Pelagic C : N : P stoichiometry in a eutrophied lake: Responses to a whole-lake food-web manipulation

Changes in the ecological stoichiometry of C, N, and P in the pelagic zone are reported from a whole-lake manipulation of the food web of Lake 227, an experimentally eutrophied lake at the Experimental Lakes Area, Canada. Add ition of northern pike eliminated populations of planktivorous minnows by t he third year (1995) after pike introduction, and in the fourth year after pike addition (1996), a massive increase in the abundance of the large-bodi ed cladoceran Daphnia pulicaria occurred. Accompanying this increase in Dap hnia abundance, zooplankton community N:P declined, seston concentration an d C:P ratio decreased, and dissolved N and P pools increased, During peak a bundance, zooplankton biomass comprised a significant proportion of total e pilimnetic phosphorus (greater than 30%), During the period of increased Da phnia abundance, concentrations of dissolved inorganic nitrogen (TIN) incre ased more strongly than dissolved phosphorus (TDP), and thus TIN:TDP ratios were elevated. Sedimentation data indicated that increased grazing led to greatly reduced residence times of C, N, and especially P in the water colu mn during 1996. Finally, previously dominant N-fixing cyanobacteria were ab sent during 1996. Our results show that strong effects of food-web structur e can occur in eutrophic lakes and that stoichiometric mechanisms play a po tentially important role in generating these effects.