Reach-scale geomorphology affects organic matter and consumer δ13C in a forested Piedmont stream

1. We investigated the spatial (longitudinal position and reach geomorphology) and seasonal (spring and autumn) influences on the variation of δ¹³C among organic matter sources and consumers in a forested Piedmont river, South Carolina, U.S.A. 2. Six sites were sampled along a continuum and varied in basin area from approximately 30 to 300 km². Sites fell into two geomorphic categories (i) high‐gradient, rock bed (‘rock’) or (ii) low‐gradient, sand bed (‘sand’) sites. 3. Variation in δ¹³C was more strongly related to reach geomorphology than longitudinal position. δ¹³C of biofilm and consumers was consistently enriched at rock sites. Leaf litter (i.e. coarse particulate organic matter, CPOM) δ¹³C did not vary with bed type. There was significant δ¹³C enrichment at rock sites for biofilm, seston, fine benthic organic matter (FBOM), and eight of nine consumer trophic guilds (e.g. grazing invertebrates, insectivorous fishes). δ¹³C of biofilm and four trophic guilds was also positively correlated with drainage area, but the magnitude of enrichment was less than between bed types. 4. δ¹³C was generally enriched in spring, but this varied among organic matter types, consumers, and by bed type. CPOM and seston were enriched in spring, FBOM was enriched in autumn, and biofilm showed no trend. Five consumer guilds were enriched in spring, and only one fish guild, generalised carnivores, showed enrichment of muscle tissue in autumn. 5. Consumer δ¹³C enrichment at rock sites suggests greater reliance on algal carbon than for consumers at sand sites, but we also found δ¹³C enrichment of biofilm at rock sites. Thus, differences in consumer δ¹³C between bed types could be related to (i) increased consumption of biofilm at rock compared with sand sites, or (ii) consumption of biofilm at rock sites that is enriched relative to biofilm at sand sites or (iii) both mechanisms. 6. δ¹³C signatures in local food webs appear to respond to processes operating at multiple spatial scales. Overall downstream enrichment of biofilm and consumers was disrupted by strong local effects related to bed morphology. These results suggest that human alteration of channel habitat will have corresponding effects on stream food webs, as assessed by changes in δ¹³C.     

Deforestation alters the resource base and biomass of endemic stream insects in eastern Madagascar

1. Rainforest streams in eastern Madagascar have species‐rich and diverse endemic insect communities, while streams in deforested areas have relatively depauperate assemblages dominated by collector‐gatherer taxa. We sampled a suite of benthic insects and their food resources in three primary rainforest streams within Ranomafana National Park in eastern Madagascar and three agriculture streams in the park's deforested peripheral zone. We analysed gut contents and combined biomass and stable isotope data to examine stream community responses to deforestation in the region, which is a threatened and globally important hotspot for freshwater biodiversity. 2. Gut analyses showed that most taxa depended largely on amorphous detritus, obtained either from biofilms (collector‐gatherers) or from seston (microfilterers). Despite different resource availability in forest versus agriculture streams, diets of each taxon did not differ between stream types, suggesting inflexible feeding modes. Carbon sources for forest stream insects were difficult to discern using δ¹³C. However, in agriculture streams dependence on terrestrial carbon sources was low relative to algal sources. Most insect taxa with δ¹³C similar to terrestrial carbon sources (e.g. the stonefly Madenemura, the caddisfly Chimarra sp. and Simulium blackflies) were absent or present at lower biomass in agriculture streams relative to forest streams. Conversely, collector‐gatherers (Afroptilum mayflies) relied on algal carbon sources and had much higher biomass in agriculture streams. 3. Our analyses indicate that a few collector‐gatherer species (mostly Ephemeroptera) can take advantage of increased primary production in biofilms and consequently dominate biomass in streams affected by deforestation. In contrast, many forest stream insects (especially those in the orders Plecoptera, Trichoptera and Diptera) depend on terrestrial carbon sources (i.e. seston and leaf litter), are unable to track resource availability and consequently decline in streams draining deforested landscapes. These forest‐specialists are often micro‐endemic and particularly vulnerable to deforestation. 4. The use of consumer biomass data in stable isotope research can help detect population‐level responses to shifts in basal resources caused by anthropogenic change. We also suggest that restoration of vegetated riparian zones in eastern Madagascar and elsewhere could mitigate the deleterious effects of deforestation on sensitive, endemic stream taxa that are dependent on terrestrial carbon sources.     

Dietary changes in predators and scavengers in a nocturnally illuminated riparian ecosystem

Aquatic and terrestrial ecosystems are linked by fluxes of carbon and nutrients in riparian areas. Processes that alter these fluxes may therefore change the diet and composition of consumer communities. We used stable carbon isotope (δ¹³C) analyses to test whether the increased abundance of aquatic prey observed in a previous study led to a dietary shift in riparian consumers in areas illuminated by artificial light at night (ALAN). We measured the contribution of aquatic‐derived carbon to diets in riparian arthropods in experimentally lit and unlit sites along an agricultural drainage ditch in northern Germany. The δ¹³C signature of the spider Pachygnatha clercki (Tetragnathidae) was 0.7‰ lower in the ALAN‐illuminated site in summer, indicating a greater assimilation of aquatic prey. Bayesian mixing models also supported higher intake of aquatic prey under ALAN in spring (34% versus 21%). In contrast, isotopic signatures for P. clercki (0.3‰) and Pardosa prativaga (0.7‰) indicated a preference for terrestrial prey in the illuminated site in spring. Terrestrial prey intake increased in spring for P. clercki under ALAN (from 70% to 74%) and in spring and autumn for P. prativaga (from 68% to 77% and from 67% to 72%) and Opiliones (from 68% to 72%; 68% to 75%). This was despite most of the available prey (up to 80%) being aquatic in origin. We conclude that ALAN changed the diet of riparian secondary consumers by increasing the density of both aquatic and terrestrial prey. Dietary changes were species‐ and season‐specific, indicating that the effects of ALAN may interact with phenology and feeding strategy. Because streetlights can occur in high density near freshwaters, ALAN may have widespread effects on aquatic–terrestrial ecosystem linkages.     

Food Web Structure and Basal Resource Utilization along a Tropical Island Stream Continuum, Puerto Rico

Tropical stream food webs are thought to be based primarily on terrestrial resources (leaf litter) in small forested headwater streams and algal resources in larger, wider streams. In tropical island streams, the dominant consumers are often omnivorous freshwater shrimps that consume algae, leaf litter, insects, and other shrimps. We used stable isotope analysis to examine (1) the relative importance of terrestrial and algal‐based food resources to shrimps and other consumers and determine (2) if the relative importance of these food resources changed along the stream continuum. We examined δ¹⁵N and δ¹³C signatures of leaves, algae, macrophytes, biofilm, insects, snails, fishes, and shrimps at three sites (300, 90, and 10 m elev.) along the Río Espíritu Santo, which drains the Caribbean National Forest, Puerto Rico. Isotope signatures of basal resources were distinct at all sites. Results of two‐source δ¹³C mixing models suggest that shrimps relied more on algal‐based carbon resources than terrestrially derived resources at all three sites along the continuum. This study supports other recent findings in tropical streams, demonstrating that algal‐based resources are very important to stream consumers, even in small forested headwater streams. This study also demonstrates the importance of doing assimilation‐based analysis (i.e., stable isotope or trophic basis of production) when studying food webs.     

Stable isotope analysis indicates microalgae as the predominant food source of fauna in a coastal forest stream,south‐east Brazil

Abstract: Stable isotope studies of food webs in floodplains, large rivers, mangroves, and seagrasses have shown that, although a large proportion of the biomass may come from higher plants, microalgae provide a disproportionate amount of carbon assimilated by metazoan consumers. Evidence is building that this may also be the case for streams, especially those in the tropics. At the level of individual consumer species we also see that the apparent diet may not be reflected in the carbon assimilated. Tropical streams commonly have omnivore‐detritivore species that potentially show this phenomenon. We tested these concepts in four moderately shaded sites in a stream in well‐preserved Atlantic rainforest at Ilha Grande, Rio de Janeiro. We sampled aquatic insects, shrimps and fish as well as potential terrestrial and aquatic primary food sources. Carbon stocks from terrestrial sources predominated over carbon of algal origin (>99% of total). The primary sources of carbon showed distinctly different isotopic signatures: terrestrial sources had δ¹³C values close to ?30‰, microalgae were ?20‰ and macroalgae were ?25‰. All fauna had δ¹³C values consistent with a carbon source derived from microalgae. Baetid mayflies and atyid shrimps exert a strong grazing pressure on periphyton and organic sediments but appear to assimilate predominantly microalgae. The palaemonid shrimp Macrobrachium olfersi also ingests large amounts of detritus of terrestrial origin, but apparently assimilates animal prey with algal δ¹³C signatures. These results support the growing view that tropical stream food chains are primarily algal based.     

Stable isotope analysis indicates trophic differences among forest floor carabids in Japan

Differences in trophic niches among carabid beetles (Coleoptera: Carabidae) co‐occurring on the forest floors of warm temperate forests in central Japan were studied using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analyses. Different carabid species showed similar δ¹⁵N values, which were higher than those of their possible invertebrate prey (herbivores and detritivores) collected from the litter layer, indicating that these species were consumers in the same trophic level. In contrast, δ¹³C values differed among carabid species, indicating interspecific differences in prey animals. The variation in the δ¹³C value was larger in summer than in autumn. In summer, δ¹³C values indicated that some carabids depended highly on either grazing (low δ¹³C values) or detrital sources (high δ¹³C values) within the food chain [Chlaenius posticalis Motschulsky and Haplochlaenius costiger (Chaudoir), respectively], although other species with intermediate δ¹³C values likely depended on both. The latter group of species comprised mostly two dominant genera (Carabus and Synuchus). Although congeners might have similar feeding habits, the stable isotope ratios indicated trophic niche differences between adults of different species and between adults and larvae of the same genus.     

Effects of stream restoration and wastewater treatment plant effluent on fish communities in urban streams

1. Fish community characteristics, resource availability and resource use were assessed in three headwater urban streams in Piedmont North Carolina, U.S.A. Three site types were examined on each stream; two urban (restored and unrestored) and a forested site downstream of urbanisation, which was impacted by effluent from a wastewater treatment plant (WWTP). Stream basal resources, aquatic macroinvertebrates, terrestrial macroinvertebrates and fish were collected at each site. 2. The WWTPs affected isotope signatures in the biota. Basal resource, aquatic macroinvertebrate and fish δ¹⁵N showed significant enrichments in the downstream sites, although δ¹³C signatures were not greatly influenced by the WWTP. Fish were clearly deriving a significant part of their nutrition from sewage effluent‐derived sources. There was a trend towards lower richness and abundance of fish at sewage‐influenced sites compared with urban restored sites, although the difference was not significant. 3. Restored stream sites had significantly higher fish richness and a trend towards greater abundance compared with unrestored sites. Although significant differences did not exist between urban restored and unrestored areas for aquatic and terrestrial macroinvertebrate abundances and biotic indices of stream health, there appeared to be a trend towards improvements in restored sites for these parameters. Additional surveys of these sites on a regular basis, along with maintenance of restored features are vital to understanding and maximising restoration effectiveness. 4. A pattern of enriched δ¹³C in fish in restored and unrestored streams in conjunction with enriched δ¹³C of terrestrial invertebrates at these sites suggests that these terrestrial subsidies are important to the fish, a conclusion also supported by isotope cross plots. Furthermore, enriched δ¹³C observed for terrestrial invertebrates is consistent with some utilisation of the invasive C4 plants that occur in the urban riparian areas.     

What does stable isotope analysis reveal about trophic relationships and the relative importance of allochthonous and autochthonous resources in tropical streams? A synthetic study from Hong Kong

1. Analysis of the stable isotope signatures of carbon (C) and nitrogen (N) of foods and consumers has led to some preliminary understanding of the relative importance of autochthonous and allochthonous resources in tropical streams. However, robust generalizations about the dynamics of food webs in these habitats, and their response to shading gradients or season, are still lacking. In addition, the feasibility of employing a baseline δ¹⁵N value for estimating trophic positions (TPs) of consumers in small tropical streams has yet to be explored. 2. We analysed data on stable isotope signatures of food sources and aquatic consumers obtained from 14 studies carried out in small streams in monsoonal Hong Kong (22°30′N, 114°10′E) between 1996 and 2006. Emphasis was placed on determining the relative importance of leaf litter and autochthonous foods in supporting consumer biomass, and the extent to which trophic base and TP vary among streams and seasons. 3. Although allochthonous leaf litter was generally ¹³C‐ and ¹⁵N‐depleted relative to autochthonous foods, there were marked isotopic shifts of food sources and consumers in response to season (dry versus wet) and stream shading. Consumer taxa were generally more ¹³C‐ and ¹⁵N‐enriched in the unshaded streams, but seasonal effects were more variable. Despite these changes, there was consistent evidence that stream food webs were based on periphytic algae and/or cyanobacteria with leaf litter serving as a minor food. 4. Heptageniidae (Ephemeroptera), Tipulidae (Diptera), Elmidae (Coleoptera) and shrimps (Atyidae) were used as a baseline for calculating the TPs of other consumer taxa. The maximum TPs in shaded streams remained fairly constant between seasons (dry = 3.93; wet = 3.97), while those in unshaded streams were higher and showed seasonal fluctuations (dry = 5.13; wet = 4.39). 5. Although variations in consumer isotope signatures in response to season and shading gradients did not confound our interpretation of the stream food base, changes in consumer δ¹⁵N did affect the calculation of consumer TPs. Misleading estimates of consumer TPs are likely if samples are collected from a narrow range of streams and/or during one season. Overestimation of the TPs of specialist herbivores (e.g. fish grazers) is also possible when autochthonous resources are substantially more ¹⁵N‐enriched than allochthonous foods.     

Small reductions in forest cover weaken terrestrial-aquatic linkages in headwater streams

1. We assessed the impacts of deforestation on the energy base of headwater food webs in seven headwater streams in the Upper Chattahoochee basin, GA, U.S.A where percentage forest in catchments ranged from 82 to 96%. We measured terrestrial organic matter standing crop and determined consumer (crayfish and insectivorous fish) dependence on terrestrial versus aquatic energy sources via gut content and stable isotope analyses. 2. Standing crop of coarse particulate organic matter (CPOM) declined with deforestation at large scales (i.e. catchment deforestation and riparian deforestation at the entire stream network scale). Terrestrial plant matter, the dominant component of crayfish guts, declined in crayfish guts with reductions in CPOM standing crop and with deforestation. 3. Crayfish and insectivorous fish δ¹³C showed enrichment trends with deforestation, indicating isotopic divergence from CPOM, the most ¹³C‐depleted basal resource, with reductions in catchment and riparian forest cover. Crayfish δ¹³C also exhibited enrichment with decreased instream CPOM standing crop. 4. A concentration‐dependent mixing model was used to calculate the relative dependence of crayfish and fish on terrestrial versus aquatic basal resources. Results suggested that both allochthonous CPOM and autochthonous production were important basal resources. Consumer dependence on CPOM decreased with reductions in canopy cover. 5. Our data suggest the importance of forest cover to headwater food webs at multiple scales, and that relatively low levels of riparian deforestation along headwater streams can lead to reductions in stream food web dependence on terrestrial subsidies.     

Terrestrial detritus supports the food webs in lowland intermittent streams of south‐eastern Australia: a stable isotope study

• 1 Large amounts of terrestrial detritus enter many low‐order forested streams, and this organic material is often the major basal resource in the metazoan food webs of such systems. However, despite their apparently low biomass, algae are the dominant food of organisms in a number of aquatic communities which conventionally would have been presumed to be dependent on allochthonous detritus, particularly those in the tropics and also in lowland intermittent streams in arid Australia.
• 2 The dual stable isotope signatures (δ¹³C and δ¹⁵N) of potential primary food sources were compared with the isotopic signatures of common aquatic animals in lowland intermittent streams in south‐eastern Australia, in both spring and summer, to determine whether allochthonous detritus was an important nutritional resource in these systems. The isotopic signatures of the major potential allochthonous plant food sources (Eucalyptus, Phalaris and Juncus) overlapped, but were distinct from algae and the dominant macrophytes growing in the study reaches. The isotopic signatures of biofilm were more spatially and temporally variable than those of the other basal resources.
• 3 Despite allochthonous detritus having relatively high C : N ratios compared to other potential basal resources, results from isosource mixing model calculations demonstrated that this detritus, and the associated biofilm, were the major energy sources assimilated by macroinvertebrate primary consumers in both spring and summer. The importance of these energy sources was also reflected in animals higher in the food web, including predatory macroinvertebrates and fish. These resources were supplemented by autochthonous sources of higher nutritional value (i.e. filamentous algae and macrophytes, which had relatively low C : N ratios) when they became more prolific as the streams dried to disconnected pools in summer.
• 4 The results highlight the importance of allochthonous detritus (particularly from Eucalyptus) as a dependable energy source for benthic macroinvertebrates and fish in lowland intermittent streams of south‐eastern Australia. This contrasts with previous stable isotope studies conducted in lowland intermittent streams in arid Australia, which have reported that the fauna are primarily dependent on autochthonous algae.

     

Terrestrial and marine trophic pathways support young-of-year growth in a nearshore Arctic fish

River discharge supplies nearshore communities with a terrestrial carbon source that is often reflected in invertebrate and fish consumers. Recent studies in the Beaufort Sea have documented widespread terrestrial carbon use among invertebrates, but only limited use among nearshore fish consumers. Here, we examine the carbon source and diet of rapidly growing young-of-year Arctic cisco (Coregonus autumnalis) using stable isotope values (δ¹³C and δ¹⁵N) from muscle and diet analysis (stomach contents) during a critical and previously unsampled life stage. Stable isotope values (δ¹⁵N and δ¹³C) may differentiate between terrestrial and marine sources and integrate over longer time frames (weeks). Diet analysis provides species-specific information, but only from recent foraging (days). Average δ¹³C for all individuals was ?25.7 ‰, with the smallest individuals possessing significantly depleted δ¹³C values indicative of a stronger reliance of terrestrial carbon sources as compared to larger individuals. Average δ¹⁵N for all individuals was 10.4 ‰, with little variation among individuals. As fish length increased, the proportion of offshore Calanus prey and neritic Mysis prey increased. Rapid young-of-year growth in Arctic cisco appears to use terrestrial carbon sources obtained by consuming a mixture of neritic and offshore zooplankton. Shifts in the magnitude or phenology of river discharge and the delivery of terrestrial carbon may alter the ecology of nearshore fish consumers.     

Feeding ecology and basal food sources that sustain the Paradoxal frog Pseudis minuta: a multiple approach combining stomach content,prey availability,and stable isotopes

In the present study, we investigated ontogenetic diet shifts, feeding strategy, prey preferences, and basal food sources that sustain the Paradoxal frog (Pseudis minuta) based on stomach content, prey availability, and stable isotope (δ¹³C, δ¹⁵N) approaches. The feeding strategy analysis showed that the population can be considered a generalist species with each individual displaying a marked opportunism for different preys. Trophic positions estimated using nitrogen isotopic ratio (δ¹⁵N) revealed that tadpoles are primary consumers, but post-metamorphic individuals shifted to secondary and tertiary trophic levels as they increase in body size. A stable isotopic mixing model revealed that most of the carbon (0.61–0.72) sustaining the post-metamorphic P. minuta is derived from the aquatic rather than the adjacent terrestrial environment. This finding suggests that the post-metamorphic P. minuta is strongly dependent on carbon sources that primarily originate in aquatic systems, regardless of the terrestrial or aquatic origins of the arthropods in its diet. Our results indicated that this species is a generalist-opportunistic predator that derives most of their carbon sources from the aquatic environment where it shows preference for aquatic preys with higher individual biomasses.     

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 isotope study of linkages between stream and terrestrial food webs through spider predation

SUMMARY 1. Transfer of carbon from freshwater to terrestrial ecosystems can occur through predation on adult aquatic insects, but the significance of this trophic pathway to the energetics of riparian communities is poorly understood. We used stable isotopes of carbon and nitrogen to explore linkages between aquatic insect production and the nutrition of web‐building and free‐living spiders alongside two streams in the North Island of New Zealand. 2. δ¹³C values for riparian tree leaves (means for each site = ?32.2 and ?30.3‰) were distinct from those of lichens collected from stream channel rocks and instream algae, both of which were similar (?23.4 to ?22.4‰). δ¹⁵N values for leaves were similar at both sites (?3.4 and ?2.7‰), but algae were considerably more depleted in δ¹⁵N atonesite suggesting significant differences in instream nitrogen sources between the twostreams. 3. Isotope values for potential aquatic prey of spiders indicated that aquatic algal production was their primary carbon source at both sites. Terrestrial invertebrates collected and assumed to be potential prey reflected a range of carbon sources and represented several trophic levels. 4. At one site, δ¹³C values indicated a primarily algae‐aquatic insect pathway of carbon transfer to both web‐building and free‐living spider guilds. The other site appeared to have a primarily terrestrial carbon pathway for the free‐living spider guild, and a mixed aquatic‐terrestrial pathway for the web‐building guild. 5. Overall, web‐building spiders were estimated to obtain around 61% of their body carbon from aquatic production compared with 55% for free‐living spiders. Our findings suggest that consumption of prey derived from aquatic sources can provide significant nutrition for spiders living along some stream channels. This pathway may represent an important feedback mechanism contributing to the energetics of riparian communities at sites where aquatic insect production is high.     

Terrestrial contributions to Afrotropical aquatic food webs: The Congo River case

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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.     

Zooplankton allochthony is spatially heterogeneous in a boreal lake

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Sources of organic carbon in the littoral of Lake Gooimeer as indicated by stable carbon isotope and carbohydrate compositions

The relative importance of potential carbon sources in the littoral of Lake Gooimeer, a lake in the centre of the Netherlands, was studied using a combination of ¹³C/¹²C-ratio analysis and carbohydrate composition analysis. The littoral is covered on the land side by a 80 m wide Phragmites australis bed. Potential carbon sources were macrophyte litter, seston and benthic algae. Samples of potential carbon sources, sediments and benthic macrofauna from inside and outside the bed were analyzed for their¹³C/¹²C-ratio and some for their carbohydrate composition. Results indicate that inside the bed, macrophyte litter was the main source of carbon for both the sediment organic matter and the benthic macrofauna, and that algal material was of minor importance. Outside the bed, production by benthic algae was the main carbon source, with seston as a second source. No macrophyte derived material could be detected outside the reed bed.Abbreviations DOC (Dissolved Organic Carbon) - SOM (Sediment Organic Matter)     

Effects of increased temperature and nutrient enrichment on the stoichiometry of primary producers and consumers in temperate shallow lakes

1. We studied the effects of increased water temperatures (0–4.5 °C) and nutrient enrichment on the stoichiometric composition of different primary producers (macrophytes, epiphytes, seston and sediment biofilm) and invertebrate consumers in 24 mesocosm ecosystems created to mimic shallow pond environments. The nutrient ratios of primary producers were used as indicative of relative nitrogen (N) or phosphorus (P) limitation. We further used carbon stable isotopic composition (δ¹³C) of the different primary producers to elucidate differences in the degree of CO₂ limitation. 2. Epiphytes were the only primary producer with significantly higher δ¹³C in the enriched mesocosms. No temperature effects were observed in δ¹³C composition of any primary producer. Independently of the treatment effects, the four primary producers had different δ¹³C signatures indicative of differences in CO₂ limitation. Seston had signatures indicating negligible or low CO₂ limitation, followed by epiphytes and sediment biofilm, with moderate CO₂ limitation, while macrophytes showed the strongest CO₂ limitation. CO₂ together with biomass of epiphytes were the key variables explaining between 50 and 70% of the variability in δ¹³C of the different primary producers, suggesting that epiphytes play an important role in carbon flow of temperate shallow lakes. 3. The ratio of carbon to chlorophyll a decreased with increasing temperature and enrichment in both epiphytes and seston. The effects of temperature were mainly attributed to changes in algal Chl a content, while the decrease with enrichment was probably a result of a higher proportion of algae in the seston and epiphytes. 4. Macrophytes, epiphytes and seston decreased their C : N with enrichment, probably as an adaptation to the different N availability levels. The C : N of epiphytes and Elodea canadensis decreased with increasing temperature in the control mesocosms. Sediment biofilm was the only primary producer with lower C : P and N : P with enrichment, probably as a result of higher P accumulation in the sediment. 5. Independently of nutrient level and increased temperature effects the four primary producers had significantly different stoichiometric compositions. Macrophytes had higher C : N and C : P and, together with epiphytes, also the highest N : P. Seston had no N or P limitation, while macrophytes and epiphytes may have been P limited in a few mesocosms. Sediment biofilm indicated strong N deficiency. 6. Consumers had strongly homeostatic stoichiometric compositions in comparison to primary producers, with weak or no significant treatment effects in any of the groups (insects, leeches, molluscs and crustaceans). Among consumers, predators had significantly higher N content and lower C : N than grazers.     

An elemental and stable isotope assessment of water strider feeding ecology and lipid dynamics: synthesis of laboratory and field studies

1. Despite the ubiquity and abundance of water striders (Hemiptera: Gerridae) in temperate streams and rivers and their potential usefulness as sentinels in contaminant studies, little is known about their feeding ecology and lipid dynamics. 2. In this study we used stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) and elemental carbon to nitrogen ratios (C/N) to assess dietary habits and lipid content, respectively, for water striders. 3. To determine diet‐tissue fractionation factors, nymphs of the most common species in New Brunswick, Canada, Aquarius remigis were reared in the laboratory for 73 days and exhibited rapid isotopic turnover in response to a switch in diet (C half‐life = 1.5 days, N half‐life = 7.8 days). Their lipid content increased towards the end of the growing season and resulted in lower δ¹³C values. Diet‐tissue fractionation factors were established after correction of δ¹³C data for the confounding effect of de novo lipid synthesis (strider δ¹³C_adj– diet δ¹³C_adj = 0.1‰, strider δ¹⁵N – diet δ¹⁵N = 2.7‰). 4. Water striders from the majority of 45 stream sites (83%) in New Brunswick had less than 50% contribution of aquatic carbon to their diets but showed a gradual increase in the contribution of this carbon source to their diet with increasing stream size. 5. These data indicate that striders exhibit a strong connection to terrestrial carbon sources, making them important users of energy subsidies to streams from the surrounding catchment. However, this dependence on terrestrial organic matter may limit their utility as indicators of contamination of aquatic systems by heavy metals and other pollutants.