Wet and Dry Climate Regimes Impact Particulate Organic Matter Quality in a Low-Inflow Subtropical Estuary

Low-inflow estuaries on the arid, subtropical Texas coast are often subject to oscillations between dry and wet climate cycles, interspersed with stochastic weather events such as prolonged drought, storms, or hurricanes. Sporadic river inflow influences the sources and composition of estuarine particulate organic matter (POM). Shifting proportions of “high quality” POM, consisting of fresh, reactive, labile material, relative to “low quality,” recalcitrant, less available fraction, may lead to ecosystem-wide changes. Between 2012 and 2020, the Mission-Aransas Estuary (MAE) experienced several significant climatic events: severe drought, flooding, and category 4 Hurricane Harvey, ultimately transitioning from a dry to wet climate regime. To assess changes in POM quantity and quality in response to these events, we quantified particulate organic carbon (POC), nitrogen (PN), natural stable isotope abundance (δ13C, δ15N), chlorophyll a, pheophytin, and pheophorbide, and total hydrolyzable amino acids (THAAs) from an 8-year (2012 to 2020) timeseries. Using a multivariate statistical approach, we constructed a degradation index to demonstrate that POM was more degraded during drought and less degraded during the wet period. Average POC, PN, and THAA concentrations increased 66%, 88%, and 88% during the wet period summers and with the degradation index demonstrate elevated proportions of high-quality, bioavailable POM seasonally, and over a climate regime shift. Low-inflow estuaries are useful examples of climate change impacts on systems increasingly stressed by freshwater inflow reduction. The quality shift of POM may play an important role in determining processing rate by bacteria or higher trophic levels, thus affecting ecosystem interactions and functions.