Identifier

etd-06292011-105102

Degree

Master of Science (MS)

Department

Oceanography and Coastal Sciences

Document Type

Thesis

Abstract

The northwestern Gulf of Mexico currently experiences a large hypoxic area (“dead zone”) during the summer. While the local effects of hypoxia on organisms have been documented, the population-level effects are largely unknown. I developed a spatially-explicit, individual-based model to analyze how hypoxia effects on Atlantic croaker reproduction, growth, and mortality in the northwestern Gulf of Mexico could lead to population-level responses. The model follows the hourly growth, mortality, reproduction, and movement of individuals on a 300 x 800 spatial grid of 1 km2 cells for 100 years. Chlorophyll-a concentration, water temperature, and dissolved oxygen were specified daily for each grid cell for an average year, which was repeated every year. A bioenergetics model was used to represent growth, mortality was assumed age- and size-dependent, and the movement behavior of juveniles and adults was modeled as an unconditioned response to external cues (kinesis) coupled with a neighborhood search algorithm that emulated hypoxia avoidance. Hypoxia effects were imposed using vitality-repair submodels that convert time-varying exposures to reduced hourly growth, increased hourly mortality, and reduced annual fecundity. Results showed that 80 years of either mild or intermediate hypoxia produced small reductions in population abundance, while severe hypoxia caused a 31% reduction in long-term population abundance. The response to severe hypoxia was added age-1 and age-2 daily mortality (8-9%) and a 5% average reduction in eggs per individual. Relatively few individuals (5%) were exposed each hour but many individuals (20-40%) experienced at least one hour of low DO each year. The effects slowly built up in the model over years; population abundance slowly declined for 40 years before the 31% reduction was realized. Under more realistic hypoxia conditions of mild, intermediate, and severe hypoxia years occurring in proportion to their historical frequency, the model predicted an 18-29% decrease in the long-term population abundance. Sensitivity analysis showed hypoxia effects via reduced growth were small, and aspects of avoidance behavior were important in determining the population response. I discuss the strengths and weaknesses of the modeling, and future plans for refining the analysis based on data from ongoing field and laboratory studies.

Date

2011

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Rose, Kenneth

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