Picoplankton carbon flux in Chesapeake Bay

Although it is increasingly clear that picoplankton play a major role in the oceanic carbon cycle, relatively little is known concerning the significance of picoplankton in coastal systems subject to significant evironmental variance on tidal to interannual scales. Here we report on seasonal and interannual patterns of variability in the productivity and biomass of phototrophic and heterotrophic picoplankton (P- and H-PICO, respectively) and on the flow of carbon from phytoplankton to H-PICO. Annual cycles in the biomass and productivity of both picoplankton trophic levels exhibit winter-spring minima and summer maxima but do not appear to be directly coupled in terms of carbon flow from P- to H-PICO. H-PICO exceeds P-PICO during spring when picoplankton productivity is low, and P-PICO exceeds H-PICO during summer when productivity is high. P-PICO productivity and biomass increase rapidly each year to an early summer peak (20 % of total phytoplankton productivity on average) immediately following the collapse of the spring diatom bloom. In contrast, H-PICO productivity and the abundance of bacterioplankton (the predominant group of H-PICO) increase slowly to a late summer peak (equivalent to 16 % of phytoplankton productivity on average). Recently released phytoplankton exudates are a major source of dissolved organic carbon with H-PICO taking up an average of 54 % during spring and 83 % during summer. Variations in H-PICO are closely coupled to the release of DOC, most of which is produced by phytoplankton > 2 μm. New nutrient input to the Bay appears to be coupled to H-PICO productivity via increases in the productivity and DOC release of phytoplankton > 2 μm in size.