Abstract
Although there are a number of single system and regional studies on the importance of picophytoplankton (PP; 0.2–2 µm), there has been little effort to quantify their importance on a larger scale and to integrate marine and freshwater studies. We developed empirical models from data in the literature to assess the absolute and relative importance of PP in marine and freshwater. The PP biomass (chlorophyll a [Chl a]) increases with trophic status, here represented by the total Chl a, in both marine and freshwater systems. In addition, PP abundance in freshwater, and primary production in marine waters, increases with trophic status. Even so, the relative importance of the PP biomass and primary production declines systematically with increasing trophic status in both marine and freshwater systems. Both the absolute biomass of PP and the relative contribution of PP biomass to the total phytoplankton community biomass are larger in freshwater at any particular value of total Chl a. In eutrophic lakes, which are typically shallow and particularly susceptible to the greatest phytoplankton resuspension and sediment recruitment, the on average small relative contribution of PP biomass appears affected by a relatively high but variable contribution of resuspended microplankton.