River transport of dissolved organic matter (DOM) provides a crucial energy subsidy to coastal ecosystems through remineralization processes. Understanding both the quantity and the molecular characteristics of riverine DOM is key to understanding the potential for coastal DOM remineralization. In South Texas, there is extreme interannual variability in hydrologic controls on freshwater inputs from rivers to coastal embayments, and thus total DOM transport. Utilizing three coastal rivers from three distinct watersheds, the Mission, Aransas, and Nueces Rivers, we sought to decipher variability in both quantity and quality of the South Texas riverine DOM matrix with a unique thermal slicing pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) technique. River water, collected in June of 2017 and January of 2018, was pre-filtered before DOM isolation by solid phase extraction (SPE). SPE-DOM was pyrolyzed at five thermal slices before chromatographic separation then molecule identification by mass spectrometry. The thermal stability of each river’s DOM matrix was similar, as ~90% of all pyrolyzates were released between 190 and 530°C. Of the pyrolyzates detected, there were 14 significant compound classes, and their proportions varied tremendously between rivers. However, methanol, acetic, benzenes, and CO2, likely sourced from polysaccharides, lignin-like materials, and carboxyl-rich alicyclic molecules, were predominant in all rivers.  Results of total hydrolysable amino acids (THAA) analysis also showed consistent amino acid composition in these DOM samples, dominated by aspartic acid, glycine, beta-alanine, and alanine. Analysis for δ13C, δ15N, C:N, and %-OC are ongoing, but the preliminary results show that the DOM isolated by SPE is compositionally similar among the three rivers, and no seasonal trend was observed between diagenetic state and molecular diversity. Despite different sources and hydrologic conditions driving differences between proportions of compound classes, DOM characteristics between these rivers showed little variability

Baffin Bay is an economically and ecologically-important estuary on the South Texas coast. In the past few decades however, it has begun to exhibit symptoms of eutrophication including high and increasing chlorophyll concentrations, episodic hypoxia/anoxia, recurring blooms of the harmful brown tide alga Aureoumbra lagunensis, and fish kills. Unfortunately, our understanding of the causes of the water quality degradation has been limited due to lack of data.  Results from a 5-year (ongoing) water quality study as well as analysis of historical TCEQ data is now providing answers.  Results show that dissolved organic nitrogen and total Kjeldahl nitrogen concentrations in Baffin Bay are 2-5 fold higher than in other Texas estuaries.  In contrast, inorganic nitrogen (ammonium, nitrate) and phosphate concentrations are relatively low except following episodic rain events.  The dominance of organic nutrients compared to inorganic nutrients, prolonged hypersaline conditions during drought, and a long-term increase in spring-summer water temperature create conditions favorable for “brown tide” persistence in this system.  Given the long residence time (> 1 year) of Baffin Bay, the system is likely sensitive to nutrient inputs and thus reductions in loadings are be needed to prevent further water quality degradation.

Estuaries are critical habitats for numerous bird, fish, and shellfish species, and they play a critical role in the economy of coastal communities. Despite this, eutrophication, with accompanying water quality and habitat degradation, is becoming pervasive in many estuarine systems. Here we report findings from a study of long-term changes in water quality in 40 bays along the Texas coast. Seasonal increases in water temperature were observed for most systems, as were annual increases in salinity. Indicators of eutrophication were found throughout Texas coastal waters, from small systems (e.g., Oso Bay and Carancahua Bay) to larger estuaries (e.g., Galveston Bay). One heavily impacted area is the Upper Laguna Madre, which includes Baffin Bay. Baffin Bay displayed multiple symptoms of eutrophication including high organic carbon, organic nitrogen, and chlorophyll concentrations. In addition, statistically significant long-term annual increases in chlorophyll a and salinity were observed, while long-term seasonal increases were observed for water temperature. Much of the increase in chlorophyll coincided with blooms of the mixotrophic “brown tide” phytoplankton species, Aureoumbra lagunensis, which is thought to be favored under high proportions of organic to inorganic nitrogen. One notable exception to the widespread eutrophic conditions found throughout coastal Texas was the Nueces-Corpus Christi Bay system, which conversely displayed symptoms of oligotrophication. Annual and seasonal decreases in phytoplankton biomass, including during the winter-spring bloom period, are occurring. These varied shifts in Texas water quality, combined with land use changes and population growth, can have serious implications for the benthos and fisheries of Texas. Identified systems of poor water quality can be used to drive future higher resolution studies to better understand potential drivers of changes in estuarine health.