IMPACT OF DISSOLVED ORGANIC-CARBON, PHOSPHORUS, AND GRAZING ON PHYTOPLANKTON BIOMASS AND PRODUCTION IN EXPERIMENTAL LAKES

Phytoplankton biomass and production in lakes tend to be increased by phosphorus input and decreased by grazing or high levels of colored, d issolved organic carbon (DOC). We estimated and compared the effects o f these three factors by using data from three lakes that were manipul ated during 1991-1995; and data from a reference lake. Multivariate pr obability distributions of chlorophyll or primary production, as predi cted by P input rate, DOG, and grazer length, were fit to the data. Al l three factors had substantial effects on chlorophyll, primary produc tion, and their variability. Comparable reductions in the mean and var iance of chlorophyll and primary production were achieved by reducing P input rate from 5 to 0.5 mg m(-2) d(-1), increasing DOC from 5 to 17 mg C liter(-1), or increasing mean crustacean length from 0.2 to 0.85 mm. The negative effect of mean crustacean length (an index of size-s elective predation) results from grazing by herbivorous zooplankton. T he negative effect of DOC on primary producers could be explained by s hading. The results suggest that natural variation in colored DOC conc entrations is a major cause of variation in primary production.