The idea that phytoplankton size structure increases with elevated nutrient content in lakes may be inaccurate because phototrophic picoplankton (Ppico) abundance was not accounted for in studies that originally described this phenomenon. The biomass, composition, and production of an entire phytoplankton community was evaluated in hypereutrophic Lake Apopka, Florida (avg chlorophyll = 105 pg liter-‘, y1 = 20) for 1 yr. Ppico cell abundances commonly exceed 10 ’ cells ml-I, and size-fractionated chlorophyll (<5 pm) and algal enumeration (cells <2 pm in size) indicate that pica-size cyanobacteria contribute significantly to phytoplankton biomass throughout the year (avg 30%, n = 15), even though clogging of small pore-sized filters used in chlorophyll fractionation studies led to underestimation of Ppico biomass estimates. Ppico production contributed nearly 30 % (n = 8) to total phytoplankton production. The quantitative importance of Ppico in Lake Apopka and eight other hypereutrophic systems in Florida (avg 50 % Chl <5 pm) indicates that Ppico can be important in waters of high nutrient content. The importance of small phytoplankton in productive waters may have been overlooked because (1) most studies focus on relatively pristine ecosystems, (2) some methodological constraints may impede studies in eutrophic environments (filtration artifacts), and (3) routine assessment of phytoplankton abundance rarely censuses all components of the community, including small algae. The contribution that phototrophic picoplankton (Ppico, algae 0.2-2 pm in size) makes to phytoplankton biomass and primary productivity in aquatic ecosystems has become evident during the past decade (e.g. Stockner 1988). Substantial information on Ppico distribution, composition, and food-web dynamics exists for marine ecosystems (e.g. Platt and Li 1986). In lakes, temporal and spatial patterns in Ppico abundance and composition have been documented (e.g
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