HAVE WE OVERLOOKED THE IMPORTANCE OF SMALL PHYTOPLANKTON IN PRODUCTIVE WATERS

The idea that phytoplankton size structure increases with elevated nut rient content in lakes may be inaccurate because phototrophic picoplan kton (Ppico) abundance was not accounted for in studies that originall y described this phenomenon. The biomass, composition, and production of an entire phytoplankton community was evaluated in hypereutrophic L ake Apopka, Florida (avg chlorophyll = 105 mu g liter(-1), n = 20) for 1 yr. Ppico cell abundances commonly exceed 10(7) cells ml(-1), and s ize-fractionated chlorophyll (<5 mu m) and algal enumeration (cells <2 mu m in size) indicate that pico-size cyanobacteria contribute signif icantly 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 estimat es. Ppico production contributed nearly 30% (n = 8) to total phytoplan kton production. The quantitative importance of Ppico in Lake Apopka a nd eight other hypereutrophic systems in Florida (avg 50% Chi <5 mu m) indicates that Ppico can be important in waters of high nutrient cont ent. The importance of small phytoplankton in productive waters may ha ve been overlooked because (1) most studies focus on relatively pristi ne ecosystems, (2) some methodological constraints may impede studies in eutrophic environments (filtration artifacts), and (3) routine asse ssment of phytoplankton abundance rarely censuses all components of th e community, including small algae.