Benthic foodweb structure in a large shallow lake studied by stable isotope analysis

The benthic foodweb structure of Lake Vortsjarv, a large (270 km(2)), shallow, and turbid Estonian lake, was evaluated based on C and N stable-isotope signatures (delta C-13, delta N-15). Variation in delta C-13 between sampling sites was not related to site proximity to the littoral zone or the more vegetated southern part of the lake, but rather appeared to be influenced by in-situ site peculiarities. delta C-13 was stable temporally and between functional feeding groups, a result implying that the whole benthic food web of the lake relies largely on the same C source admixture, essentially particulate organic matter (POM). Thus, the foodweb composition of Lake Vortsjarv is remarkably homogeneous given the lake's large surface area. Apparent trophic-level delta N-15 fractionation between total collectors and total predators (mean 1.7 parts per thousand) was lower than the value of 3.4 parts per thousand generally adopted in foodweb studies, but the higher value was valid for specific prey-predator links. The low delta C-13 signature of some chironomid samples indicated probable assimilation of methane-oxidizing bacteria (MOB) by these sediment-dwelling invertebrates. However, the lack of similar C-13 depletion in benthic filterers (mussels) indicated that the MOB layer is essentially confined to the sediments and does not reach the water column, which probably constrains transfer of methane-derived C through the food web to fish in this lake. Our study demonstrates that the benthic food web of shallow turbid lakes like Vortsjarv is simplified and is mostly sustained by phytoplanktonic C sources.