AMMONIUM UPTAKE BY HETEROTROPHIC BACTERIA IN THE DELAWARE ESTUARY AND ADJACENT COASTAL WATERS

Uptake of NH4+ by heterotrophic bacteria and the relative importance of NH4+ and dissolved free amino acids (DFAA) as nitrogen sources for bacterial production were examined in the Delaware estuary and adjacent coastal waters during 1988 and 1990. Although total uptake of NH4+ and bacterial production were similar to 4-fold higher in 1988 than in 1990, percent NH4+ uptake by bacteria in the upper and lower estuary was similar for both years, Bacterial uptake rates were highest at the mouth of the bay in summer and represented 10-25% of total uptake of NH4+. Less than 5% of NH4+ uptake was by bacteria at salinities <20 parts per thousand. In contrast to NH4+ uptake, DFAA uptake was greatest in the upper estuary and often exceeded nitrogen requirements for bacterial growth. Bacteria accounted for 15-35% of total NH4+ uptake at coastal and offshore stations in 1990. About 50% of bacterial nitrogen demand was supported by NH4+ at the mouth of the bay and in coastal waters during summer, when DFAA concentrations were generally lowest. Although DFAA concentration and uptake did not explain all variability, they appeared to explain large-scale features in NH4+ uptake by heterotrophic bacteria. Ammonium uptake by bacteria was lowest in the estuary, where DFAA concentrations and uptake were highest; at an offshore station, where DFAA concentrations and uptake were low, relative NH4+ uptake by bacteria was highest. These and other results suggest that NH4+ uptake by bacteria is relatively high in oligotrophic water and low in eutrophic systems, which has important implications on the role of heterotrophic bacteria in the N cycling of marine environments.