The Role of Plankton Community Metabolism in Regulating Sedimentation of Organic Matter in a Coastal Environment
We compared temporal variations in the production and respiration of coastal plankton community in the mesohaline region of Chesapeake Bay at time-scales of weeks with contemporaneous measurements of plankton trophic structure and deposition of particulate organic matter (POM) for the spring to summer. Seasonal cycles were punctuated by two major events of POM deposition, which were observed repeatedly over six years. The first event occurred in mid-April in association with the sinking of intact diatoms from the spring bloom, while the second occurred in mid-August at the time of peak phytoplankton production. Cool springtime temperatures retarded grazing and heterotrophic metabolism during March to April (despite a relatively high abundance of copepods), allowing the sedimentation of a large fraction of phytoplankton production. In late spring and early summer (May–July), the phytoplankton community was dominated by small flagellates, and most of the primary production was consumed and respired by rapidly growing communities of bacteria and protozoa, with little POM sinking to bottom waters. A comparison of rates of POM collection in traps deployed near the water surface, with those at the pycnocline, suggested that there is tightly coupled nutrient cycling in the upper euphotic zone during this period, with little new production. However, by mid-summer, heterotrophic activity was reduced; these relatively low rates of community respiration rates, coupled with high rates of primary production, result in increased rates of POM deposition. Net diel metabolism of the surface plankton community (NCM) was estimated as the sum of net daytime production (O2) minus nighttime respiration (O2). POM (total chl) sedimentation was highly correlated with NCM (r2 = 0.87), but unrelated to gross primary production (r2 = 0.20). Thus, we conclude that the timing and magnitude of organic matter losses from the euphotic zone are controlled as much by heterotrophic activity as by new rates of primary production.