Phytoplankton dynamics in a eutrophying south Texas estuary

Abstract

Low freshwater inflow estuaries are common worldwide, yet data is lacking on phytoplankton dynamics in these systems. This dissertation advances our understanding of phytoplankton dynamics in low-inflow estuaries through analysis of field data collected at multiple temporal scales throughout Baffin Bay, Texas, an estuary that is frequently hypersaline and that has been experiencing symptoms of eutrophication. These symptoms include long-term increases in nutrients and chlorophyll as well as recurring blooms of the “brown tide” phytoplankton, Aureoumbra lagunensis. Variability in phytoplankton biomass and community composition were related to climatic/hydrologic shifts associated with El Niño-Southern Oscillation. During drought conditions, high chlorophyll concentrations were observed (25.7 ± 11.7 µg L-1) and the phytoplankton community consisted of a near monoculture of A. lagunensis (87% of phytoplankton biovolume). These results point to the importance of regenerated nutrients in supporting phytoplankton growth during this time. When El Niño conditions developed, high rainfall took place and corresponded with increased dissolved inorganic nitrogen concentrations (ammonium, nitrate + nitrite) in the bay, but chlorophyll concentrations were lower during this time (16.2 ± 14.3 µg L-1). This suggests that the potential stimulatory effects of nutrient pulses were counter balanced by decreased residence times associated with increased inflow. This study also identified sub-monthly variability as a dominant timescale of chlorophyll variability in Baffin Bay. Wind speed was found to be an important factor related to short-term (timescales of days) bloom events, often positively correlating with chlorophyll concentrations. Results additionally demonstrate that traditional monitoring frequencies are often are not adequate to capture short-term process and bloom events. Lastly, this study quantified the distribution of, and potential controls upon, A. lagunensis in Baffin Bay. Persistently high abundances of A. lagunensis were observed in Laguna Salada, indicating that this tertiary bay may serve as a reservoir for A. lagunensis in the system. High abundances of A. lagunensis (> 1×106 cells mL-1) were found at salinities as low as 20, indicating that hypersalinity is not a prerequisite for A. lagunensis blooms. Instead, results suggest a variety of physical, chemical and biological factors (top-down controls, nutrient concentrations and forms, salinity) drive A. lagunensis population dynamics in the bay. This research improves our ecological understanding of low-inflow estuaries, specifically pertaining to the ecology of phytoplankton in them. Results also suggest that as climate pressures increase, causing systems like Baffin Bay to become warmer and drier, they will be more conducive for harmful taxa such as A. lagunensis. The finding that Laguna Salada may be a reservoir for A. lagunensis presents an opportunity for targeted bloom mitigation efforts.