Constraining alkalinity sources to a secondary bay in South Texas
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Submarine groundwater discharge (SGD) is an important pathway for solute transfer from land to sea. With recent atmospheric carbon dioxide (CO2) increase, global concerns over oceanic and estuarine acidification have ascended. To better understand the natural ability of estuaries to buffer decreasing pH levels and assist in ocean acidification mitigation studies, overlooked sources of total alkalinity (TA) need to be considered. This study investigates TA changes in Nueces Bay (a secondary bay adjacent to Corpus Christi Bay in the Gulf of Mexico), at scales spanning from hours to seasonal, by incorporating submarine groundwater discharge (SGD) rates. TA concentrations within the bay fluctuated from December 2014 to December 2015 (December, 2014: 2527.6 µM; March, 2015: 2341.0 µM; June, 2015: 2765.1 µM; September, 2015: 2869.8 µM; December, 2015: 2638.6 µM). These observed changes in TA among the sampling events were mostly affected by different rates of SGD and biogeochemical reactions, while discharge from Nueces River was only important after significant rain events and flooding (June 2015), increasing the average TA (March, 2015: 2341.0 µM to June, 2015: 2765.1 µM) of the bay. Dry periods (March 2015) indicated consumption of DIC in the form of calcification may occur at some locations. SGD rates are the highest during the dry seasons (March 2015: 9.2 · 10-1 m3/d), thus there is an increased export of TA from groundwater. SGD, also has a lag effect post flooding (September-2015 to December-2015), as discharge at the shoreline increased (north shoreline of the Nueces Bay: 6.5 · 10-1 to 9.0 · 10-1 m3/d; south shoreline: 4.5 · 10-1 to 8.8 · 10-1 m3/d) and elevated SGD-derived TA input. Under unimpaired conditions, Nueces River would likely have a significant role on the bay’s alkalinity given the high groundwater contribution of alkalinity (1.2 · 107 µM/m2/d). This research shows that influences on TA fluctuations in an estuary may be spotty and not necessarily revealed by simple spatial analyses regardless of the sampling resolution. Hourly time series analyses are necessary to understand endogenous processes influencing alkalinity especially in estuaries with limited riverine inflows.
A thesis Submitted in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE in ENVIRONMENTAL SCIENCE from Texas A&M University-Corpus Christi in Corpus Christi, Texas.