions.     

Habitat utilization by sympatric arctic charr Salvelinus alpinus L. and brown trout Salmo trutta L. in Lake Atnsjø, south-east Norway

SUMMARY. 1. Habitat utilization, as well as inter- and intraspecific relations of different size groups of arctic charr (Salvelinus alpinus (L.)) and brown trout (Salmo trutta L.) in Lake Atnsjø, south-east Norway, were investigated by analysing food and spatial niches from monthly benthic and pelagic gillnet catches during June-October 1985.
2. Small individuals (150–230 mm) of both arctic charr and brown trout occurred in shallow benthic habitats. However, they were spatially segregated as arctic charr dominated at depths of 5–15 m and brown trout at depths of 0–5 m.
3. Larger (>230 mm) arctic charr and brown trout coexisted in the pelagic zone. Both species occurred mainly in the uppermost 2-3 m of the pelagic, except in August, when arctic charr occurred at high densities throughout the 0–12 m depth interval. On this occasion, arctic charr were segregated in depth according to size, with significantly larger fish in the top 6 m. This was probably due to increased intraspecific competition for food.
4. The two species differed in food choice in both habitats, Arctic charr fed almost exclusively on zooplankton, whereas brown trout had a more variable diet, consisting of surface insects, zooplankton. aquatic insects and fish.
5. The data suggest that the uppermost pelagic was the more favourable habitat for both species. Large individuals having high social position occupied this habitat, whereas small individuals lived in benthic habitat where they were less vulnerable to agonistic behaviour from larger individuals and less exposed to predators. The more aggressive and dominant brown trout occupied the more rewarding part of the benthic habitat.     

Isotopic analysis of the sources of organic carbon for zooplankton in shallow subarctic and arctic waters

Shallow high-latitude lakes and ponds are usually characterized by an oligotrophic water column overlying a biomass-rich, highly productive benthos. Their pelagic food webs often contain abundant zooplankton but the importance of benthic organic carbon versus seston as their food sources has been little explored. Our objectives were to measure the δ¹³C and δ¹⁵N isotopic signatures of pelagic and benthic particulate organic matter (POM) in shallow water bodies in northern Canada and to determine the relative transfer of this material to zooplankton and other aquatic invertebrates. Fluorescence analysis of colored dissolved organic matter (CDOM) indicated a relatively strong terrestrial carbon influence in five subarctic waterbodies whereas the CDOM in five arctic water columns contained mostly organic carbon of autochthonous origin. The isotopic signatures of planktonic POM and cohesive benthic microbial mats were distinctly different at all study sites, while non-cohesive microbial mats often overlapped in their δ¹³C signals with the planktonic POM. Zooplankton isotopic signatures indicated a potential trophic link with different fractions of planktonic POM and the non-cohesive mats whereas the cohesive mats did not appear to be used as a major carbon source. The zooplankton signals differed among species, indicating selective use of resources and niche partitioning. Most zooplankton had δ¹³C values that were intermediate between the values of putative food sources and that likely reflected selective feeding on components of the pelagic or benthic POM. The results emphasize the likely importance of benthic-pelagic coupling in tundra ecosystems, including for species that are traditionally considered pelagic and previously thought to be dependent only on phytoplankton as their food source.     

Diet of Murray cod (<Emphasis Type="Italic">Maccullochella peelii peelii</Emphasis>) (Mitchell) larvae in an Australian lowland river in low flow and high flow years

Researchers have hypothesised that influxes of pelagic zooplankton to river channels after floods and high flows are necessary for strong recruitment of some native fish species, including Murray cod (Maccullochella peelii peelii) (Mitchell), in the Murray–Darling river system, Australia. This study investigated the composition of the diet and gut fullness of drifting Murray cod larvae weekly during two spawning seasons with contrasting flows, to determine if pelagic zooplankton comprised a greater proportion of the gut contents and guts were fuller in a high flow (2000) than in a low flow (2001) year. Gut fullness and yolk levels of 267 larvae were ranked, and prey identified to family level. Approximately 40 and 70% of individuals had been feeding in 2000 and 2001, respectively. Gut fullness increased with declining yolk reserves. Larvae in both the years had an almost exclusively benthic diet, irrespective of the flow conditions at the time. Substantial inundation of dry ground in 2000, albeit restricted to in-channel benches, anastomosing channels and oxbow lakes, did not lead to an influx of pelagic, floodplain-derived zooplankton subsequently exploited by Murray cod larvae. These results have the implications for the management of regulated temperate lowland rivers: high flows cannot automatically be assumed to be beneficial for the fish larvae of all species and their food resources, and caution should be exercised with the timing of flow releases.     

Spatial and Temporal Variation of Cestode Infection and its Effects on Two Small Barbs (Barbus humilis and B. tanapelagius) in Lake Tana, Ethiopia

Pseudophyllidean cestodes as Ligula have a complex life cycle with cyclopoid copepods as first intermediate host, zooplanktivorous fish as second, and piscivorous birds as final host. We studied the effects of diet, season and habitat occupation on the prevalence of plerocercoid larvae of the tapeworm Ligula intestinalis in two closely related small barbs and the effects of the parasites on the barbs life histories in Lake Tana (Ethiopia) during 1 year. In all affected barbs L. intestinalis caused retardation in gonad development, maturation at reduced size and lower absolute fecundity. Infection rate, averaged over all habitats was significantly higher in B. tanapelagius (10%) than in B. humilis (6%). Below a threshold of 48 mm the infection rate was zero for both barbs, this coincided with a very low proportion of copepods in their diets, increasing up to 90 and 55%, respectively, for their largest size class (81–90 mm). The relatively high infection rate in B. tanapelagius is explained by its obligatory zooplanktivorous feeding behaviour, ingesting a relatively high proportion of infected cyclopoid copepods. This is in contrast with B. humilis, which is a polyphagous species, feeding both on zooplankton and benthic invertebrates. Significant seasonal effects in infection rates were observed. In both barb species infection rates were lower during the breeding season. Only for B. tanapelagius a significant negative correlation was observed between rain fall and infection rate, probably caused by an increased turbidity that decreases feeding efficiency on zooplankton. Habitat type had also a significant effect on infection rate. Barbus humilis showed a much higher infection rate in shallow clear water (10%) than in shallow turbid water (3%), whereas B. tanapelagius showed much higher infection rates in the shallow sublittoral (13%) than in the deeper pelagic (7%). Most likely, birds predate more efficiently on barbs in shallow clear waters than in shallow turbid and deep waters.     

Phenotype-specific feeding behaviour of two arctic charr Salvelinus alpinus morphs

The feeding performance of two morphs of a trophically polymorphic fish, the arctic charr Salvelinus alpinus, feeding on zooplankton, was studied in the laboratory. Limnetic morphs, with a fusiform body, terminal mouth, short pectoral fins, and relatively long and dense gill rakers, fed at significantly higher rates on Daphnia and Cyclops than did benthic morphs. The benthic morphs were characterized by a chunky body form, subterminal mouth, long pectoral fins, and relatively short and spaced-out gill rakers. The limnetic morphs attacked the prey at significantly higher rates and were more successful in capture. Benthic morphs were more reluctant to start feeding on zooplankton and reaction time was longer than in limnetic morphs. In the absence as well as presence of zooplankton food, the position of the two morphs in the aquaria differed markedly. The results agree with dietary differences found in field-collected fish. The limnetic morphs consume zooplankton, whereas benthic morphs specialize on Lymnaea gastropods. The results suggest that benthic morphs are phenotypically inferior at using the pelagic food niche. This implies that the current role of competitive or predatory interactions in segregating the morphs may be minor.     

Growth, biomass, and production of two small barbs (Barbus humilis and B. tanapelagius, Cyprinidae) and their role in the food web of Lake Tana (Ethiopia)

Growth, biomass and production of two small barbs (Barbus humilis and Barbus tanapelagius) and their role in the food web of Lake Tana were investigated. From length–frequency distribution of trawl monitoring surveys growth coefficient, Φ′ values were estimated at 3.71–4.17 for B. humilis and 3.70–4.14 for B. tanapelagius, respectively. Values for B. humilis were confirmed in pond experiments. Mean biomass of the small barbs was 13.3 kg fresh wt ha⁻¹, with B. humilis being most abundant in the littoral and sub-littoral zones, whereas B. tanapelagius was most abundant in the sub-littoral and pelagic zones. The two small barbs had a production of 53 kg fresh wt ha⁻¹ year⁻¹. Although their P/B ratios of about 4.0 were relatively high for small cyprinids, both their biomass and production were low in comparison with other small fish taxa in other tropical lakes. Of the zooplankton production only about 29% was consumed by the small barbs. However, they did not utilize calanoid copepods, which were responsible for approximately 57% of the zooplankton production and it is likely that small barb production was food limited during certain periods of the year. Piscivorous labeobarbs consumed about 56% of the small barbs production annually, but additionally, Clarias gariepinus, and many bird species were also preying on them. Therefore, limitation of Barbus production by predation during certain periods in the year cannot be excluded.     

The influence of predation on horizontal distribution of zooplankton species

SUMMARY. 1. The horizontal distributions of Daphnia longispina and Bosmina tongispina in Lake Kvernavatn (Norway) were investigated twice in 1982. In late spring, when populations were small, the two species inhabited the same areas, and they were evenly distributed from the littoral to the pelagic. At high population densities, during midsummer, the species were spatially segregated, D. longispina being pelagic and B. longispina littoral in distribution.
2. The distribution and feeding of three-spined sticklebacks (Gasteros-teus aculeatus) were also studied. The sticklebacks were apparently forced into littoral areas by larger piscivorous predators in the pelagic and they were consequently restricted to foraging primarily on B. longispina, which formed dense swarms during daytime in summer.
3. We suggest that predation and competition influence the spatial distribution of zooplankton species. The feeding efficiency of fish foraging on high-density zooplankton populations can be reduced by spatial segregation of zooplankton species. Where high local densities occur, due to swarm formation, predation is changed from size-selective feeding to consumption of spatially isolated individuals.     

Light and nutrients regulate energy transfer through benthic and pelagic food chains

The amount of energy flowing to top trophic levels depends on primary production and the efficiency at which it is converted to production at each trophic level. In aquatic systems, algal production is often limited by light and nutrients, and the nutritional quality of algae depends on the relative balance of these two resources. In this study, we used a mesocosm experiment to examine how light and nutrient variation affected food chain efficiency (FCE, defined as the proportion of primary production converted to top trophic level production), using a food web with benthic and pelagic food chains. We also related variation in benthic and pelagic efficiencies to the nutritional quality of primary producers, i.e. carbon:nitrogen:phosphorus stoichiometry. As predicted, pelagic and benthic FCEs were highest under low light/high nutrient conditions, the treatment with the best algal food quality, i.e. the lowest C:nutrient ratios. Pelagic FCE and pelagic herbivore efficiency (HE_P) were more responsive than benthic FCE to variation in light and nutrients. Furthermore, pelagic FCE and HE_P were highly correlated with algal C:P, suggesting ‘carryover effects’ of algal food quality on carnivores (larval fish) via effects on herbivore (zooplankton) quality. Benthic (tadpole) production was primarily explained by primary production rate, suggesting food quantity rather than quality drives their production. However, benthic FCE was also highest at low light/high nutrients and was significantly correlated with food quality. The stronger effect of food quality in mediating pelagic compared to benthic efficiencies, is consistent with differences in the stoichiometric mismatches between algae and consumers. Pelagic FCE and HE_P were more likely to be P‐limited, whereas benthic FCE was more likely N‐limited. This study is the first to examine both pelagic and benthic FCE within the same system, and highlights the importance of differential consumer needs in determining how food quality affects energy transfer efficiency.     

Experimental evidence for trait utility of gill raker number in adaptive radiation of a north temperate fish

North temperate fish in post‐glacial lakes are textbook examples for rapid parallel adaptive radiation into multiple trophic specialists within individual lakes. Speciation repeatedly proceeded along the benthic–limnetic habitat axis, and benthic–limnetic sister species diverge in the number of gill rakers. Yet, the utility of different numbers of gill rakers for consuming benthic vs. limnetic food has only very rarely been experimentally demonstrated. We bred and raised families of a benthic–limnetic species pair of whitefish under common garden conditions to test whether these species (i) show heritable differentiation in feeding efficiency on zooplankton, and (ii) whether variation in feeding efficiency is predicted by variation in gill raker numbers. We used zooplankton of three different size classes to investigate prey size dependency of divergence in feeding efficiency and to investigate the effect strength of variation in the number of gill rakers. Our results show strong interspecific differences in feeding efficiency. These differences are largest when fish were tested with the smallest zooplankton. Importantly, feeding efficiency is significantly positively correlated with the number of gill rakers when using small zooplankton, also when species identity is statistically controlled for. Our results support the hypothesis that a larger number of gill rakers are of adaptive significance for feeding on zooplankton and provide one of the first experimental demonstrations of trait utility of gill raker number when fish feed on zooplankton. These results are consistent with the suggested importance of divergent selection driven feeding adaptation during adaptive radiation of fish in post‐glacial lakes.     

Food- and habitat-segregation in sympatric grayling and brown trout

Grayling in the large reservoir Aursjøen, Norway, did not use the pelagic habitat and only trout larger than 187 mm and older than 4+ were caught there. Both species preferred the upper 8 m in the benthic habitat, but grayling were caught deeper than trout in August and September. Grayling were more closely associated with soft substratum than trout. Association with soft substratum decreased with increasing size in grayling, whereas this relationship was reversed in trout. A low number of empty stomachs indicated a low degree of food competition, but despite this the two species segregated in different key prey items—probably due to differences in mouth shapes. The key prey item for grayling was the benthic Eurycercus lamellatus , with zooplankton dominating the diet of the smallest individuals. Large trout (>300 mm) preferred the benthic Lepidurus arcticus , whereas medium trout (160–300 mm) ate predominantly the limnic-pelagic Bythotrephes longimanus , irrespective of habitat. The distribution of key prey items seemed to be the major factor controlling the habitat use, except in small trout which showed a generalist feeding pattern throughout the period. A generalist feeding pattern, a close association to coarse substratum and absence from the pelagic habitat indicated that small trout were negatively influenced by large individuals. Small grayling seemed less affected by aggressive encounters as they were less associated with coarse substratum and ate predominantly high-risk food such as zooplankton. These individuals probably did shoal in order to reduce predation risk.     

Benthic algae support zooplankton growth during winter in a clear-water lake

We used stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes to assess the importance of benthic algae for the zooplankton individual growth in winter in a shallow, clear subarctic lake. The δ¹³C values of calanoid ( Eudiaptomus graciloides ) and cyclopoid ( Cyclops scutifer ) zooplankton in autumn suggest a food resource of pelagic origin during the ice-free period. The zooplankton δ¹³C values were high in spring compared to autumn. E. graciloides did not grow over winter and the change in δ¹³C was attributed to a decrease in lipid content during the winter. In contrast, the increase in δ¹³C values of C. scutifer over the winter was explained by their growth on organic carbon generated by benthic algae. The δ¹⁵N of the C. scutifer food resource during winter was low compared to δ¹⁵N of the benthic community, suggesting that organic matter generated by benthic algae was mainly channelled to zooplankton via ¹⁵N-depleted heterotrophic bacteria. The results demonstrate that benthic algae can sustain zooplankton metabolic demands and growth during long winters, which, in turn, may promote zooplankton growth on pelagic resources during the summer. Such multi-chain omnivory challenges the view of zooplankton as mainly dependent on internal primary production and stresses the importance of benthic resources for the productivity of plankton food webs in shallow lakes.     

Interdemic variation in the foraging ecology of the African cyprinid, Barbus neumayeri

In the African cyprinid, Barbus neumayeri, populations from hypoxic waters have larger gills than populations from well-oxygenated streams. Differences in trophic morphology and feeding performance between these populations suggest a reduction in feeding efficiency in large-gilled fish that may reflect spatial constraints of the gills. However, this variation may also reflect interdemic variation in diets. In this study, we describe patterns of variation in diet, gut morphology, and prey availability for populations of B. neumayeri from swamp (low-oxygen) and stream (high-oxygen) sites in Kibale National Park, Uganda. Our results indicate that B. neumayeri are omnivorous, feeding primarily on benthic prey items; however, diets differed among swamp and stream sites for certain prey types. The observed dietary differences do not provide direct support for predictions based on variation in trophic musculature; hard-bodied prey were more common in low-oxygen sites. Prey availability also differed among sites; in particular, insect abundance and richness was generally lower in the swamp sites. Gut length was longer in one of the four populations, but did not conform to expectations based on diet differences. Condition and growth rates did not differ between populations from hypoxic and well-oxygenated sites, despite observed differences in prey availability and diet, suggesting that B. neumayeri may be distributed in a way that equalizes fitness among populations in different habitats.     

Bottom‐up and top‐down effects of browning and warming on shallow lake food webs

Productivity and trophic structure of aquatic ecosystems result from a complex interplay of bottom‐up and top‐down forces that operate across benthic and pelagic food web compartments. Projected global changes urge the question how this interplay will be affected by browning (increasing input of terrestrial dissolved organic matter), nutrient enrichment and warming. We explored this with a process‐based model of a shallow lake food web consisting of benthic and pelagic components (abiotic resources, primary producers, grazers, carnivores), and compared model expectations with the results of a browning and warming experiment in nutrient‐poor ponds harboring a boreal lake community. Under low nutrient conditions, the model makes three major predictions. (a) Browning reduces light and increases nutrient supply; this decreases benthic and increases pelagic production, gradually shifting productivity from the benthic to the pelagic habitat. (b) Because of active habitat choice, fish exert top‐down control on grazers and benefit primary producers primarily in the more productive of the two habitats. (c) Warming relaxes top‐down control of grazers by fish and decreases primary producer biomass, but effects of warming are generally small compared to effects of browning and nutrient supply. Experimental results were consistent with most model predictions for browning: light penetration, benthic algal production, and zoobenthos biomass decreased, and pelagic nutrients and pelagic algal production increased with browning. Also consistent with expectations, warming had negative effects on benthic and pelagic algal biomass and weak effects on algal production and zoobenthos and zooplankton biomass. Inconsistent with expectations, browning had no effect on zooplankton and warming effects on fish depended on browning. The model is applicable also to nutrient‐rich systems, and we propose that it is a useful tool for the exploration of the consequences of different climate change scenarios for productivity and food web dynamics in shallow lakes, the worldwide most common lake type.     

Behavioural responses of benthic and pelagic Arctic charr to substratum heterogeneity

Responses to environmental heterogeneity were studied in laboratory-reared offspring of two morphs of Arctic charr Salvelinus alpinus from Loch Rannoch, Scotland, one occupying the pelagic zone and feeding predominantly on zooplankton and the other being benthic in habit and feeding mainly on macroinvertebrates. When housed in groups in tanks with a black-and white striped base, benthic charr demonstrated a clear preference for dark areas, whereas pelagic fish positioned themselves at random with respect to substratum colour. In general, pelagic charr were much less aggressive than benthic charr. In pelagic fish, neither spacing nor aggression was affected by the visual heterogeneity of the substrate. In contrast, benthic charr swam closer together and fought more when housed over a uniform as opposed to a non-uniform substratum. The results are discussed in the context of habitat-specific visual requirements and of an interaction between visual complexity and territoriality previously described for Atlantic salmon Salmo salar .     

Fatty acid composition of consumers in boreal lakes – variation across species,space and time

1. Fatty acids (FAs) have been widely applied as trophic biomarkers in aquatic food web studies. However, current knowledge of inter‐ and intraspecific variation in consumer FA compositions across spatial and temporal scales is constrained to a few pelagic taxa. 2. We analysed the FAs of 22 taxa of benthic macroinvertebrates, zooplankton and fish collected from the littoral, pelagic and profundal habitats of nine boreal oligotrophic lakes over spring, summer and autumn. We quantified and compared the FA variance partitions contributed by species identity (i.e. an integrative effect of phylogenetic origin, life history and functional feeding guild of individual taxa), site and season using partial redundancy analysis both on all consumers and on benthic arthropods alone. 3. Species identity alone contributed 84.4 and 72.8% of explained FA variation of all consumers and benthic arthropods, respectively. Influences of site, season and all joint effects accounted for 0–11.3% only. Fatty acid profiles of primary consumers differentiated below class level, but those of predators were distinguishable only when they became more taxonomically distinct (i.e. among classes or higher). 4. Pelagic and profundal consumers showed stronger reliance on autochthonous resources than did their littoral counterparts as reflected by their higher ω3 to ω6 FA ratios. Polyunsaturated FAs (PUFAs) were increasingly retained with trophic levels, and saturated FAs (e.g. FA 16 : 0) gradually reduced. Ecologically, this trade‐off enhances the trophic transfer efficiency and confirms the importance of PUFA‐rich autotrophs in aquatic food webs. 5. Our findings indicate strong interspecific differences in FA requirements and assimilation among aquatic consumers from a wide range of taxonomic levels, habitats and lakes. Consumers were able to maintain homoeostasis in FA compositions across spatial and temporal changes in resource FAs, but consumer homoeostasis did not limit the effectiveness of FAs as trophic biomarkers.     

Foraging capacities and effects of competitive release on ontogenetic diet shift in bream, Abramis brama

Bream (Abramis brama) undergo ontogenetic diet shift from zooplankton to benthic macroinvertebrates, but the switching size may be highly variable. To unravel under what conditions bream are pelagic versus benthic foragers, we experimentally determined size‐dependent foraging capacities on three prey types from the planktivory and benthivory niche; zooplankton, visible and buried macroinvertebrates. From these data we derived predictions of size‐dependent diet preferences from estimates of prey value and competitive ability, and tested these predictions on diet data from the field. Planktivorous foraging capacity described a hump‐shaped relationship with bream length that peaked for small bream of 67 mm total length. Benthivory capacity increased with increasing bream size, irrespective if benthic prey were visible on the sediment surface or buried in the sediment. From the experimental data and relationships of metabolic demand we calculated minimum resource requirement for maintenance (MRR) for each of the prey categories used in experiments. MRR increased with bream size for both zooplankton and visible chironomids, but decreased with bream size for buried chironomids, suggesting that intermediate sized bream (120–300 mm) may be competitively sandwiched between small and large bream that are more competitive planktivores and benthivores, respectively. Prey value estimates and competitive abilities qualitatively predicted diet shift in a bream population being released from competition. Competitive release did not change the diet of the largest size‐class feeding on an optimal diet of benthic invertebrates both before and after competitive release. However, profound diet shifts towards benthic macroinvertebrates were recorded for intermediate size‐classes that fed on a suboptimal diet prior to competitive release. Thus, laboratory estimates of size‐dependent foraging capacity of bream in planktivorous and benthivorous feeding niches provided useful information on size‐specific competitive ability, and successfully predicted diet preference in the field.