Patterns of primary production and herbivory in 25 North American lake ecosystems
The effects of nutrients and herbivory on phytoplankton biomass and production were examined, using data from 25 lakes studied for 2 to 6 years each. Variance among lakes was substantially greater than variance among years, for all physical, chemical, phytoplankton, and zooplankton variates studied. Experimentally manipulated lakes had coefficients of variation within the range exhibited by nonmanipulated lakes. Graphical, correlative, and regression analyses illustrated the significant joint effects of both nutrients and herbivory on phytoplankton biomass and production. A Bayesian analysis of sensitivity to new information showed that the statistical models for chlorophyll are quite robust. Statistical models for primary production were deemed less conclusive, because primary production was measured in fewer lakes. We provide a list of common challenges in comparative statistical analysis of ecosystems and explain their implications for our study. The major pattern apparent in our data—that summer chlorophyll responds positively to nutrients and negatively to herbivore size — is congruent with results of whole-lake experiments in which nutrients or predators were manipulated.