Summer nutrient dynamics of the Middle Atlantic Bight: nitrogen uptake and regeneration

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Abstract
Nitrate and ammonium uptake and ammonium regeneration rates (by zooplankton, microplankton and benthos) were measured on the Atlantic continental shelf (Middle Atlantic Bight) during summer, 1980. Euphotic zone profiles of NO3− and NH4+ uptake rates were similar in magnitude and vertical structure over a large geographical area. Microplankton NH4+ regeneration rates, although measured less frequently, also showed a relatively consistent vertical structure; rates were positively correlated with uptake rates. Nitrate assimilation (‘new’ production) was used to estimate vertical eddy diffusivity and paniculate sinking rates. Eddy diffusion estimates ranged from 2.0 cm2 s−1 and were positively related to areal primary production. Estimated particulate sinking rates averaged 5 mg at Nm−2d−1 and compared favorably with sedimentation rates measured from free-floating and moored sediment traps. Benthic nitrogen regeneration rates represented 4+ assimilation (‘regenerated’ production) represented 50–80% of the total (NO3− + NH4+ ) nitrogen productivity and 33% for the euphotic zone. Of this, 30% was attributed to zooplankton, 63% to microplankton (4+ regeneration was associated with organisms passing 1 μm filters.

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