The trophic importance of epiphytic algae in a freshwater macrophyte system (Potamogeton perfoliatus L.): stable isotope and fatty acid analyses

Stable isotope and fatty acid analyses were used to study carbon sources for animals in a submerged plant bed. Epiphytes growing on Potamogeton perfoliatus, sand microflora, and alder leaves were the most important carbon sources. The most abundant macrophyte, P. perfoliatus was unimportant as a food source. Modelling (IsoSource) showed that epiphytes were the most important food source for the most abundant benthic invertebrates, the isopod Asellus aquaticus (annual mean contribution 64%), the amphipod Gammarus pulex (66%), and the gastropod Potamopyrgus antipodarum (83%). The mean annual contributions of sand microflora were, respectively, 21, 19, and 9%; and of alder leaves, 15, 15, and 8% for these three species. The relative importance of carbon sources varied seasonally. The relative contribution of epiphytes was lowest for all three grazer species in July: A. aquaticus 38%, G. pulex 43%, and P. antipodarum 42%. A decline in epiphyte biomass in summer may have caused this switch to less attractive food sources. P. perfoliatus provided habitat and shelter for consumers, but food was mainly supplied indirectly by providing space for attached epiphytes, which are fast-growing and provide a highly nutritious food source

Forest-Stream Linkages: Effects of Terrestrial Invertebrate Input and Light on Diet and Growth of Brown Trout (Salmo trutta) in a Boreal Forest Stream

Subsidies of energy and material from the riparian zone have large impacts on recipient stream habitats. Human-induced changes, such as deforestation, may profoundly affect these pathways. However, the strength of individual factors on stream ecosystems is poorly understood since the factors involved often interact in complex ways. We isolated two of these factors, manipulating the flux of terrestrial input and the intensity of light in a 2×2 factorial design, where we followed the growth and diet of two size-classes of brown trout (Salmo trutta) and the development of periphyton, grazer macroinvertebrates, terrestrial invertebrate inputs, and drift in twelve 20 m long enclosed stream reaches in a five-month-long experiment in a boreal coniferous forest stream. We found that light intensity, which was artificially increased 2.5 times above ambient levels, had an effect on grazer density, but no detectable effect on chlorophyll a biomass. We also found a seasonal effect on the amount of drift and that the reduction of terrestrial prey input, accomplished by covering enclosures with transparent plastic, had a negative impact on the amount of terrestrial invertebrates in the drift. Further, trout growth was strongly seasonal and followed the same pattern as drift biomass, and the reduction of terrestrial prey input had a negative effect on trout growth. Diet analysis was consistent with growth differences, showing that trout in open enclosures consumed relatively more terrestrial prey in summer than trout living in covered enclosures. We also predicted ontogenetic differences in the diet and growth of old and young trout, where we expected old fish to be more affected by the terrestrial prey reduction, but we found little evidence of ontogenetic differences. Overall, our results showed that reduced terrestrial prey inputs, as would be expected from forest harvesting, shaped differences in the growth and diet of the top predator, brown trout

Changes in community composition, carbon and nitrogen stable isotope signatures and feeding strategy in epilithic aquatic nematodes along a depth gradient

Peters L, Faust C, Traunspurger W. Changes in community composition, carbon and nitrogen stable isotope signatures and feeding strategy in epilithic aquatic nematodes along a depth gradient. Aquatic Ecology. 2012;46(3):371-384.Periphyton is an important component within the littoral zones of lakes, but it is known to vary dramatically on small (cm-m) and large (km) spatial scales, showing differences in composition and abundance. Until relatively recently, changes in periphyton composition along depth gradients have not been studied sufficiently and the response of small meiobenthic invertebrates inhabiting the periphyton to vertically changing environmental conditions such as light are poorly understood. To investigate the changing community composition of epilithic (on stones) nematodes along a depth gradient, we conducted a field study at Lake Erken, Sweden, with the specific objective to investigate whether changes in periphyton composition (algae, bacteria) are reflected in changing nematode feeding types and what the consequences are for nematodes and their resource consumption. We analysed the abundance, species composition and feeding type distribution of epilithic nematodes along 11 depth levels, from 5 to 300 cm water depth. Our study resulted in the first measurements of carbon (C-13) and nitrogen (N-15) stable isotopes in free living nematodes in lakes. Nematode community composition and feeding type distribution exhibited dramatic changes along the depth gradient. Nematode feeding types changed from a dominance of algae-feeding species in the shallow littoral zone to one of bacteria-feeding species in the deep littoral zone. The C-13 stable isotope signatures of nematodes and the small (< 20 mu m) periphyton fraction changed with increasing depth, with nematodes shown to feed on this small periphytic component. Nematodes were identified as primary consumers by means of trophic level calculations based on N-15 stable isotopes