The quantitative importance of light-mediated, dissolved organic nitrogen (DON) utilization in relation to overall nitrogen-assimilation in Aureococcusanophagefferens Hargraves et Sieburth was assessed during a brown tide event in Shinnecock Bay, Long Island, 24 through 26 July 1995. The growth response of A . anophagefferens was maximal in organic-rich Bay water and decreased proportional to the organic:inorganic nutrient ratio of the water. Short-term uptake measurements with six nitrogenous substrates revealed that reduced nitrogen could potentially represent 95% of overall nitrogen uptake of which 70% was due to organic nitrogen alone. Potential uptake of urea by the A . anophagefferens -dominated bloom was substan tially greater than uptake of the other substrates tested during the study, contributing the largest percentage of total nitrogen uptake (58 to 64%; ρ ′ max(urea) 4.4 μg atom N l −1 h −1 ), followed by NH 4 + (18 to 26%; ρ′ max(NH4+) 2 μg atom N l −1 h −1 ). The combined rates of uptake of algal extract, lysine and glutamic acid contributed between 11 and 16% of total uptake, whereas NO 3 − contributed 5 to 8%. Based on the kinetic determinations from this study we suggest an ecological framework for the events leading to the dominance and abundance of A . anophagefferens in coastal bays.
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