Stable isotope analysis of the origins of zooplankton carbon in lakes of differing trophic state

Abstract

Carbon stable isotope analysis was carried out on zooplankton from 24 United Kingdom lakes to examine the hypothesis that zooplankton dependence on allochthonous sources of organic carbon declines with increasing lake trophy. Stable isotope analysis was also carried out on particulate and dissolved organic matter (POM and DOM) and, in 11 of the lakes, of phytoplankton isolates. In 21 of the 24 lakes, the zooplankton were depleted in C relative to bulk POM, consistent with previous reports. δC for POM showed relatively little variation between lakes compared to high variation in values for DOM and phytoplankton. δC values for phytoplankton and POM converged with increasing lake trophy, consistent with the expected greater contribution of autochthonous production to the total organic matter pool in eutrophic lakes. The difference between δC for zooplankton and that for POM was also greatest in oligotrophic lakes and reduced in mesotrophic lakes, in accordance with the hypothesis that increasing lake trophic state leads to greater dependence of zooplankton on phytoplankton production. However, the difference increased again in hypertrophic lakes, where higher δC values for POM may have been due to greater inputs of C-enriched organic matter from the littoral zone. The very wide variation in phytoplankton δC between lakes of all trophic categories made it difficult to detect robust patterns in the variation in δC for zooplankton.

Seasonal changes in the importance of the source of organic matter to the diet of zooplankton in Loch Ness, as indicated by stable isotope analysis

“…Different species within the community exhibited varying reliance on the two potential carbon sources, a phenomenon often overlooked in stable isotope studies by the amalgam-ation of zooplankton into a single trophic functional group (Grey and Jones 1999;Grey et al 2000). Both the carbon and nitrogen isotopic signatures of D. hyalina suggest virtually no assimilation from allochthonous sources during summer in Loch Ness, this species apparently being almost completely reliant on algal production.…”

Comparison of methods to determine algal δ13C in freshwater

“…Stable isotopes analysis (SIA) is a popular tool used by ecologists to address a wide range of questions related to food sources of organisms, the length of food chains, or the transfer of contaminants (Cabana and Rasmussen 1994, Post et al 2000, Grey et al 2000, Pace et al 2004. These applications are possible because the stable isotopic composition of a consumer's tissues is related to that of its food (Rounick andWinterbourn 1986, Vander Zanden andRasmussen 2001).…”

Contributions of autochthonous and allochthonous sources to dissolved organic matter in a large, shallow, eutrophic lake with a highly calcareous catchment

“…The composition and sources of dissolved organic matter (DOM) in an aquatic ecosystem may vary spatially and temporally (Finlay and Kendall 2007). In lakes, the relative importance of allochthonous sources should increase with decreasing lake trophy (Grey et al 2000). In many lakes, especially within the cool boreal region, net ecosystem production is negative (Jansson et al 2007), suggesting that appreciable Al-DOM runoff from the catchments increases the respiration within the lake ecosystem to a level that exceeds photosynthetic carbon sequestration by phytoplankton and aquatic plants.…”