Buoyant river plumes are highly dynamic and often characterized by marked physical and chemical gradients that interact to drive biological responses. For example, interactions among factors resulting in algal growth (e.g., nutrient and light availability) and algal loss (e.g., sinking and zooplankton grazing) vary with spatiotemporal changes in physics and chemistry. The nature of these interactions profoundly influences the transfer and transformation of materials carried by the plume, including nutrients and metals. In April 2005, during the Lagrangian Transport and Transformation Experiment (LaTTE), water from the Hudson River recirculated in a nearshore eddy before moving southward to mix with relatively saline water along the New Jersey coast. Within the recirculating eddy, phytoplankton rapidly assimilated nutrients, resulting in extremely high rates of productivity (> 10 mg C m-3 h-1), with approximately 75% of carbon fixed by large, chain-forming diatoms. Sampling of phytoplankton and mesozooplankton, along with experimental estimates of microzooplankton and mesozooplankton grazing rates, indicated that these large phytoplankton escaped grazing and sank. The subsequent decomposition of this organic material contributed to decreased oxygen concentrations in bottom waters along the edges of the buoyant plume. In contrast to carbon, particulate metal concentrations derived from the smaller size class of phytoplankton were twice as high as those derived from larger phytoplankton. Relatively efficient grazing on the smaller size class led to bioaccumulation of metals in mesozooplankton. The interactions among chemistry, physics, and biology in the dynamic Hudson River plume serve as an example of how anthropogenic activities in urbanized watersheds can influence and potentially alter coastal food webs.

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