Plankton dynamics on the outer southeastern U.S. continental shelf. Part III: A coupled physical-biological model

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Abstract
Gulf Stream frontal eddies and bottom intrusions provide an almost continuous input of nutrients to the waters of the outer southeastern U.S. continental shelf. However, the two types of upwelling differ in physical dynamics, frequency of occurrence and duration. To investigate the effect of these differences on biological production associated with the two upwellings, a ten-component biological model was coupled to circulation and temperature fields constructed from an optimal interpolation of current meter data obtained from moorings deployed on the mid- to outer southeastern U.S. continental shelf as part of GABEX I (February to June, 1980) and GABEX II (June to September, 1981). Model output gives the spatial and temporal distributions of nitrate, ammonium, phytoplankton, and a copepod lin an alongshore plane located at a nominal depth of 37 m. These distributions were used to investigate the flux of properties to and from the outer southeastern U.S. shelf and the role of upwelling features in the formation of large-scale plankton patches on the outer southeastern U.S. continental shelf. The simulated distributions show basic differences in the biological response to frontal eddy and bottom intrusion upwelling. For frontal eddies most of the upwelled nitrate is advected northward along the outer shelf, with only occasional transport of the upwelled nitrate across the shelf to the 40-m isobath. The primary production resulting from this upwelling is also transported northward. The net transport of primary production at the 75-m isobath is offshore. Copepod densities associated with the frontal eddies are low. Bottom intrusions have time scales that are longer than those associated with the frontal eddies and bottom intrusions tend to move onshore rather than remaining only on the outer shelf. Consequently, these events result in a net onshore flux of nitrate and phytoplankton at the 75-m isobath over the region included in the model. The longer lifetime of these events results in the development of copepod blooms that are characterized by a biomass that is approximately twice that associated with frontal eddies. A large portion of the primary production resulting from bottom intrusions is consumed by the shelf zooplankton community. Also, much of the primary and secondary production associated with bottom intrusions is transported across the shelf to onshore areas. These results imply a strong seasonal signal in the carbon and nitrogen fluxes on the outer southeastern U.S. continental shelf.