Rates of loss sustained by selected populations of natural phytoplankton husbanded in large limnetic enclosures (“Lund Tubes”) are derived, in terms of specific biomass units (cells or colonies), from direct measurements of the rates of net increase, of sedimentation and zooplankton filtration, corrected for selectivity (“grazing”). Partitioning between the losses was found to vary interspecifically between almost complete removal by grazers (in the case of nanoplankton) and almost complete elimination by sinking (in the case of some diatom populations). These differences are argued to be critical in influencing the specific composition of the phytoplankton and its seasonal dominance. A model simulating the dynamics of three species experiencing differential loss rates is employed to demonstrate the outcome of interspecific competition. Evidence contrary to the widely-held contentions that zooplankton controls phytoplankton biomass and that blue-green algae (cyano-bacteria) dominate by virtue of being “ungrazed” is discussed.

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