Primary production on the continental shelf off Cape Hatteras, North Carolina

Distributions of primary production and chlorophyll size structure were examined in relationship to hydrographic properties during March and July 1996 in continental shelf waters off Cape Hatteras, North Carolina. Chlorophyll concentrations ranged highest (>10 mg m−3) during March. Peak values were associated with subsurface maxima near the shelf edge, which were dominated by larger (>8 μm) phytoplankton. In July, chlorophyll concentrations were generally lower with higher proportions associated with the <8 μm size fraction. Primary production was determined using a wavelength-resolved photosynthesis–irradiance model. Model performance was evaluated by comparison of modeled and measured estimates of optical properties and simulated in situ primary production. Regional patterns of primary production determined using the model revealed high water-column-integrated primary production during March (>2.0 g C m−2 d−1) in association with the shelf–slope front. In contrast, highest values in July (>3.0 g C m−2 d−1) were associated with an intrusion of high-salinity, high-nutrient water into the southern portion of the study region. Average shelf-wide productivities were 1.3 g C m−2 d−1 in March and 2.1 g C m−2 d−1 in July. Contemporaneous Advanced Very High-Resolution Radiometer sea-surface temperature imagery revealed that regions of high productivity were proximal to Gulf Stream circulation features. In March, cross-shelf sections of hydrographic properties and nutrients provided evidence for onshore displacement of the shelf–slope front, apparently related to the combined effects of the strong winds prior to the cruise and the offshore presence of energetic Gulf Stream processes. We speculate that variable forcing, related to the dynamics of the Gulf Stream and wind, resulted in significant frontal displacement and frontal velocity fluctuations leading to enhanced mixing and injection of nutrients across the frontal boundary. In July, Gulf Stream circulation, in conjunction with upwelling favorable conditions, resulted in a subsurface intrusion of high-nutrient water onto the shelf analogous to similar phenomena observed in the South Atlantic Bight. Localized regions of enhanced productivity appear to be quantitatively important in the overall carbon/nitrogen budget of the southern Mid-Atlantic Bight shelf.