Date of Award

2006

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

School

School of Natural Sciences

Faculty

Faculty of Computing, Health and Science

First Advisor

Dr Kathryn McMahon

Second Advisor

Paul Lavery

Third Advisor

Dr Anne Brearley

Fourth Advisor

Dr Matt Vandekift

Abstract

One of the main anthropogenic disturbances to seagrass meadows in Australia is reduction in light availability, through nutrient enrichment or suspended sediments. Dredging can create suspended sediment plumes from the expulsion of particulates into the water column and in tum reduces light penetration to seagrass ecosystems. Preliminary investigations have demonstrated that light reduction for different intensities and durations results in reduced seagrass and epiphytic algae biomass. The main aim of this study was to determine the effects of different intensities and durations of light reduction on epifaunal assemblages in Amphibolis griffithii seagrass meadows in Jurien Bay, Western Australia. This was achieved by reducing light availability to an A. griffithii meadow by shading plots with 50- 80% (moderate and high intensity light reduction) light-reducing shade cloth for three and six months durations. Samples were collected in December 2005 and March 2006 and epifauna abundance, biomass, production and composition were assessed. Based on a visual census, fish abundances in the main experiment were also analysed and found to be a confounding factor as shade screens increased fish abundance. In an attempt to assess the mechanisms driving changes in epifauna abundance, a second experiment was conducted to examine the effect of reduced structural complexity on epifauna, mimicking the changes observed in the main experiment but without the presence of shade screens that were creating increased fish abundances. In the main experiment, after three months of light reduction the abundance, biomass and production of epifauna showed decreasing trends, which intensified and were statistically significant after six months of light reduction. This interactive effect of intensity and duration of light reduction was also apparent on assemblage composition. The second experiment, although no significant result was detected, also showed a reduction in epifauna abundance with reduction in seagrass structural complexity. Therefore, although fish abundance may have contributed to a change in epifauna abundance in the main experiment, components of seagrass complexity are fundamentally important in maintaining epifaunal assemblages in seagrass meadows. Crustaceans and molluscs dominated the taxa found in both experiments. Within these groups, taxa showed varying changes to light reduction treatments. In the crustacean taxonomic group, amphipods and tanaids showed the most dramatic decline after three months of light reduction, while copepods declined after six months. The abundance of ostracods did not appear to change with light reduction treatments after three months, although a decline was observed after six months. In the mollusc taxonomic group gastropods declined after three months, but declined most dramatically after six months. Bivalves were not affected negatively by light reduction treatments and showed slightly higher abundance after six months of light reduction. Overall, the results indicate that light reduction in seagrass meadows results in reduced epifaunal assemblages. However, not all epifauna respond in the same way. This study has revealed that different groups of fauna respond in different ways and at different rates. Therefore, there is a complex set of interactions involving many factors explaining epifauna abundance and composition in seagrass meadows exposed to light reduction. In addition, these changes m epifauna with light reduction may have significant trophic consequences in A. griffithii habitats. Additional studies and management implications are discussed.

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