Sulphur stable isotopes can distinguish trophic dependence on sediments and plankton in boreal lakes

1. Stable isotopes of carbon are useful for differentiating between freshwater food chains based on planktonic algae or benthic algae, but are reported to be of limited use for identifying food chains based on sedimentary detritus. Because data from marine systems suggest that stable isotopes of sulphur (δ³⁴S values) have potential in this regard, we tested their utility in freshwater lakes.
2. We found that sulphate in the water column of four boreal lakes was enriched in ³⁴S compared to the sulphur in bulk sediments from these lakes. Furthermore, within a given lake, insects known to feed on sediment (directly or via predation) had δ³⁴S values similar to those of sediment, whereas planktonic and benthic invertebrates known to feed on suspended particles had δ³⁴S values similar to those of sulphate in the water column.
3. Using the stable S isotope values of invertebrates that obtain their S from either the sediment or the water column as end members in a two-source mixing model, we show that two fish species obtain their food from both planktonic and sedimentary sources. Furthermore, model results suggest that, as expected, the more benthic-feeding fish species obtains more of its S from the sediment compartment than does the species that feeds in the water-column.
4. Our results suggest that measurements of stable sulphur isotopes provide a means of distinguishing between members of food chains that are based in the water column from those based on sedimentary detritus. As such, they would be a useful complement to stable C isotopes that are used to distinguish between food chains based on planktonic or benthic algae.     

Invertebrate community patterns in Mediterranean temporary wetlands along hydroperiod and salinity gradients

1. Temporary aquatic habitats often are inhabited by a unique fauna and flora and contribute significantly to regional diversity. Temporary wetlands around the world are disappearing rapidly. The individual and interacting impacts of factors influencing community structure and dynamics in temporary wetlands are not always well known.
2. Camargue wetlands are mainly characterized by variable salinity and hydroperiod. The individual and combined impacts of these local factors, together with regional variables, on invertebrate communities remain unknown. We therefore characterized and sampled invertebrates in 30 temporary wetlands along salinity and hydroperiod gradients in the Camargue (Southern France) 3, 5 and 7 months after inundation.
3. Over the three sampling occasions, a total of 17 cladoceran species and 49 macroinvertebrate taxa were identified. Hydroperiod and salinity were the most important variables explaining variation in taxonomic composition and can be considered key factors shaping the invertebrate communities in Camargue wetlands. The impact on taxon richness was significantly positive for hydroperiod but significantly negative for salinity. Regional factors had no significant effect on the structure of the studied invertebrate communities, suggesting that dispersal was not limiting and that species sorting was the most important structuring process.
4. The results of this study suggest that the combined and interacting effects of salinization and hydrological modification of Mediterranean temporary wetlands (due to water management, climate change, etc.) can result in reduced diversity in large numbers of Mediterranean wetlands and induce a considerable decline in regional diversity of aquatic invertebrates.     

13C natural abundance variations in carbonates and organic carbon from boreal forest wetlands

¹³C natural abundance variations were measured in peat soil and vegetation from two contrasting boreal forest wetlands: an upland watershed basin and a permanently saturated lowland mire. Evidence of methane oxidation was shown in the permanently saturated wetland with δ¹³C values as low as -97 ‰ in carbonate minerals found in floating peat mats. It is postulated that¹³C depleted CH₄ is oxidized in the mat and reacts with calcium ions to form calcite (identified through x-ray diffraction). Methane flux measurements during the summer of 1992 showed much lower fluxes in areas with floating peat mats relative to open water. Secondary carbonates in the basin peat have isotope compositions close to the δ¹³C values of the peat organic carbon (-25 ‰), indicating their origin from fermentation and possibly from sulfate-reduction. In the upland basin peat deposits, the δ¹³C_PDB values of organic C were constant with depth, while the permanently saturated mire had localities of¹³C enrichment in deeper layers of the peat. The¹³C enrichment may reflect areas of intense CH₄ production in which¹³C enriched residual substrate is left behind during the production of highly¹³C depleted CH₄.     

A comparison of the trophic ecology of the crayfishes (Orconectes nais (Faxon) and Orconectes neglectus (Faxon)) and the central stoneroller minnow ( Campostoma anomalum (Rafinesque)): omnivory in a tallgrass prairie stream

Omnivorous fish, such as the central stoneroller minnow (Campostoma anomalum(Rafinesque)), and crayfish often play important roles in the trophic dynamics of streams. The trophic role of these two omnivores has not been compared within a system even though they both consume algae, detritus and invertebrates and often co-occur in streams in the Midwestern United States. Natural abundance of ¹⁵N and ¹³C isotopes and a whole stream ¹⁵N-labeled ammonium chloride release were used to compare the trophic ecology of the central stoneroller minnow (Campostoma anomalum (Rafinesque)) and two species of crayfish (Orconectes neglectus (Faxon) and Orconectes nais (Faxon)) in a tallgrass prairie stream. The ¹⁵N and ¹³C values of Orconectes spp. were more similar to coarse benthic organic matter (CBOM) and filamentous green algae than to invertebrates, fine benthic organic matter (FBOM), and periphyton. Values for ¹⁵N and ¹³C in C. anomalum were more similar to grazer and collector invertebrates and filamentous green algae than to FBOM and periphyton. Results from a ¹⁵N tracer release also indicated a portion of algae and/or invertebrates were a component of nitrogen assimilated in Orconectes spp. and C. anomalum diets. Gut contents of C. anomalum were also analyzed. In contrast to stable isotope data, amorphous detritus was a significant component of C. anomalum guts, followed by diatoms and filamentous green algae. A significant percentage of invertebrate material was found in C. anomalum guts sampled in the spring. Experiments were conducted in artificial streams to determine if Orconectes spp. and C. anomalum could reduce epilithic algal biomass in small streams. Algal biomass on clay tile substrata was decreased relative to controls in artificial stream channels containing O. neglectus (3.4 fold, p=0.0002), C. anomalum (2.1 fold, p=0.0012), and both species combined (3.0 fold, p=0.0003). Results indicate that Orconectes spp. are functioning more as algal and detrital processors than as predators in Kings Creek. Isotope and gut content data show that C. anomalum includes invertebrates as well as algae and detritus in its diet. Both species have the potential to affect algal biomass and are important omnivores in the stream food web.     

The influence of productivity and width of littoral zone on the trophic position of a large-bodied omnivore

Omnivory is common in many food webs. Omnivores in different habitats can potentially change their feeding behaviour and alter their trophic position and role according to habitat conditions. Here we examine the trophic level and diet of the omnivorous signal crayfish (Pacifastacus leniusculus) in gradients of trophic status and lake size, both of which have been previously suggested to affect trophic position of predators separately or combined as productive space. We found the trophic position of omnivorous crayfish to be positively correlated with lake trophic status, but found no evidence for any influence of lake size or productive space on crayfish trophic position. The higher trophic position of crayfish in eutrophic lakes was largely caused by a shift in crayfish diet and not by an increase in trophic links in basal parts of the food web. Hence, our results support the “productivity hypothesis,” suggesting that food chains can be longer in more productive systems. Furthermore, stable isotope data indicated that larger crayfish are more predatory than smaller crayfish in lakes with wider littoral zones. Wider littoral zones promoted the development of intrapopulation differences in trophic position whereas narrow littoral zones did not. Hence, differences in habitat quality between and within lakes seem to influence the trophic positions of omnivorous crayfish. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Spatial heterogeneity of trophic pathways in the invertebrate community of a temperate bog

Summary 1. To examine spatial heterogeneity of trophic pathways on a small scale (<5 m diameter), we conducted dual stable isotope (δ¹³C and δ¹⁵N) analyses of invertebrate communities and their potential food sources in three patchy habitats [sphagnum lawn (SL), vascular‐plant carpet (VC) and sphagnum carpet] within a temperate bog (Mizorogaike Pond, Kyoto, Japan). 2. In total, 19 invertebrate taxa were collected from the three habitats, most of which were stenotopic, i.e. collected from a single habitat. Amongst the habitats, significant variation was observed in the isotopic signatures of dominant plant tissues and their detrital matter [benthic particulate organic matter (BPOM)], both of which were potential organic food sources for invertebrates. Site‐specific isotopic variation amongst detritivores was found in δ¹³C but not in δ¹⁵N, reflecting site‐specificity in the isotopic signatures of basal foods. The eurytopic hydrophilid beetle Helochares striatus was found in all habitats, but showed clear site variation in its isotopic signatures, suggesting that it strongly relies on foods within its own habitat. 3. The most promising potential foods for detritivores were the dead leaf stalks of a dominant plant in the VC and BPOM in the SL and carpet. An isotopic mixing model (IsoSource version 1.3.1) estimated that aquatic predators rely on unknown trophic sources with higher δ¹³C than detritus, whereas terrestrial predators forage on allochthonous as well as autochthonous prey, suggesting that the latter predators might play key roles in coupling between habitats. 4. Our stable isotope approach revealed that immobile detritivores are confined to their small patchy habitats but that heterogeneous trophic pathways can be coupled by mobile predators, stressing the importance of habitat heterogeneity and predator coupling in characterising food webs in bog ecosystems.     

Tracing energy flow in stream food webs using stable isotopes of hydrogen

1. Use of the natural ratios of carbon and nitrogen stable isotopes as tracers of trophic interactions has some clear advantages over alternative methods for food web analyses, yet is limited to situations where organic materials of interest have adequate isotopic separation between potential sources. This constrains the use of natural abundance stable isotope approaches to a subset of ecosystems with biogeochemical conditions favourable to source separation. 2. Recent studies suggest that stable hydrogen isotopes (δD) could provide a robust tracer to distinguish contributions of aquatic and terrestrial production in food webs, but variation in δD of consumers and their organic food sources are poorly known. To explore the utility of the stable hydrogen isotope approach, we examined variation in δD in stream food webs in a forested catchment where variation in δ¹³C has been described previously. 3. Although algal δD varied by taxa and, to a small degree, between sites, we found consistent and clear separation (by an average of 67‰) from terrestrial carbon sources. Environmental conditions known to affect algal δ¹³C, such as water velocity and stream productivity did not greatly influence algal δD, and there was no evidence of seasonal variation. In contrast, algal δ¹³C was strongly affected by environmental factors both within and across sites, was seasonally variable at all sites, and partially overlapped with terrestrial δ¹³C in all streams with catchment areas larger than 10 km². 4. While knowledge of isotopic exchange with water and trophic fractionation of δD for aquatic consumers is limited, consistent source separation in streams suggests that δD may provide a complementary food web tracer to δ¹³C in aquatic food webs. Lack of significant seasonal or spatial variation in δD is a distinct advantage over δ¹³C for applications in many aquatic ecosystems.     

A stable carbon and nitrogen isotope study of lake food webs in Canada's Boreal Plain

1.  Stable carbon and nitrogen isotope and fish stomach content analyses were used to investigate food webs in five relatively undisturbed lakes on the Boreal Plain of Canada. Stable isotope analysis was also used to determine the importance of external and internal carbon sources.
2.  Overlap in the carbon and nitrogen signatures of primary producers made it difficult to determine unambiguously the feeding habits of many invertebrates. However, isotope analysis suggested that external carbon inputs were detectable in the aquatic food chains of the one lake with a short water residence time («1 year). In the other four lakes, with water residence times ≥1 year, autochthonous carbon was the only detectable carbon source in the food webs.
3.  Food webs in these lakes spanned a range of four to five trophic levels. Both invertebrates and fish appeared to eat a variety of food, often feeding at more than one trophic level.
4.  With the exception of one lake (SPH20), top predators in these lakes, northern pike ( Esox lucius ) and fathead minnows ( Pimephales promelas ), occupied similar trophic positions despite large differences in body size and trophic morphology. In SPH20, where there were two additional fish species, pike occupied a higher trophic position. However, all the top predators in each lake appeared to be omnivores and generalists.
5.  The prevalence of omnivory and the apparent generalist feeding habits of fish in these lakes suggest that organisms are flexible in their feeding habits and that these food webs will be resilient to disturbance.     

Influence of lateral gradients of hydrologic connectivity on trophic positions of fishes in the Upper Mississippi River

1. Riverscapes consist of the main channel and lateral slackwater habitats along a gradient of hydrological connectivity from maximum connection in main channel habitats to minimum connection in backwaters. Spatiotemporal differences in water currents along this gradient produce dynamic habitat conditions that influence species diversity, population densities and trophic interactions of fishes. 2. We examined the importance of lateral connectivity gradients for food web dynamics in the Upper Mississippi River during spring (high flow, moderately low temperatures) and summer (low flow, higher temperatures). We used literature information and gut contents analyses to determine feeding guilds and stable isotope analysis to estimate mean trophic position of local fish assemblages. During June and August 2006, we collected over 1000 tissue samples from four habitats (main channel, secondary channels, tertiary channels and backwaters) distributed within four hydrologic connectivity gradients. 3. Mean trophic position differed among feeding guilds and seasons, with highest values in spring. Mean trophic position of fish assemblages, variability in trophic position and food chain length (maximum trophic position) of the two dominant piscivore species (Micropterus salmoides and M. dolomieu) in both seasons were significantly associated with habitat along the lateral connectivity gradient. Food chain length peaked in tertiary channels in both seasons, probably due to higher species diversity of prey at these habitats. We infer that food chain length and trophic position of fish assemblages were lower in backwater habitats in the summer mainly because of the use of alternative food sources in these habitats. 4. A greater number of conspecifics exhibited significant among‐habitat variation in trophic position during the summer, indicating that low river stages can constrain fish movements in the Upper Mississippi River. 5. Results of this study should provide a better understanding of the fundamental structure of large river ecosystems and an improved basis for river rehabilitation and management through knowledge of the importance of lateral complexity in rivers.     

Trophic relationships of juvenile blue crabs (Callinectes sapidus) in estuarine habitats

Salt marshes and shallow-water macroalgal beds are known to provide nursery habitat for many species of fish and invertebrates. The role of these habitats as refuge from predation is well established, but the degree to which indigenous primary production within the nursery provides food for growth and development of estuarine species remains unresolved. In this study, we tested the hypothesis that juvenile blue crabs depend on indigenous primary production, directly or indirectly, during their entire stay within the nursery. To test this hypothesis, we conducted isotopic studies and stomach content analyses of juveniles from habitats near the mouth of Delaware Bay and from an adjacent lagoonal estuary (ca. 39.5° N, 75.1° W). Primary producers, marsh detritus, various life-history stages of blue crabs and potential prey species were sampled in the main estuary and in an adjacent marsh during the summer and early fall of two consecutive years. Newly settled juveniles (<15 mm carapace width) from the marsh were about 1.8‰ lighter in carbon (−17.2‰) relative to larger juveniles from the marsh (15–30 mm carapace width) and appeared to have retained a carbon isotopic signature indicative of the phytoplankton-based food web associated with larval stages. However, the signature of juveniles changed as a function of size. Large juveniles and crabs >60 mm were enriched in δ¹³C (−14.7 ± 0.1‰) compared to small crabs, suggesting a gradual shift in diet from a planktonic to a detritus-based food web with increasing size. As with crabs from Delaware Bay, the δ¹³C signature of juvenile crabs sampled from macroalgal beds in the lagoonal estuary (Rehoboth Bay) changed as a function of size. Also, δ¹³C ratios of crabs varied among the various species of macroalgae. The δ¹⁵N composition of primary producers in the marsh and main estuary also was reflected in the δ¹⁵N values of crabs and other benthic consumers in the respective habitats. Results of stomach-content analysis in this study were consistent with isotope data. Observed changes in prey preferences were related to changes in size of juvenile crabs and also differed among habitats. Gut content analyses of the three size classes of juveniles in macroalgal beds from Rehoboth Bay indicated that the crabs depend heavily on various amphipod species that occur on the seaweeds. These amphipods graze directly on the macroalgae and are among the most abundant invertebrates in the macroalgal beds. This implies a direct trophic relationship between the juvenile crabs and the macroalgae. In summary, our study provides strong evidence that the value of nursery areas such as salt marshes and macroalgal beds goes beyond that of providing refuge from predation, and that species using these nurseries (e.g. juvenile blue crabs) are ultimately dependent on primary production originating in benthic plants indigenous to the nursery.     

Trophic fractionation of carbon and nitrogen stable isotopes in Chironomus riparius reared on food of aquatic and terrestrial origin

1. Trophic fractionation was studied in short‐term laboratory feeding experiments with larvae of the deposit‐feeding midge Chironomus riparius. Larvae were fed food of terrestrial (oats, peat) and aquatic origin (Spirulina, Tetraphyll^®). 2. By analysing both whole larvae and isolated gut contents we were able to distinguish between the isotopic signature of recently ingested food and that of assimilated carbon and nitrogen in body tissue. Additionally we studied the effects of microbial conditioning, i.e. the colonisation and growth on food particles of microbes, on the isotopic signal of food resources. 3. Nitrogen fractionation for the different food types ranged from 0.67‰ to 2.68‰ between consumer and diet and showed that isotopic fractionation can be much lower than the value of 3.4‰ that is commonly assumed. 4. Microbial degradation of food particles resulted in an approximate doubling of the δ¹⁵N in 8 days, from 6.24 ± 0.05‰ to 11.36 ± 0.56‰. Values for δ¹³C increased only marginally, from ?20.66 ± 0.11‰ to ?20.34 ± 0.12‰. These results show that microbial conditioning of food may affect dietary isotope signatures (in particular N) and, unless accounted for, could introduce an error in measures of trophic fractionation. Microbial conditioning could well account for some of the variation in fractionation reported in the literature.     

Carbon source accounting for fish using combined DNA and stable isotope analyses in a regulated lowland river weir pool

Determining the source and flow of carbon, energy and nutrients through food webs is essential for understanding ecological connectivity and thus determining the impact of management practices on biodiversity. We combined DNA sequencing, microarrays and stable isotope analyses to test whether this approach would allow us to resolve the carbon flows through food webs in a weir pool on the lower Murray River, a highly impacted, complex and regulated ecosystem in southern Australia. We demonstrate that small fish in the Murray River consume a wide range of food items, but that a significant component of carbon and nitrogen entering the food web during dry periods in summer, but not spring, is derived from nonconventional sources other than in-channel primary producers. This study also showed that isotopic analyses alone cannot distinguish food sources and that a combined approach is better able to elucidate food-consumer dynamics. Our results highlight that a major river ecosystem, stressed by reduced environmental flows, can rapidly undergo significant and previously undetected changes that impact on the ecology of the system as a whole.     

13C and 15N in microarthropods reveal little response of Douglas-fir ecosystems to climate change

Understanding ecosystem carbon (C) and nitrogen (N) cycling under global change requires experiments maintaining natural interactions among soil structure, soil communities, nutrient availability, and plant growth. In model Douglas-fir ecosystems maintained for five growing seasons, elevated temperature and carbon dioxide (CO₂) increased photosynthesis and increased C storage belowground but not aboveground. We hypothesized that interactions between N cycling and C fluxes through two main groups of microbes, mycorrhizal fungi (symbiotic with plants) and saprotrophic fungi (free-living), mediated ecosystem C storage. To quantify proportions of mycorrhizal and saprotrophic fungi, we measured stable isotopes in fungivorous microarthropods that efficiently censused the fungal community. Fungivorous microarthropods consumed on average 35% mycorrhizal fungi and 65% saprotrophic fungi. Elevated temperature decreased C flux through mycorrhizal fungi by 7%, whereas elevated CO₂ increased it by 4%. The dietary proportion of mycorrhizal fungi correlated across treatments with total plant biomass (n= 4, r²= 0.96, P= 0.021), but not with root biomass. This suggests that belowground allocation increased with increasing plant biomass, but that mycorrhizal fungi were stronger sinks for recent photosynthate than roots. Low N content of needles (0.8–1.1%) and A horizon soil (0.11%) coupled with high C : N ratios of A horizon soil (25–26) and litter (36–48) indicated severe N limitation. Elevated temperature treatments increased the saprotrophic decomposition of litter and lowered litter C : N ratios. Because of low N availability of this litter, its decomposition presumably increased N immobilization belowground, thereby restricting soil N availability for both mycorrhizal fungi and plant growth. Although increased photosynthesis with elevated CO₂ increased allocation of C to ectomycorrhizal fungi, it did not benefit plant N status. Most N for plants and soil storage was derived from litter decomposition. N sequestration by mycorrhizal fungi and limited N release during litter decomposition by saprotrophic fungi restricted N supply to plants, thereby constraining plant growth response to the different treatments.     

Carnivory and resource-based niche differentiation in anuran larvae: implications for food web and experimental ecology

1.  Food webs represent the paths of material and energy flow through organisms in an ecosystem. Anuran larvae are important components of pond food webs: they are abundant, consume large quantities of food and serve as prey for many organisms. However, there are very basic uncertainties about the feeding ecology of anuran larvae; for instance, as to which trophic level they belong and whether species differ in resource use. Because anuran larvae have been employed in model systems in experimental ecology for decades, these uncertainties could lead to misinterpretation of published experiments, or inadequate designs of experiments directed at general, conceptual issues in ecology.
2.  Using ¹³C and ¹⁵N stable isotope and gut content analyses of free-ranging and enclosed tadpoles of four ranid species ( Lithobates sylvaticus , L. pipiens , L. clamitans , L. catesbeianus ) in the food webs of six wetlands, we tested the following null hypotheses: (i) that anuran larvae are strict primary consumers; (ii) that they are non-selective feeders and therefore exhibit little feeding niche differentiation; (iii) that they are opportunistic consumers and (iv) that their diet remains unchanged through ontogeny.
3.  All four species consumed and assimilated substantial amounts of animal food; bullfrog larvae, in particular, appear to be predatory. Significant feeding niche differentiation among species occurred with respect to the sources of carbon, consumption of animal matter and nutritional quality of food ingested. We further documented opportunistic feeding habits and ontogenetic shifts in diet.
4.  Collectively, these studies revealed complex trophic relationships that might require a reconsideration of the role of anuran larvae in pond food webs, as well as a reinterpretation of results of previous studies employing anuran larvae in model experimental systems.     

The influence of a predatory ostracod,Australocypris insularis,on zooplankton abundace and species composition in a saline lake

The influence of the predatory ostracod Australocypris insularis on zooplankton abundances and species composition in an Australian saline lake was monitored over 8 months in aquaria and tank microcosms. Control, Predator (double A. insularis Control densities) and No Predator (all A. insularis removed) treatments were set up in small 11.5l aquaria and large 883.5l tanks. Microcosm salinities were manipulated to accord with seasonal lake salinities and ranged from 35 to 110 gl^-1. Densities of the calanoid copepods Calamoecia clitetellata and C. salina, the small ostracods Diacypris compacta and D. dietzi, and juvenile ostracods (excluding A. insularis) were significantly reduced by A. insularis predation in the aquaria experiments. Daphniopsis sp. was only present (in low numbers) in the No Predator aquaria. Results from the large tank experiments were not as clear-cut, due to difficulties in the manipulation of predator numbers. Differing salinity tolerances of predator and prey may confound the predation impact of A. insularis.     

Presence or absence of fish as a cue to macroinvertebrate abundance in boreal wetlands

Waterfowl that eat macroinvertebrates must select among potential nesting or brood-rearing habitats that may vary in food abundance over the season. We compared the reliability of predicting the relative abundance of macroinvertebrates in boreal wetlands using either the number of macroinvertebrates collected at one sampling period, or presence or absence of fish. Wetlands with fish had fewer macroinvertebrates than fishless wetlands in all five sampling periods. Predictions of the relative abundance of invertebrates in a wetland at other sampling periods based on the presence or absence of fish, were equal to or better than predictions based on the actual number of macroinvertebrates collected during one sampling period. These results suggest that fish status of a wetland is a reliable cue to invertebrate abundance in boreal wetlands.     

Assessing the net effect of anthropogenic disturbance on aquatic communities in wetlands: community structure relative to distance from canals

Anthropogenic alterations of natural hydrology are common in wetlands and often increase water permanence, converting ephemeral habitats into permanent ones. Since aquatic organisms segregate strongly along hydroperiod gradients, added water permanence caused by canals can dramatically change the structure of aquatic communities. We examined the impact of canals on the abundance and structure of wetland communities in South Florida, USA. We sampled fishes and macroinvertebrates from marsh transects originating at canals in the central and southern Everglades. Density of all aquatic organisms sampled increased in the immediate proximity of canals, but was accompanied by few compositional changes based on analysis of relative abundance. Large fish ( >8 cm), small fish ( <8 cm) and macroinvertebrates ( >5 mm) increased in density within 5 m of canals. This pattern was most pronounced in the dry season, suggesting that canals may serve as dry-down refugia. Increases in aquatic animal density closely matched gradients of phosphorus enrichment that decreased with distance from canals. Thus, the most apparent impact of canals on adjacent marsh communities was as conduits for nutrients that stimulated local productivity; any impact of their role as sources of increased sources of predators was not apparent. The effect of predation close to canals was overcompensated by increased secondary productivity and/or immigration toward areas adjacent to canals in the dry season. Alternatively, the consumptive effect of predatory fishes using canals as dry-season refuges is very small or spread over the expanse of marshes with open access to canals. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Food web of macroinvertebrate community in a Yangtze shallow lake: trophic basis and pathways

No detailed food web research on macroinvertebrate community of lacustrine ecosystem was reported in China. The present study is the first attempt on the subject in Lake Biandantang, a macrophytic lake in Hubei Province. Food webs of the macroinvertebrate community were compiled bimonthly from March, 2002 to March, 2003. Dietary information was obtained from gut analysis. Linkage strength was quantified by combining estimates of energy flow (secondary production) with data of gut analysis. The macroinvertebrate community of Lake Biandantang was based heavily on detritus. Quantitative food webs showed the total ingestion ranged from 6930 to 36,340 mg dry mass m⁻² bimonthly. The ingestion of macroinvertebrate community was higher in the months with optimum temperature than that in other periods with higher or lower temperature. Through comparison, many patterns in benthic food web of Lake Biandantang are consistent with other detritus-based webs, such as stream webs, but different greatly from those based on autochthonous primary production (e.g. pelagic systems). It suggests that the trophic basis of the web is essential in shaping food web structure. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.