C-13 and N-15 depletion in components of a foodweb from an ephemeral boreal wetland compared to boreal lakes: putative evidence for microbial processes

Stable carbon and nitrogen isotope ratios were used to posit the relative i mportance of microbial processes on energy pathways in an ephemeral, humic boreal wetland compared to four clearwater lakes in northwestern Ontario, C anada. In addition to algae and dipteran larvae, odonate larvae were sample d as these latter organisms are known to predate indiscriminately on smalle r invertebrates and are thus likely to have average isotope ratios reflecti ve of their habitats. Similarities in delta C-13 and delta N-15 values betw een lake insect larvae and emerged adults suggested that littoral foodwebs in these oligotrophic lakes may rely to a considerable degree upon terrestr ial carbon. Wetland insect larvae and algae were depleted in both C-13 and N-15 compared to biota in lakes. Carbon isotope analysis implied a substant ial presence of microbial respiration from decomposition in the humic wetla nd, whereas nitrogen isotope analysis suggested the prevalence of microbial ly modified nitrogen dynamics, including the possibilty of N-fixation.