Semester of Graduation

Fall 2023

Degree

Master of Science (MS)

Department

Renewable Natural Resources

Document Type

Thesis

Abstract

Living shorelines present one restoration approach using biotic elements (via ecosystem engineers) to stabilize shorelines and increase valuable habitat. Gulf ribbed mussels (Geukensia granosissima) are important ecosystem engineers within brackish and salt marshes in the northern Gulf of Mexico, and can increase nutrient availability, enhance marsh vegetation production, and potentially improve shoreline stability. It is unknown whether mussels will maintain these functions as future climate change predicts increases in temperature and water level, which may exceed the mussel’s tolerance. A field survey conducted during the summer of 2022 identified the daily maximum temperature at the marsh surface, where mussels naturally occur, was 2 °C higher than adjacent water daily maximum temperatures and experienced twice the variation and range. In contrast, marsh sub-surface temperatures tracked closely with water temperatures. Two lab experiments explored the acclimated and acute thermal tolerance of mussels to recorded maximum field temperatures (28-42 °C). In the acclimated experiment, 100% cumulative mortality was reached after 4 days at 40 °C, and 50% mortality after 34 days at 36 °C. Lower treatments (28 and 32 °C) remained below 50% mortality throughout the experiment. Cumulative mortality with acute exposure reached 100% after 12 hours at 42 °C and after 24 hours at 40 °C. Mussels exposed to 38 °C reached 50% mortality after 5.5 days. While the range of maximum marsh temperatures is nearing the mussels’ thermal limits when continually exposed, mussels have behavioral and physiological adaptive strategies that may enable them mitigate thermal stress, potentially extending their thermal tolerance. A field experiment using marsh organs was designed with descending levels simulating modest RSLR rates (~5mm yr-1) for the next 30 years with increasing mussel densities based on previous field surveys (0, 254, 509, and 1018 individuals m- 2). We measured variables of plant growth biweekly through the growing season and quantified final Spartina alterniflora biomass after 120 days. S. alterniflora productivity increased throughout the experiment, with higher increases associated with increasing mussel density. However, increasing inundation resulted in reduced biomass and production, and this reduction was not offset by the presence of mussels. Combined, the conclusions from both experiments provide support for the potential utility of G. granosissima/S. alterniflora living shorelines as a possible restoration tool under current conditions, however, their function could be limited as our climate continues to change.

Date

8-4-2023

Committee Chair

La Peyre, Megan K.

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