Skidaway Institute of Oceanography

Ocean margins are highly productive ecosystems. Continental shelf and adjacent slope waters represent 10-20% of the surface area of the global oceans, but contribute 25-50% of total oceanic primary production. These coastal oceans have been stimulated by increased anthropogenic nutrient loadings during the twentieth century, with resulting increases in phytoplankton biomass and, likely, primary production. The magnitude and fate of this new production is unknown. It has been argued that this enhancement is sufficient to affect anthropogenic carbon dioxide concentrations. Determining the factors, which influence the magnitude of carbon exchanges between the ocean margins and atmosphere, and the fate of that carbon, is essential to understanding and predicting the ocean's capacity to store anthropogenic CO2.

The Cape Hatteras shelf lies at the confluence of the Middle (MAB) and South Atlantic Bights (SAB), a watershed impacted by the activities of 100 million people. Productive shelf waters from north and south exit at Cape Hatteras, and the particles they contain enter the interior ocean or slope sediments. In slope waters off Cape Hatteras, carbon fluxes to the sediments and carbon concentrations within the sediments are among the highest in the western North Atlantic Ocean. However, the source and fate of that material, and its magnitude relative to other carbon fluxes, is unknown.

Along the S.E. U.S. continental margin south of Cape Canaveral to Cape Canaveral, deep nutrient-rich waters directly contact shelf waters, and rapid exchanges take place due to baroclinic instability of the Gulf Stream front. The displacement of these nutrients into the euphotic zone results in new primary production. This phytoplankton either remains on the shelf, is returned to the Gulf Stream where it decomposes in the water column and reenters the North Atlantic Deep Water, or is deposited on the continental slope or abyssal plain. Studies of the region have discovered preferred regions for nutrient flux onto and off the shelf, organic carbon production and removal to the Gulf Stream, and have identified key physical and biological processes involved in controlling the carbon cycle. Subsurface intrusions of upwelled water onto the shelf are the most important process affecting summer plankton productivity. In winter, intrusions of Gulf Stream water also occur, but override the colder, denser shelf water. Both types of events result in highly productive but episodic phytoplankton blooms. However, the magnitude and fate of this newly produced particulate organic carbon (POC) is poorly understood.

Three fates potentially consume primary production occurring on ocean margins: portions can be oxidized within the water column, portions can sediment to shelf/slope depots, and portions can be exported to the interior ocean. Zooplankton mediate all three of these processes and thus can alter the pathway and residence time of particulate organic carbon. The fate of newly produced POC depends, to a large extent, on the size structure and composition of the zooplankton and phytoplankton. If protozoans dominate the zooplankton community, then grazing rates and gross growth efficiencies will be high but fecal matter will remain in suspension. Thus protozoopankton grazing would be a moderately large sink for primary production, but considerable suspended POC would follow the path of water into the Atlantic basin.

We contribute by determining the role of microzooplankton in these processes, using two basic components: measuring their growth and ingestion, and coupling those to biomass to estimate community-level impacts. My lab has and continues to specialize in improved methods to determine composition and biomass. Samples are analyzed using an imaging cytometry system which includes a state-of-the-art cooled integrating color CCD detector, mounted on an epifluorescence microscope, and interfaced to a Dual-Pentium II computer. This is interfaced to a motorized stage and associated modular automation controller which permits the operator to scan transects of variable length across the plankton slides while the computer records the fraction of the surface area of the slide which has been examined. The entire process (moving to a given location, focussing, opening an electronic shutter, grabbing an image, closing the shutter, and moving to a new location) can also be automated and computer-controlled. We have used this system at sea on several cruises, in conjunction with vibration-dampening tables.

We determine the role of microzooplankton in these processes, by measuring phytoplankton biomass production and its consumption by microzooplankton. Grazing and growth rates are measured via dilution incubation experiments using chlorophyll a (chl a) as a proxy for phytoplankton (prey) biomass. Chl a production and grazing are determined for the <200um phytoplankton community and also the <8um size class. Primary production is also determined using 14C incubations. Data generally support the notion that, contrary to traditional paradigms about shelf ecosystems, small autotrophs contribute the majority of production, and that this carbon is actively incorporated into the microbial food web via grazing by microzooplankton.