Influence of Drought, Periodic Storm Events, and Hurricane Harvey on the Phytoplankton Community in a South Texas Estuary

Estuaries in subtropical south Texas, including the Mission-Aransas Estuary (MAE) are characterized by low base freshwater inflow interspersed with sporadic major storm and inflow events, driving dynamic salinity, nutrient availability, and particulate organic carbon (POC) concentration. Coupled with high water residence times and limited exchange with the Gulf of Mexico, these estuaries can exhibit dynamic shifts in productivity and phytoplankton community structure. From 2012-2019, the MAE transitioned from exceptional drought to wet conditions, punctuated by several major storm events. Previous work has demonstrated that phytoplankton community composition changed dramatically following a storm event during drought conditions, but the 2017 direct impact of category 4 Hurricane Harvey on the MAE’s productivity and phytoplankton community structure was unknown. We analyzed suspended particulates from four sites across the MAE taken biweekly from 2012-2019, using High Performance Liquid Chromatography (HPLC) pigment analysis to measure chlorophyll a concentration and characterize major phytoplankton taxa based on accessory pigments. We found a distinct shift between the period of drought (2012-mid 2015) and the wet period (mid 2015-2019), characterized by a decrease in bay salinity (35-40psu to 10-20psu), increase in POC and chlorophyll a concentration, and a shift from a diatom-dominated phytoplankton community to a cyanobacteria-dominated community. Enhanced in-situ production within the estuary during the sustained wet period is likely driven by riverine-delivered nutrients. Inflow events under wet conditions, including Hurricane Harvey, did not significantly impact phytoplankton community composition compared to the 2015 inflow event under dry conditions. The “flood or famine” paradigm of the MAE during wet/dry extremes causes dynamic responses in estuarine particulate organic matter composition, although antecedent conditions predicate the acute impacts of individual storm events. The resilience of the phytoplankton community post-hurricane may be an indication of overall system resilience to major storm disturbances during wet antecedent conditions.