Graduation Year

2014

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Marine Science

Major Professor

Cameron H. Ainsworth

Keywords

brevetoxin, Ecopath with Ecosim, fish kills, Gag grouper, red tides

Abstract

Karenia brevis dinoflagellate blooms off the west coast of Florida can create devastating effects on marine communities when they release a neurotoxin known as a brevetoxin. These blooms, informally referred to as red tides, can cause massive fish kills, necessitate closures of shellfish fisheries, and can even leave lingering toxins that impact shelf communities long after the bloom has dissipated. As a result, much effort has been put into studying K. brevis bloom initiation and dynamics. However, how K. brevis blooms impact Florida's fisheries is not fully understood because the relationship between K. brevis cell counts and fish mortality is poorly described. To study this relationship and the ecosystem response to K. brevis blooms, Ecopath with Ecosim (EwE) modeling is used to force K. brevis bloom mortality on the shelf ecosystems by using a recently developed time series that indexes K. brevis bloom severity. This index dynamically drives K. brevis bloom mortality in EwE in a historical reconstruction scenario from 1980 to 2009. Three hypotheses on ecosystem response are explored using Gag grouper as a case study. We postulate a) that K. brevis blooms impose bottom-up and top-down effects on the food web, b) that episodic perturbations by these blooms shape the community structure and c) that fishing pressure exacerbates those effects. Results support the hypothesis that K. brevis blooms pose top-down food web pressures, which is seen by evidence of trophic cascading. Changes in community structure with bloom mortality are also evidenced by changes seen in biodiversity and richness. An exacerbation of those effects as a result of heavy fishing pressure is evident, however, is only seen during severe bloom events. Little to no changes were found in the mortality from K.brevis blooms during blooms of average severity, and less mortality was imposed on the system during blooms of particularly low severity. However, this may be an artefact of the mode of action of K. brevis in EwE. Investigation of bloom effects on Gag showed that natural mortality rates of Gag appear to be largely influenced by mortality incurred during K. brevis blooms relative to the low rate of predation on Gag. Moreover, consumption rates of Gag on its prey were found to increase under a realistic schedule of these blooms. This may be due to a combination of effects, including increased mortality on competitors (making more prey available for Gag) and a lowering of the mean age of the Gag stock, which increases population productivity.

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