Abstract. Relative to inorganic nitrogen, concentrations of dissolved organic nitrogen (DON) are often high, even in regions believed to be nitrogen-limited. The persistence of these high concentrations led to the view that the DON pool was largely refractory and therefore unimportant to plankton nutrition. Any DON that was utilized was believed to fuel bacterial production. More recent work, however, indicates that fluxes into and out of the DON pool can be large, and that the constancy in concentration is a function of tightly coupled production and consumption processes. Evidence is also accumulating which indicates that phytoplankton, including a number of harmful species, may obtain a substantial part of their nitrogen nutrition from organic compounds. Ongoing research includes ways to discriminate between autotrophic and heterotrophic utilization, as well as a number of mechanisms, such as cell surface enzymes and photochemical decomposition, that could facilitate phytoplankton use of DON components.

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Two different approaches to measuring phytoplankton nitrogen (N) use were compared in late summer 2004 along the main axis of Chesapeake Bay. Uptake of 15 N-labeled ammonium and nitrate and dual-labeled ( 15 N and 13 C) urea and dissolved free amino acids (DFAA) were measured in surface water samples from upper, mid, and lower bay stations. Two distinct methods were used to assess the relative uptake of N substrates by phytoplankton and correct for bacterial artifacts: (1) traditional filtration using Whatman glass fiber (GF/F) filters and (2) flow cytometric (FCM) sorting of chlorophyll-containing cells. The concentration of dissolved inorganic N (DIN) decreased with distance south along the bay, whereas dissolved organic N (DON) concentrations were relatively constant. Absolute N uptake rates measured using the traditional approach exceeded those of FCM-sorted phytoplankton, thereby suggesting the possibility of bacterial "contamination." Ammonium was the dominant N form used throughout the transect, although FCM-sorted phytoplankton relied more on urea and DFAA as the ratio of DON/DIN increased toward the bay mouth. Overall, ammonium comprised 74±17%, urea 10±9%, DFAA 9±7%, and nitrate 7±12% of total measured N uptake by phytoplankton. Results suggest that bacteria relied primarily on DFAA and ammonium for N nutrition but also used N from urea at a rate similar to that of phytoplankton, whereas bacterial nitrate uptake was insignificant. On average, phytoplankton uptake of ammonium, urea, and DFAA was overestimated by 61%, 53%, and 135%, respectively, as a result of bacterial retention on GF/F filters.

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