Organic nitrogen uptake and growth by the chrysophyte Aureococcus anophagefferens during a brown tide event

The quantitative importance of light-mediated, dissolved organic nitrogen (DON) utilization in relation to overall nitrogen-assimilation in Aureococcus anophagefferens 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 substantially greater than uptake of the other substrates tested during the study, contributing the largest percentage of total nitrogen uptake (58 to 64%; rho'(max(urea)) 4.4 mu g atom N l(-1) h(-1)), followed by NH4+ (18 to 26%; rho'(max(NH4+))) 2 mu g 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 NO3- 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.