Speciation and concentrations of dissolved nitrogen as determinants of brown tide Aureococcus anophagefferens bloom initiation

Growth responses of the brown tide organism Aureococcus anophagefferens and 4 cooccurring microalgal species to varying concentrations of nitrate, ammonium, urea, and glutamate were assayed in laboratory experiments. Analogous to seasonal shifts in nutrient regimes in local bays, growth functional responses were used to predict relative species abundances in simulated communities supported by a reduced nitrogen source after cultivation on NO ₃^-. Simulations were based on Monod kinetic parameters, lagged growth responses and threshold nutrient concentrations. Presented with NO₃^- only, rank order of biomass production was Thalassiosira pseudonana > Nannochloris atomus > Synechococcus bacillaris > Prorocentrum minimum > A. anophagefferens at all concentrations. In contrast, model communities offered NH₄⁺ or glutamate at most environmental concentrations (1 to 50 μM N) tended to be dominated by A. anophagefferens and P. minimum. Over environmental ranges of urea concentrations (1 to 50 μM N), T. pseudonana grew fastest, followed by A. anophagefferens. In field validation experiments, bay water with bloom concentrations of A. anophagefferens (10⁵ cells ml^-1) amended with equimolar (total N) nitrate-rich groundwater or reduced N-rich sediment porewater supported equal increases in total chlorophyll a through time. However, porewater selectively stimulated A. anophagefferens growth more than groundwater, and the converse was observed for cyanobacteria. Extrapolation of all experimental results to conditions in Long Island embayments suggests that phytoplankton communities supplied primarily with NO₃ ^- will be dominated by diatoms, such as T. pseudonana, and perhaps by chlorophytes and cyanobacteria like N. atomus and S. bacillaris. Conversely, phytoplankton communities primarily supplied with low-to-moderate concentrations of reduced N-species, NH₄⁺, and dissolved organic nitrogen (DON) will be dominated by A. anophagefferens and, to a lesser extent, dinoflagellates like P. minimum. Our results are consistent with published field observations of A. anophagefferens bloom dynamics in Long Island estuaries.