Degree

Doctor of Philosophy (PhD)

Department

Department of Oceanography and Coastal Sciences

Document Type

Dissertation

Abstract

The nearshore region off Louisiana’s coast is one of the most productive areas in the United States. Nutrient-rich discharge from the Mississippi and Atchafalaya rivers forms the base of this productivity, but it also contributes to the annual formation of a large hypoxic zone (DO < 2.0 mg l-1). This region contains >900 oil and gas platforms (platforms) that are de facto artificial reefs and support fish and invertebrate communities. In this dissertation, I examined how select platform-associated fishes and invertebrates responded to river-driven productivity and hypoxia. Settlement plates were used to compare barnacle production at depths of 2 and 7-m along a 43.9 km landward-seaward transect with decreasing river influence. Amphibalanus reticulatus settlement and growth decreased with increased distance from shore and was generally higher at 2 than 7-m. Total accumulation of fouling organisms decreased at rates of -9.8 to -1.2 g m-2 d-1 km-1, depending on depth and year. A video array was used to estimate abundances and depth distributions of fishes before, during, and after summer hypoxia at platforms experiencing intense (seaward) and mild hypoxia (shoal). Occupation of bottom waters by fishes was consistent throughout the study period at shoal platforms, but fishes were rarely observed in the bottom 3-m when hypoxia was present at seaward platforms. However, patterns of fish abundances were not driven by the presence or absence of hypoxia. A different camera array was used to characterize fish foraging at platforms. Sheepshead, Gray Snapper, Horse-eye Jack, Atlantic Spadefish, and Black Drum were observed foraging on fouling organisms, but Sheepshead was the only fish that consistently foraged on platforms (88.9% of samples). Mixing-models of Sheepshead stable isotopes (δ13C and δ15N) indicated their diet was not negatively affected by hypoxia, but gut contents suggested they temporarily exploited prey that were vulnerable because of hypoxia. Barnacles represented 64.1% of gut contents and stable isotopes suggested that barnacles and filamentous algae were 79.2 ­– 83.0% of their prey. Secondary production at platforms was increased by high primary productivity associated with river discharge. However, hypoxia influenced distributions of platform-associated organisms and temporarily increased Sheepshead exploitation of benthic prey.

Date

3-19-2018

Committee Chair

Baltz, Donald

DOI

10.31390/gradschool_dissertations.4507

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