Stable Carbon and Nitrogen Isotopic Analysis of the Plankton Food Web in a Subarctic Lake

Carbon and nitrogen stable isotopes (δ 13 C and δ 15 N) were used to track energy flow and nutrient cycling pathways in the plankton food web of a subarctic Alaskan lake. Results indicated that planktonic primary production was the major energy source fueling the zooplankton community. In spring, δ 15 N of Daphnia was strongly influenced by atmospheric nitrogen derived from a N 2 -fixing blue-green algal bloom. In winter, δ 13 C evidence suggested that phytoplankton comprised a small fraction (~15%) of particulate organic matter (POM) in the water column, largely due to low primary productivity. The disparity between δ 13 C of POM and Daphnia in winter may result from preferential assimilation of isotopically light algal carbon from POM. Nitrogen isotope values showed that Heterocope, a presumed carnivore, probably relied heavily on POM as a nutrient source. In common with some arctic lakes, the δ 15 N data showed less than three trophic levels in this plankton food web. The energy transfer pathways and trophic levels revealed a simple plankton trophic structure in this subarctic lacustrine system.