| The potential linkage between harmful algal blooms (HABs) and the growing eutrophication of coastal waters has often been suggested. This dissertation research focused on several bloom forming estuarine phytoplankton species, with emphasis on Prorocentrum minimum, and examined their nutritional preference, the physiological responses of nitrogen uptake for different nitrogen components under different light conditions, and the enzyme regulation that reflects the patterns of utilization.; Four nitrogenous substrates (NO3-, NH 4+, urea, and a mixed amino acids substrate) were used to determine uptake rate and substrate preference. The results indicated a preference for NH4+ by P. minimum assemblages. NH4+ was the primary nitrogen source supporting the blooms, but temperature strongly affected the affinity for different nitrogen substrates. Experiments were also carried out to characterize carbon and nitrogen uptake of P. minimum as a function of irradiance. Short-term nitrogen uptake rates (half hour) were not substantially different over the irradiance range measured for each nitrogenous substrate for the P. minimum-dominated bloom. Over the duration of the P. minimum bloom, however, ambient nitrogen uptake rates showed a strong relationship to irradiance. Uptake of 13C bi-carbonate showed typical light-dependent photosynthetic characteristics for the P. minimum bloom, and the measured photosynthetic parameters suggested that at least on the short time scale of mid-day measurements, P. minimum cells were adapted to high light during the bloom.; Urease activity was detected, regardless of the growth nitrogen source in three bloom forming phytoplankton species: Aureococcus anophagefferens , Prorocentrum minimum, and Thalassiosira weissflogii. However, physiological regulation of urease within these phytoplankton species varied. For A. anophagefferens and for P. minimum, a positive relationship between the cell growth rate and urease activity was found, and urease activity exceeded or matched the nitrogen demand for growth. However, for T. weissflogii , the urease activity did not vary significantly with growth rate, its urease activity was always less than the growth nitrogen demand. These variations on urease activity may play a role in their ecological success in varying environments.; These data have important implications on the role of nutrient input in regulations of dinoflagellate blooms. NH4+ and other reduced nitrogen substrates (urea, dissolved free amino acid) could be important in the development of HABs. |
