Date of Award

2004

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Computing, Health and Science

First Supervisor

Dr Paul Lavery

Second Supervisor

Catherine Collier

Abstract

Declines in seagrass health and distribution are commonly caused through human induced reductions in the availability of photosynthetically active radiation (PAR). These reductions can result from a variety of human-induced perturbations, including channel dredging. The impetus for the research was driven by the broad-scale degradation of the ecologically important southern-Australian endemic seagrass Amphibolis griffithii (Black) den Hartog in Champion Bay, Geraldton, Western Australia. The study investigated the affects of reduced PAR on A. griffithii and identified responses that may be useful in developing management triggers to minimise the impact of PAR limitation events. The study was carried out during late summer and winter at Jurien Bay on the Midwest coast of Western Australia. Replicate plots of Amphibolis griffithii meadow were subjected to 90% reduction in PAR availability for 106 days using shade screens suspended over the meadow. A variety of morphological and physiological variables were monitored in control and treatment plots at approximately monthly intervals during this time and after 42 days of recovery. There was a noticeable meadow-scale response in A. griffithii with significant reductions in leaf biomass measurements, such that the number of leaves per stem (-12 leaves per stem) and total leaf biomass (DWm-2) were approximately half that of ambient levels after 106 days of shading. This resulted in a dramatic change in the light attenuation coefficients between shaded (0.59 m-1) and control plots (2.38 m-1) allowing greater penetration of PAR through the canopy, effectively reducing self-shading in the lower canopy. These changes were paralleled by marked physiological responses with increases in chlorophyll and decreases in rhizome sugar concentrations in the shaded plants. Chlorophyll levels responded consistently in the upper canopy with highly significant increases after 106 days of treatment and a return to ambient levels after 42 days of recovery. Rhizome sugars depleted quickly and consistently with treatment, culminating in highly significant differences after 106 days of shading with concentrations at less than one third (-1) when compared with ambient levels. The apparent reduction in canopy self-shading was likely to have aided the considerable recovery of most variables, such as leaf extension which fully recovered after 42 days following shade removal. This research identified a suite of specific responses to reduced PAR in A. griffithii and has assessed their inherent potential for future development of Environmental Quality Criteria (management trigger values) to high intensity, short duration impact events on the mid-west coast of Western Australia, including recommendations for further research. The study has highlighted the species specific nature of seagrass responses to reduced PAR climates; contributions to the broader ecological knowledge were made with specific reference to ecologically and morphologically unique species that do not necessarily conform to known responses in the blade-like species, such as Posidonia. The application of these research outcomes will ultimately help environmental managers minimise the impacts of broad-scale PAR induced degradation events like the dredging program at Champion Bay, Geraldton from re-occurring.

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