Dissertation Defense

Coastal communities rely heavily on seagrasses because meadows support economically valuable fisheries, improve water quality, and protect shorelines from erosion. However, global meadow coverage has significantly declined over decades because of anthropogenic and environmental stressors. Therefore, there is a growing need to study these vital coastal habitats to aid in the preservation and restoration of ecosystem function. The goals of this dissertation were to: 1) establish baselines for evaluating ecosystem change, 2) evaluate methods for assessing ecosystem function, and 3) identify agents of ecosystem change.

Texas serves as a seagrass monitoring model because of decades of concerted efforts between academic and government researchers. Currently, over 800 stations are surveyed semi-regularly to assess changes in seagrass cover, but relatively little information is known on the function of the plants. Therefore, we leveraged ongoing sampling efforts to obtain 1138 sediment cores from across the entire Northern Gulf of Mexico for assessing surficial carbon and nitrogen stocks. These biogeochemically active pools can be remobilized or sequestered depending on the health of the ecosystem. We found the upper 10 cm of sediments contained over 11.5 million Mg C, ten times larger than the carbon released from driving a car to Pluto and back, as well as 770 thousand Mg N, twice the amount of N that enters Texas waters from rivers each year. Roughly half of the organic carbon in sediments was created by seagrasses, reflecting their high primary productivity in the region. Multiple methods exist for measuring productivity, so we tested a variety of techniques to determine their applicability to seagrasses. Baseline measurements using a traditional single-logger method suggested that the seagrass ecosystem was releasing more carbon dioxide than was fixed in photosynthesis because of high air-sea exchange rates, contrary to their known ecological function. Aquatic eddy covariance provided additional insight into processes by directly measuring benthic oxygen flux, improving our oxygen budget estimates. However, these techniques were unable to measure oxygen bubble formation, a common occurrence in Texas seagrass meadows. Acoustic measurements of this ebullition revealed it significantly contributed to productivity budgets, nearly double the traditional method. Infrared gas analysis was able to provide deeper insight into carbon fixation during desiccation, an understudied realm of seagrass biology. Finally, we synthesized over three decades of seagrass monitoring data from the Upper Laguna Madre to understand contemporary threats to seagrass productivity. We found that rapid rates of sea level rise caused seagrasses to retreat from our long-term monitoring site because of insufficient light. These findings were confirmed over the entirety of the region by utilizing data from an additional 144 locations. We predict that sea level rise will shift the distribution of seagrasses to newly flooded areas in the southern Laguna and may serve as a net benefit if all available land is colonized.