Turbidity and wave energy affect community composition and trophic interactions
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Abiotic variables are well known community regulators and can strongly influence species distributions when they are outside of a species physiological tolerance limits. However, environmental variables within tolerance limits may also alter species distributions, morphology, predator-prey interactions, and influence the structure and function of communities. The purpose of this study was to determine how abiotic variables (notably turbidity) alter diversity, species distributions and abundances, predation rates on and species morphology. Texas Parks and Wildlife Department Fisheries Independent Survey Data from 1991-2008 were used in addition to field surveys of St. Charles Bay to determine the effects of turbidity on fish and crab diversity and abundance. Feeding assays were conducted in the field using groups of 5 mud crabs and 10 juvenile oysters to assess feeding rates in high and low turbidity. Juvenile oysters were also allowed to grow in the field to test the effects of turbidity on oyster growth. In addition, the effects of wave energy on oyster reef species composition and size were assessed using field surveys. I found that turbidity affects top-down control and biodiversity in estuaries and has similar effects to salinity and temperature. Elevated turbidity reduced fish diversity and abundance (p < 0.01), while increasing the diversity and abundance of crabs (p < 0.01). Predation by visual fish predators was also reduced in elevated turbidity (p = 0.02), which leads to an increase in the abundance of crabs and increased predation on mud crabs in high turbidities (p = 0.03). Juvenile oysters respond to increased crab abundance by growing heavier shells, which may lower fecundity. In laboratory assays, increased turbidity decreased the predation efficiency of visual predators (fish) but not of chemosensory predators (crabs). Differences were found between wave exposed and wave protected areas. Areas with higher wave energy had fewer species (p < 0.001) and the average size of species was smaller. This research identifies turbidity as an important variable within estuarine systems and extends the effects of wave energy from rocky intertidal systems to oyster reef communities. Both of these variables should be considered for effective management and restoration of estuarine communities.
Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy.