Extreme climate events influence particulate organic matter quality, quantity, and composition in the Mission-Aransas Estuary

Estuaries are sites where rivers meet the ocean, marked by dynamic physical, chemical, and biological variability, making them particularly vulnerable to the effects of global climate change and shifting freshwater inflow regimes. Arid subtropical climates oscillate between drought and wet conditions, leading to a “flood or famine” paradigm for estuarine freshwater inflow, in which sporadic storm events drive dynamic changes in salinity and nutrient availability. In turn, these physical inputs can alter particulate organic matter (POM) sources, composition, and quality, with cascading consequences for trophic webs and overall carbon production and storage. Over the past decade, the Mission-Aransas Estuary (MAE), located on the south Texas coast, experienced several significant climatic events: severe drought, sequential flooding events, and the direct impact of category 4 Hurricane Harvey in 2017. Multiple biomarkers from an eight-year twice monthly sample set (2012-2019) were used to assess POM quantity, quality, and composition. POM was quantified using elemental analysis of organic carbon and nitrogen. POM sources, including phytoplankton community composition, were determined using pigments, stable isotopes (δ¹³C, δ¹⁵N), and neutral and polar fatty acids. POM quality was assessed using total hydrolyzable amino acids (THAAs), phospholipid linked fatty acids (PLFAs), and chloropigments (chlorophyll a, pheophorbide, and pheophytin). This research shows the transition from drought to wet conditions was more influential on POM source, composition, quantity, and quality than any individual storm event, including Hurricane Harvey. Drought conditions dampened phytoplankton biomass growth and elevated POM degradation. Following sequential flooding events, phytoplankton biomass and highly labile biomolecules (THAAs and polyunsaturated PLFAs) increased, improving overall POM quality. The phytoplankton community, dominated by diatoms during the drought, switched to a stable cyanobacteria-dominated community following estuarine freshening, driven by salinity and nutrient availability with seasonal growth oscillations between taxa. The drought-to-wet transition event represented a climate-driven regime shift of the estuarine phytoplankton community and POM quality to a new steady state. Characterizing POM sources and composition at high sampling resolution across timescales long enough to capture extreme climatic events is critical to understanding the weathering capacity of estuarine systems, the importance of antecedent conditions on disturbance events, and the potential outcomes for primary producers, fisheries, and organic carbon storage in a climatically uncertain future.