Equilibrium Models for Seasonal Dynamics of Plankton Biomass in Four Oligotrophy Lakes

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Abstract
Plankton biomass dynamics were monitored over a 11-yr period in four coastal British Columbia lakes while they were disturbed by salmonid introductions, fertilization, and zooplankton harvesting. Except for dramatic zooplankton responses to fertilization, the lakes had relatively simple and stable seasonal biomas patterns, with midsummer zooplankton peaks and no clear seasonal cycles in biomass of unicellular phytoplankton. Simple models predict that equilibrium biomasses should follow the observed pattern, provided zooplankton grazing and metabolic rates are temperature independent; experimental measurments of these rates did not show clear temperature edependence. Enclosure studies showed that phytoplankton biomass can return quickly (48-72 h) to equilibrium after disturbance, but zooplankton biomass responds more slowly (2- to 3-wk recovery times), yet fast enough to track a seasonally varying equilibrium. We conclude that the biomass equilibrium of unicellular phytoplankton is set by grazing and metabolic rates of zooplankton, while the zooplankton biomass equilibrium is set by phytoplankton productivity.