Coastal Ocean Observatories Enable Biological Investigations in a Buoyant Plume

Rivers that flow through urban watersheds represent major conduits for the transport of anthropogenically derived nutrients and chemical contaminants to the coastal ocean. The fate of these materials is controlled not only by the physical dynamics of the riverine plume as it moves over the coastal shelf, but also by associated biological and chemical processes. The resulting interactions are complex and often difficult to capture using traditional oceanographic methods and resources. The advent of coastal observation systems, however, enables ocean scientists to react quickly and adaptively sample dynamic coastal environments to more fully understand key physical, chemical and biological processes. In April 2005, during the Lagrangian Transport and Transformation Experiment (LaTTE), nutrient-laden water discharged from the Hudson River was retained nearshore in a recirculating eddy before moving southward along the New Jersey coast and mixing with relatively saline coastal water. Biological sampling of the recirculating zone and southward moving plume water was facilitated by a shelf-wide coastal observatory system. Rapid nutrient uptake and assimilation by phytoplankton within the eddy resulted in extremely high rates of productivity (> 10 mg C m^-3 h^-1). Approximately 75% of the fixed carbon within the eddy could be attributed to phytoplankton in the > 20 um size class determined subsequently to be comprised primarily of large (~200 um) chain forming diatoms. Characterization of the zooplankton assemblage revealed a dominance of small copepods, approximately 200-400 um in size, and shipboard grazing experiments indicated a negligible feeding impact of these mesozooplankton on the phytoplankton. The apparent mismatch in size between producers and consumers, coupled with the high rates of primary production, resulted initially in a pronounced accumulation of phytoplankton biomass (>25 ug L^-1 chl a) within the recirculation- - zone with subsequent declines in dissolved oxygen concentration in bottom waters coincident with nutrient reduction and bloom senescence