Composition and spatial heterogeneity of submersed vegetation in a softwater lake in Wisconsin

A mechanistic, size-structured model was used to analyze effects of grazers on lacustrine primary productivity. Dependence of grazing and nutrient excretion rates on herbivore size, and dependence of algal metabolic rates on cell size, interacted to produce strong responses in primary productivity. Productivity was maximum at intermediate concentrations of chlorophyll and zooplankton biomass. This unimodal response is consistent with data from aquatic systems, terrestrial systems (where the underlying mechanisms are different), and theoretical expectations. Substantial variability in lake productivity (2 to 3 orders of magnitude) exists that cannot be explained by nutrient supply. Food web interactions can regulate herbivorous zooplankton size and abundance, independent of nutrient supply. The resultant variability in herbivory influences nutrient recycling and the size distribution of the phytoplankton, and can alter lake primary productivity by as much as two orders of magnitude.