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 of Chaetoceros sp. in low nitrate waters. Biphasic kinetics, with transition points b etween 10 and 50 mu M, are shown by blooms of Skeletonema costatum and flagellates, while unsaturated kinetics (up to 100 mu M) are shown by Thalassiosira blooms and flagellate blooms. The latter also exhibit s urge uptake of nitrate in situations of negative growth rates and rath er high ammonium levels. The uptake patterns of the three diatom gener a are similar to those observed in cultures or consistent with known i nternal nitrate accumulation characteristics. Based on the kinetic pat terns presented, it is apparent that, for nitrate concentrations near 50 mu M such as in upwelling areas, nitrate uptake will be underestima ted in present models of nitrate assimilation by a factor of almost 2 for S. costatum and a factor of about 3 for Thalassiosira sp. (C) 1997 Academic Press Limited.