Seasonal changes in the spatial distribution of phytoplankton in small, temperate-zone lakes

Abstract

Seasonal sampling across two small lakes shows that phytoplankton patchiness is greatly enhanced during winter ice-cover relative to the open-water seasons of exposure to wind stress and rapid turbulent mixing. A fundamental property of plankton populations is their spatial heterogeneity or patchiness. Phytoplankton patchiness can result from spatial variations of biological processes such as growth, grazing, regulated buoyancy and vertical migration (e.g. Reynolds, 1984; Mackas et al., 1985) or from advective transports (George and Edwards, 1976), and it is disrupted by turbulent mixing. These processes are dynamic, so we expect temporal variability of phyto- plankton patchiness to accompany fluctuations in physical and biological processes across a spectrum of time scales. For example, fluctuations in the wind stress across small lakes can produce measurable change in the spatial distribution of phytoplankton, at time scales of: hours, associated with the passage of cold fronts (Stauffer, 1982); days, from the cumulative effects of antecedent wind stress over periods of 1-10 days (Small, 1963); and weeks, associated with events of enhanced vertical mixing from weather systems, which have 10-20 days periodicity (Trimbee and Harris, 1983). These observations demonstrate a dynamic coupling between the input of turbulent kinetic energy from wind and the pattern of phytoplankton spatial distribution, operating at time scales of weeks or less. Can this generality be extended to longer time scales? For example, if phytoplankton spatial patterns are responsive to hourly- daily fluctuations in wind stress, then we might expect very large changes in the distribution of phytoplankton when temperate lakes freeze over and become isolated, for months, from the input of wind energy. To test this hypothesis we mapped phytoplankton biomass (as chlorophyll a concentration) seasonally within two small lakes situated in the glacial terrain of