Ph.D. Dissertation Defense Seminar - Alyssa Outhwaite

MARINE BIOLOGY PROGRAM
DEPARTMENT OF LIFE SCIENCES
TEXAS A&M UNIVERSITY-CORPUS CHRISTI

TITLE: The influence of habitat context on community structure, organic matter composition, and consumer diets of subtidal oyster reefs in Matagorda Bay, Texas 

GRADUATE COMMITTEE: Dr. Jennifer Pollack (Chair), Dr. Benoit Lebreton, Dr. Kim Withers, Dr. Paul Montagna, Dr. Lynn Hemmer (Graduate Faculty Representative)  

ABSTRACT
Estuaries are mosaics of habitats interconnected through flows of organic matter. The spatial arrangement of these habitats within the broader estuarine environment, known as habitat context, can influence the composition organic materials and faunal communities they support. Although the significance of estuarine habitat context on ecological functioning is acknowledged, its impact on subtidal oyster reefs remains underexplored. This dissertation examines the influence of habitat context on subtidal oyster reefs by focusing on three key aspects: (1) oyster populations and faunal community structure, (2) organic matter composition and assimilation, (3) and bay-scale organic matter dynamics. Using a combination of traditional faunal metrics and stable isotope analyses, results demonstrate that different habitat contexts (such as bar and patch reefs) produce distinct effects on oyster populations and faunal communities. For example, bar reefs support higher densities of oysters and greater biomass of motile nekton, while patch reefs host fewer oysters but a more abundant infaunal community. However, the influence of habitat context on organic matter composition and consumer diets among oyster reefs was limited. Regardless, it is clear that oyster reefs act as unique spaces that promote locally produced organic materials—especially phytoplankton and benthic microalgae—that are important for supporting community-wide faunal biomass. Although organic matter composition was similar between oyster reef types, zooming out to the scale of the bay revealed variations in both the quantity and quality of organic matter, with higher biomass observed near the river delta and higher quality near the mouth of the bay. Additionally, sediments within seagrass beds had higher organic matter quantity and quality compared to non-vegetated sediments. Regardless of habitat context, locally produced benthic microalgae emerged as a primary component of the oyster diet. These findings highlight the significant implications of habitat context on ecological functioning at both local and ecosystem scales. Understanding these influences, ranging from individual habitats to bay-wide dynamics, can inform creation of targeted goals and strategies for restoration and conservation efforts.