STABLE CARBON AND NITROGEN ISOTOPIC ANALYSIS OF THE PLANKTON FOOD-WEB IN A SUB-ARCTIC LAKE

Carbon and nitrogen stable isotopes (delta(13)C and delta(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, delta(15)N of Daphnia was strongly influenced by atmospheric nitrogen derived from a N-2-fixing blue-green algal bloom. In winter, delta(13)C evidence suggested that phytoplankton comprised a small fraction (similar to 15%) of particulate organic matter (POM) in the water column, largely due to low primary productivity. The disparity between delta(13)C oi 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 delta(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.