Variability in Nitrate Uptake Kinetics of Phytoplankton Communities in a Mediterranean Coastal Lagoon

Nitrate uptake kinetics of different phytoplankton communities exhibit great variability over a 1-year period. Classical saturation kinetics accompanied by an induction phase are shown by blooms ofChaetocerossp. in low nitrate waters. Biphasic kinetics, with transition points between 10 and 50 μM, are shown by blooms ofSkeletonema costatumand flagellates, while unsaturated kinetics (up to 100 μM) are shown byThalassiosirablooms and flagellate blooms. The latter also exhibit surge uptake of nitrate in situations of negative growth rates and rather high ammonium levels. The uptake patterns of the three diatom genera are similar to those observed in cultures or consistent with known internal nitrate accumulation characteristics. Based on the kinetic patterns presented, it is apparent that, for nitrate concentrations near 50 μM such as in upwelling areas, nitrate uptake will be underestimated in present models of nitrate assimilation by a factor of almost 2 forS. costatumand a factor of about 3 forThalassiosirasp.