The Temporal Scale of Variance in Limnetic Primary Production

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Abstract
Substantial variability in the productivity of the world''s lakes cannot be explained by methodology, weather, hydrology, or nutrient supply. A deterministic model shows that variable piscivory, cascading through the food web and causing fluctuations in planktivory, herbivory, and primary production, can bring about variability in primary production comparable in magnitude and time scale to the variability that cannot be explained by physical or chemical factors. The variance of productivity and the correlation between primary production and zooplankton biomass are functions of the time scale over which model results were averaged. The scatter of data around regressions of annual production versus nutrient loading represents internal system dynamics at a host of time scales, some of which may be regulated by fish populations. An important consequence of these dynamics in that variances and covariances of limnological variables are strongly related to sampling scale. Nutrient loading, hydrology, and spring mixing establish the potential summer productivity of lakes. The realized productivity depends in part on the configuration and dynamics of the food web, which are largely determined by the top carnivore in the system.