Nitrogen and carbon uptake kinetics and the influence of irradiance for a red tide bloom off southern California

The kinetics of nitrogen (nitrate, ammonium, urea) and carbon uptake by a red tide bloom consisting almost exclusively of the dinoflagellate Lingulodinium polyedrum (Stein) Dodge were determined with N-15- and C-13-tracer techniques, as a function of substrate concentration (for nitrogen) and irradiance (for both carbon and nitrogen). Samples were collected from Newport Beach, California, in late March 1995, during a massive red tide bloom which occurred off the California coast. At the collection site, surface concentrations of L. polyedrum reached 1.1 x 10(6) cells l(-1), with chlorophyll a = 125 mu g l(-1)'. Maximal uptake rates of urea-N were approximately twice the maximal rates for either ammonium or nitrate during both the uptake versus substrate and uptake versus irradiance experiments, and the affinity for nitrate was much greater than previously demonstrated: half-saturation constant (K-s) = 0.47 mu g-at N l(-1). Carbon and nitrogen uptake rates as a function of irradiance were well described by a 3-parameter P versus E relationship (photosynthesis vs irradiance) proposed by Platt & Gallegos (1980), although dark-uptake of nitrogen compounds accounted for ca 50% of V-max. These results demonstrate that L. polyedrum is capable of utilizing a broad range of both nitrogen concentrations and light fluences, and that urea could potentially provide a large percentage of the nitrogen demand at ambient urea concentrations and across the entire spectrum of light fluences. These data represent a more complete quantification of the N uptake dynamics of this bloom-forming species and contrast markedly compared to previous studies of L. polyedrum.