Picoplankton and Nanoplankton Biomass in Lake Ontario: Relative Contribution of Phototrophic and Heterotrophic Communities

The seasonal and vertical abundance of phototrophic and heterotrophic nanoplankton (2–20 μm) and picoplankton (0.2–2 μm) was estimated in Lake Ontario during 1982 by epifluorescence microscopy. Phototrophic and heterotrophic nanoplankton abundance differed by less than an order of magnitude and was typically present at densities of 103 cells∙mL−1. Heterotrophic nanoplankton was, on average, only half as abundant as phototrophic nanoplankton, but on several dates was more abundant than the latter. Both populations peaked in late June to early July. Phototrophic picoplankton, primarily chroococcoid cyanobacteria, increased rapidly during midsummer, reaching a maximum epilimnetic concentration of 4 × 105 cells∙mL−1 in late August. Heterotrophic picoplankton (bacteria) showed a similar seasonal pattern, reaching maximum abundance in September (6 × 106) cells∙mL−1). The concentrations of both picoplankton populations were significantly correlated with temperature. By late summer, picoplankton biomass represented 74% and pico-cyanobacteria alone 50% of the total weight biomass of microorganisms < 20 μm; these populations are generally missed by inverted microscope techniques.