INORGANIC AND ORGANIC NITROGEN CYCLING IN CHESAPEAKE BAY - AUTOTROPHIC VERSUS HETEROTROPHIC PROCESSES AND RELATIONSHIPS TO CARBON FLUX

To define the role of dissolved organic nitrogen (DON) in Chesapeake B ay (USA) nitrogen cycling, we performed experiments in the mesohaline Chesapeake Bay during 3 seasons: May, during the mid to late spring bl oom, August, when patchy summer blooms often develop, and October, aft er the fall overturn. Every 3 h for 30 h we measured ambient nitrogen concentrations and used N-15 tracers to determine uptake and regenerat ion rates of NH4+ and urea, uptake rates of NO3- (NO2- was measured in October only), and rates of DON release due to NH4+ uptake. Autotroph ic production and dissolved organic carbon (DOC) release were determin ed simultaneously using C-14 tracer techniques to compare with nitroge n flux rates. We found that first, the water column nitrogen demand ex ceeded nitrogen regeneration in May but nitrogen regeneration was over 3 times greater than the nitrogen demand in October. Second, mean rat es of autotrophic DON release were highest in May but heterotrophic re lease was more important in summer and fall. Third, during all 3 studi es, the C:N ratio of DOC to DON release rates was 3.4 to 4.5, indicati ng release of nitrogen-rich compounds. Release of dissolved organic ma tter (DOM) with a low C:N ratio was corroborated by an observed increa se in DON concentrations from 7 to 42 mu g-at. N l(-1) from May to Aug ust and a concomitant decrease in the C:N ratio of the ambient DOM poo l from 36.6 to 8.8. These data suggest that Chesapeake Bay is primaril y autotrophic in the spring, but becomes progressively more heterotrop hic later in the year. These data also suggest that DON is an importan t intermediate in this transition and serves as a Link between the all ochthonous nitrogen-based autotrophic production in the spring and the autochthonous nitrogen-based heterotrophic system in the summer and f all.