Inorganic and organic nitrogen cycling in Chesapeake Bay: autotrophic versus heterotrophic processes and relationships to carbon flux

Abstract

To define the role of dissolved organic nitrogen (DON) in Chesapeake Bay (USA) nitrogen cycling, we performed experiments in the mesohaline Chesapeake Bay dunng 3 seasons: May, during the mid to late spring bloom, August, when patchy summer blooms often develop, and October, after the fall overturn. Every 3 h for 30 h we measured ambient nitrogen concentrations and used tracers to determine uptake and regeneration rates of NH,' and urea, uptake rates of NO,(NO; was measured in October only), and rates of DON release due to NH,' uptake. Autotrophic production and dissolved organic carbon (DOC) release were determined sunultaneously using 'v tracer techniques to compare with nitrogen flux rates We found that first, the water column nitrogen demand exceeded nitrogen regeneration in May but n~ t rogen regeneration was over 3 times greater than the nitrogen demand in October. Second, mean rates of autotrophic DON release were highest in May but heterotrophic release was more important in summer and fall. Third, dunng all 3 studies, the C N ratio of DOC to DON release rates was 3.4 to 4.5, indicating release of nitrogen-rich compounds. Release of dissolved organic matter (DOM) \nth a low C:N ratio was corroborated by an observed increase in DON concentrations from 7 to 4 2 pg-at. N I-' from May to August and a concomitant decrease in the C . N ratio of the ambient DOM pool from 36.6 to 8.8. These data suggest that Chesapeake Bay IS primarily autotrophic In the spring, but becomes progressively more heterotrophic later in the year. These data also suggest that DON is an Important intermediate in this transition and serves as a link between the allochthonous nitrogen-based autotrophic production In the spring and the autochthonous nitrogen-based heterotrophic system in the summer and fall.