Cycling of colloidal organic carbon and nitrogen during an estuarine phytoplankton bloom

To establish the influence of phytoplankton blooms on the cycling of dissolved and particulate species of organic carbon and nitrogen, we conducted a field study during a series of blooms in a coastal embayment on Shelter Island, NY. Using cross-flow filtration, we collected highand low-molecular-weight (HMW and LMW) dissolved organic matter (DOM), along with particulate organic matter (POM). There was a significant and near equivalent enhancement in levels of particulate organic carbon (POC) and dissolved organic carbon (DOC) during phytoplankton blooms. HMW organic carbon was responsible for most (80%) of the DOC increase. In contrast, substantial amounts of organic nitrogen were produced in all size fractions (particulate organic nitrogen [PON], HMW, and LMW) during blooms. POC : PON and HMW C : N ratios exceeded Redfield stoichiometry and were well correlated with chlorophyll concentrations, which suggests that phytoplankton were the primary source of C-enriched particles and colloids in this system. DOM C : N ratios were higher during periods of elevated nitrate than during low nitrate conditions, which were dominated by phytoplankton with heterotrophic capabilities. This suggests that, in some coastal systems, the accumulation of C-enriched organic matter may be more dependent on algal species composition than ambient inorganic nitrogen levels. After the collapse of algal blooms, bacterial densities rose markedly, and all organic pools rapidly decreased to near prebloom levels. Despite substantial production and turnover rates of HMW organic carbon during blooms, longer residence times of other, more refractory, organic carbon pools such as the LMW fraction indicated that considerable portions of organic matter produced during estuarine phytoplankton blooms may be exported to continental shelves. Coastal environments are regions of high biological productivity. It has been estimated that, although continental margins occupy ,10% of the total surface area of the world ocean, up to one third of the global marine primary production occurs within these regions (Wollast 1991). The fate of this production (whether it is consumed by the shelf microbial communities, buried in the shelf sediments, or exported to deep ocean waters) is of considerable interest, particularly with regard to its impact on the global carbon budget (Hedges 1987; del Giorgio and Duarte 2002). Although recent mass balance estimates have suggested that the input of dissolved organic matter (DOM) from continental margins to the open ocean could be as much as 100 times greater than the contribution of material from surface waters (Bauer and Druffel 1998), the importance of coastal environments in the export of organic matter remains the subject of diverging of opinions (e.g,, Falkowski et al. 1988; Liu et al. 2000; del Giorgio and Duarte 2002). Our current understanding of the fate of the marine organic pools in coastal waters (e.g., ex-