CANNIBALISM AND TROPHIC STRUCTURE IN A HIGH ARCTIC LAKE - INSIGHTS FROM STABLE-ISOTOPE ANALYSIS

Stable-nitrogen (N-15/N-14) isotope ratios (from 2.1 parts per thousand in moss to 14.5 parts per thousand in Arctic char, Salvelinus alpinus), showed enrichment with trophic level in the food web of Char Lake, Northwest Territories, and may be used to infer trophic position. The average N-15 enrichment of 1.5 parts per thousand between moss or algae and invertebrates suggests input to the food web of isotopically lighter nitrogen than that measured for these sources of primary production. Stable-carbon (C-13/C-12) isotope ratios differed between moss and algae by almost 10 parts per thousand and indicate that carbon in the Char Lake food web is derived from a blend of these sources of primary production. Arctic char delta(15)N values for muscle tissue were positively correlated with fork length and clustered into three distinct groups: small fish (1-3 cm, mean delta(15)N = 5.7 parts per thousand), which possibly consumed more benthic particles than previously assumed; intermediate-size fish (10-35 cm, mean delta(15)N = 10 parts per thousand), which likely depended on larval char as well as their primary zooplankton and chironomid prey; and larger Arctic char, which showed a mean stepwise increase in delta(15)N of 3.7 parts per thousand. This suggests that complete cannibalism in this population generally occurs abruptly and is exercised by a relatively small number of large individuals.