CHLOROPHYLL VARIABILITY, NUTRIENT INPUT, AND GRAZING - EVIDENCE FROM WHOLE-LAKE EXPERIMENTS

A Planktivore Lake with small herbivorous zooplankton and a Piscivore Lake with large grazers were monitored for 2 yr and then enriched for 2 yr with inorganic phosphorus and nitrogen. An unenriched lake with a n undisturbed fish community served as a reference ecosystem. Added nu trients increased biomass of both phytoplankton and zooplankton. In th e Planktivore Lake, phytoplankton were stimulated more than zooplankto n. This situation was reversed in the Piscivore Lake. Time series mode ls predicting chlorophyll from P input rate and crustacean length fit well for total chlorophyll and the edible fraction (<35 mu m), but wer e less successful for the large fraction (>35 mu m). A 1-mm change in mean crustacean length had about the same effect on chlorophyll as a d ecrease in P input rate of 1 mg . m(-3) . d(-1). There was no evidence of interaction between grazer and P input effects. Although effects o f grazers and P input are equal and additive, the range of mean crusta cean lengths among lakes is approximate to 1 mm, while the range of P input rates is substantially >1 mg . m(-3) . d(-1). Therefore, the pot ential for increasing eutrophication by P input exceeds the potential for controlling eutrophication by food web manipulation. Nevertheless, biomanipulation can reduce chlorophyll concentration at P input rates of 1.5 mg . m(-3) . d(-1) or more; these rates characterize a large f raction of the world's stratified lakes.