Phytoplankton Dynamics in Galveston Bay: Assessing Responses to Freshwater Inflows

Abstract

Increased freshwater use in estuarine watersheds is a concern for productivity downstream in ecologically and economically important estuaries worldwide. In Galveston Bay (TX), the seventh largest estuary in the United States, population growth in two large metropolitan areas (Houston and Dallas-Fort Worth), continues to alter the quantity and quality of freshwater inflows (FWI). We report here on the influence of FWI on pelagic and benthic phytoplankton in Galveston Bay in spring and summer over 3 years (2010 to 2012), intended to capture periods of high and low FWI, respectively. A year of severe drought that persisted throughout 2011 allowed us to also examine consequences of prolonged low flows. We followed the response of pelagic phytoplankton (biomass, community composition) to the addition of nutrients using assays, and the response of benthic phytoplankton (biomass, community composition), in addition to corresponding nutrient fluxes and sediment oxygen consumption via core incubation methods.

Log response ratios indicated bay-wide nitrate+ammonium (NA) and nitrate+phosphate (NP) co-limitation of pelagic phytoplankton, in addition to recurrent N or A limitation. Further, nutrient limitation of phytoplankton standing stock was more frequently observed during drought than non-drought years. Diatoms, cyanobacteria, and chlorophytes were dominant in 2010 and 2011, but dinoflagellates became particularly prominent in spring 2012 as FWI alleviated prolonged drought conditions. We also observed resilience of the benthic microalgal (BMA) community to drought, but not in the benthic boundary layer (BBL) phytoplankton community. BMA communities primarily consisted of diatoms throughout, while BBL phytoplankton communities differed with each sampling event. Fluxes differed before and after the drought, and the results here imply that resilience of the water column system is at risk in future drought events, though further study is necessary. We observed that drought itself does not have a significant effect on pelagic or benthic phytoplankton community composition, though timing of the beginning of the drought in relation to annual phytoplankton growth cycles could play a role. Rather, the increase in availability of freshwater inflows following the drought appeared to be more influential on community structure, than the lack of inflows and the resources they bring.