The fate of organic matter produced by phytoplankton constrains the response of algal production and biomass to nutrient inputs, influences food web structure, and determines the export of nutrients, contaminants and organic matter from the euphotic zone to sediments. Two potentially important fates of phytoplankton production, the extracellular release of organic carbon (ER) and the sinking loss from the euphotic zone, were compared between marine and freshwater planktonic ecosystems of varying productivity.

Published ER estimates were most closely related to primary productivity and not algal biomass. This pattern does not support the notion that extracellular products diffuse passively from algal cells in natural systems. Differences exist between lakes and marine environments. In marine and estuarine ecosystems, ER averaged 12% and did not vary systematically along a trophic gradient. In lakes, ER was non-linearly related to productivity, averaging about 30% of primary production in oligotrophic lakes and 5% in eutrophic lakes. A comparison of regression models implies that dissolved organic material released by phytoplankton can support less than 50% of bacterial production in natural ecosytems.

Sinking fluxes of carbon, nitrogen, phosphorus and pigments were closely related (r^2=0.87-0.92) to metalimnetic pigments in lakes. Sinking velocities of particle communities are not systematically related to chlorophyll concentration. Since the amount of production per unit chlorophyll increases with productivity in lakes, a negative relationship between primary productivity and the percent of production lost to sinking (export ratio) was inferred and then validated by literature data. This pattern is opposite to that previously expected for lakes and that observed for the ocean. However, it is consistent with models relating chlorophyll concentration to primary productivity, sinking fluxes of carbon, and light attenuation in lakes and the ocean.

The difference between marine and freshwater export ratio-productivity relationships appears to stem from 12-fold lower production per unit chlorophyll in oligotrophic lakes than in ocean regions of similar chlorophyll concentrations. Particle community sinking velocities are also average 2- to 3-fold higher in the ocean. These discrepancies between marine and freshwater ecosystems indicate fundamental differences in the way planktonic communities process organic
matter.