| Galveston Bay is a large, sub-tropical, shallow estuary. The annual nitrogen loading to the estuary is high, yet phytoplankton biomass is moderate to low with infrequent large blooms. In many estuaries, phytoplankton growth is mainly limited by the bottom-up factors of nutrients, light and/or temperature. Microalgal species compete for these resources and small phytoplankton are usually considered better competitors in resource-limited environments. Therefore, the primary objective of my research was to characterize the mechanistic processes that regulate the spatial and annual variability of phytoplankton community structure in Galveston Bay.; Nanophytoplankton dominated the phytoplankton biomass and diatoms, cryptophytes, chlorophytes and cyanobacteria were the most abundant algal groups. Biomass and relative abundance of the most abundant phytoplankton groups showed weak correlations with environmental parameters. Picoeukaryotes, picocyanobacteria, and bacteria all demonstrated a winter minimum and a summer or late fall maximum. The HPLC-CHEMTAX approach provided a useful tool for resolving the spatio-temporal distribution of phytoplankton in the presence of a Karenia brevis bloom in the fall 2000.; Nutrient addition bioassays were used to assess short-term (1–2 day) phytoplankton responses to limiting nutrients. Phytoplankton biomass increased in the nitrate (10 μM) additions in 11 of the 13 bioassays conducted over the 3-year study, but no significant increases were detected in the phosphate (3 μM) only additions. Bioassay results suggest that the phytoplankton community was usually not phosphate limited. All major groups increased in biomass following nitrate additions but diatoms increased in biomass at a faster rate than other groups, shifting the community composition toward higher relative abundance of diatoms.; Phytoplankton demonstrated a rapid (24h) growth rate when exposed to increased concentrations of limiting nutrients independent of the surface irradiances used in the incubations (12%, 30%, 41% of surface irradiance). Growth rates were not significantly different between incubations at light levels of 12–41% of surface irradiance. Observed and estimated phytoplankton biomass changes during incubations differed, emphasizing the possible structural role of grazers on the phytoplankton community. Rapid phytoplankton community growth rate responses and coupled growth and grazing, suggest that the diverse phytoplankton community in Galveston Bay is maintained through frequent nutrient pulses. |
