Larger cell or colony size in winter, smaller in summer – a pattern shared by many species of Lake Kinneret phytoplankton

We examined an 8.5-year record (2004-2012) of cell size data for phytoplankton species from Lake Kinneret, Israel, sampled weekly or at 2-week intervals and determined microscopically by the same person. Many of the species abundant enough to be counted year-round showed a typical seasonal cell size pattern that repeated annually: cell diameter was maximal in winter and minimal in summer. This pattern was shared by species from different taxonomic groups including cyanobacteria, chlorophyta, and dinoflagellates. Similarly, in colonial species of diatoms, chlorophyta, and cyanobacteria the number of cells per colony was larger in winter and smaller in summer. We postulated that the seasonal changes in cell or colony size constituted an adaptation that enabled species to overcome temperature-dependent changes in water density and viscosity and adjust their sinking velocities in the different seasons. A series of computations based on Stokes' law supported our hypothesis. If this phenomenon of larger cells or colonies of the same species at lower temperatures is widespread in lake phytoplankton, then (1) phytoplankton biomass estimates should be based on seasonally determined biovolume per species data rather than fixed values; and (2) we would expect to see smaller-sized organisms in the future as global warming changes ambient temperatures.