Dependent coupling of inorganic and organic nitrogen uptake and regeneration in the plume of the Chesapeake Bay estuary and its regulation by large heterotrophs

Abstract

Nitrogen uptake and regeneration in the plume of the Chesapeake Bay estuary was studied during a series of four cruises (1985–1986). During each season we followed the short‐term patterns in inorganic (NH4+, NO3‒, NO2‒), and organic (urea and dissolved free amino acids—DFAA) nitrogen uptake and regeneration as plume water aged and became incorporated into ambient coastal water over 1–3 d. There was little influence of seasonally changing temperature on inorganic nitrogen uptake rates: the highest specific and absolute hourly rates of uptake occurred in April, when the availability of total N was at a seasonal high; rates were somewhat lower in early and late summer and were lowest in February. NH4+ regeneration rates, on the other hand, were highly correlated with temperature, with the highest rates occurring in late summer. Rates of DFAA uptake were highest in June. Urea contributed up to 70–80% of the total N utilized during winter and summer; in spring most nitrogen uptake was in the forms of NO3 and NH4+. The seasonal differences in the patterns of release of dissolved organic N, and the role of zooplankton in mediating this flux, paralleled the patterns previously reported for the release of dissolved organic C in the Chesapeake Bay plume. That is, in winter most organic N release was not grazer mediated, but instead was more likely due to nutrient‐deficient phytoplankton. In contrast, in late summer, there seemed to be more dissolved organic N release by grazers, either directly or from “sloppy feeding.” At all seasons, in the transition from the bay environment to coastal waters, autotrophic populations appeared to become nutrient deficient, and a system dominated by heterotrophic processes rapidly became established.