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Seasonal hypoxia (dissolved oxygen-DO, < 2.0 mg/l) develops and extends over large regions (> 20,000 km2) of the northwestern Gulf of Mexico shelf in the summer (May-August) as a result of nutrient inputs and salinity stratification associated with the Mississippi-Atchafalaya watershed. Hypoxia can lead to mortality, enhance metabolic costs, lead to habitat loss, and alter species interactions by modifying the behavior and spatial distributions of resident species. The direction and magnitude of these effects depends on the intensity and duration of the hypoxic event, and the species-specific DO tolerances and behavioral responses to low DO. Prior studies have shown that demersal fishes in the Gulf avoid hypoxic bottom water by aggregating near the edges of hypoxic zones. However, few studies have investigated how species interactions at upper trophic levels respond to hypoxic events. It may be that the spatial distributions of apex predators (e.g., dolphins) and other marine megafauna (e.g., sharks, sea turtles, rays) are altered by hypoxia-induced shifts in the distribution of their prey. To address this possibility, our team conducted synoptic aerial and shipboard (hydrographic) surveys of marine megafauna at both local, high resolution scales (2003-2004) and at shelf-wide lower resolution scales (2011-2012) during peak hypoxic periods of summer. Bottlenose dolphins, loggerhead and Kemp's ridley sea turtles, sharks, and cownose rays comprised most of the sightings. I found a higher probability of sighting dolphins in water with low levels of bottom DO or high standard deviation-SD of bottom DO. The presence of fish schools in the upper level of the water column also tended to improve the fit of models for dolphin sightings. These results suggest that dolphins are responding to hypoxia-induced behavioral responses of prey such that their trophic interactions are indirectly mediated by hypoxia. More broadly, I found higher probabilities of detecting marine megafauna and higher numbers of megafauna sightings in areas with bottom water hypoxia or on the edges of hypoxic events, though these trends were less consistent than the trends in dolphin distributions. The importance of DO varied as megafaunal community composition changed and across different months within the summer. Other water quality (i.e., fluorescence, salinity, temperature) and spatial (i.e., latitude, longitude, distance to shore) factors were also significant predictors of dolphin and megafauna sightings; however, the strength and direct of these relationships were not as consistent as those with hypoxia. This work helps elucidate the indirect effects of hypoxia on trophic interactions within the northwestern Gulf of Mexico.
A Thesis submitted to the Department of Biological Science in partial fulfillment of the requirements for the degree of Master of Science.
Includes bibliographical references.
Felicia Coleman, Professor Co-Directing Thesis; J. Kevin Craig, Professor Co-Directing Thesis; Brian Inouye, Committee Member; Charlotte Lee, Committee Member.
Florida State University
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