Date of Award

1-1-2013

Document Type

Open Access Thesis

Department

Biological Sciences

First Advisor

James L Pinckney

Abstract

The trophic link between benthic microalgae (BMA) and fiddler crabs is critical for the ecosystem functioning of estuaries and alterations in this linkage by anthropogenic activities could have cascading impacts on food webs and biogeochemical cycling in these sensitive habitats. Singular and interactive effects of two common pollutants in aquatic ecosystems, polycyclic aromatic hydrocarbons (PAHs) and copper (Cu), were investigated by exposing field collected sediment communities to the contaminants and measuring changes in BMA biomass and community composition in a bioassay design. The consequential impacts on the food web were then explored by examining the effects of PAHs and copper on food preference and feeding rates of sand fiddler crabs (Uca pugilator). No significant overall change in BMA biomass (as chlorophyll a) after 10 days was observed between treatments. However, the trends in the algal biomass responses throughout the experiment, as well as the significantly greater change in BMA biomass between 4 and 10 days after exposure in the Cu treatment compared to the controls, suggest a complex sediment community response. The abundance of diatoms relative to cyanobacteria (the fucoxanthin to zeaxanthin ratio) increased significantly in Cu and Mix (PAHs + Cu) treatments compared to controls, possibly due to cyanobacterial sensitivity. Fiddler crabs grazed on sediments of the Mix (PAHs + Cu) treatment significantly less than they did upon controls during a food choice experiment. In addition, the feeding rates of crabs exposed to Cu only, PAHs only and to PAHs + Cu (the Mix treatment) were significantly lower than those in the Water control when no alternative food choice was provided. Because fiddler crabs are important bioturbators, a reduction in feeding, (and therefore sediment processing) in contaminated areas could have significant impacts on the chemistry of surficial sediments which subsequently influences sediment communities and marsh grass growth. Reduced feeding would likely also reduce resources for crab growth and reproduction, effect crab health, and eventually could result in a reduction in crab abundance. Sublethal exposure of intertidal communities to PAHs and Cu may result in subtle alterations in the trophodynamics of BMA and fiddler crabs that have the potential to affect multiple levels of biological organization.

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