Hydrological and biogeochemical controls on estuarine carbonate chemistry along a climate gradient

A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Coastal and Marine Systems Science.Increasing global atmospheric CO2 concentrations drive a net flux of CO2 into the oceans, mitigating the impacts of anthropogenic greenhouse gas emissions on the climate. This results in a reduction in pH and carbonate saturation state, a.k.a. ocean acidification, of marine waters. The acidified ocean water may advect into estuaries, leading to estuarine acidification. Many estuaries are highly sensitive to this acidification due to low buffer capacity. Because estuaries provide many important ecosystem services, alterations in their carbonate systems may have significant consequences on ecosystems and the economy. Despite the current understanding that estuaries may play a disproportionately important role in global air-sea CO2 flux, little is known about carbonate systems in subtropical estuaries. Further comprehension of estuarine carbonate systems is vital for quantification of the global carbon cycle. Specifically, subtropical estuaries in the northwestern Gulf of Mexico (nwGOM) exhibit a general long-term decrease in pH and total alkalinity (TA), with lower latitudes experiencing more extreme acidification than higher latitudes. In Chapter II, sediment cores and slurries from the semiarid Mission-Aransas Estuary of the nwGOM were incubated and surface waters were analyzed for contributions of biogeochemical processes to TA change. Changes in total TA as well as calcium and sulfate ion concentration were examined following known reaction stoichiometry. Ratio of TA: ion changes suggested that carbonate dissolution co-occurred with oxidation of reduced sulfur species, and the latter consumed TA during drought periods in Mission-Aransas Estuary. This biogeochemical (sulfide oxidation) TA consumption has been poorly studied yet may affect TA budget in other semiarid estuaries worldwide. In Chapter III, river alkalinity total load and concentration were calculated using the United States Geological Survey’s Fortran Load Estimator Program (LOADEST) and long-term trends in alkalinity and discharge of six major nwGOM rivers were determined. Stepwise multiple linear regression methods were used to generate models for predicting estuarine TA based on river alkalinity, year, and net evaporation (evaporation-precipitation). Some rivers were found to have long-term (multidecadal) declines in freshwater discharge, area-weighted alkalinity yield, of alkalinity flow-weighted concentration, with most declines occurring in the southern end of the study region. Freshwater flow-weighted alkalinity concentration (annual alkalinity load for an area divided by discharge) appeared in many of the predictive models for estuarine TA and may play a major role in regulating estuarine TA of the nwGOM. Methods for linking freshwater and estuarine carbonate dynamics are lacking in the scientific literature; this study provides a potentially useful approach for predicting estuarine carbonate chemistry based on freshwater quality and input. In Chapter IV, CO2 flux of the Trinity-San Jacinto Estuary (Galveston Bay) was calculated and compared to results from discrete samples for carbonate parameters. Inferences about spatial and temporal patterns in CO2 flux as well as ecosystem metabolism were made based on results. The Trinity-San Jacinto Estuary was found to be a net sink for atmospheric CO2, but with high seasonal and spatial variability. Specifically, large freshwater inflows in spring stimulated photosynthesis in the estuary, which increased the sink behavior. Seasons with less freshwater inflow resulted in higher heterotrophy and CO2 emission in some regions of the estuary. This research increases knowledge and research capacity in the nwGOM region on estuarine acidification and carbonate chemistry. Causes of acidification in major estuaries within the region were addressed along a latitudinal climatic gradient. This will aid with better management of fresh and estuarine water resources in the nwGOM. The results of this research will also clarify the role of semiarid, subtropical estuaries in the global carbon cycle and expand our range of knowledge on carbonate system analyses of estuaries.Physical and Environmental SciencesCollege of Scienc