Date of Award

2000

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Communications, Health and Science

First Advisor

Dr Paul Lavery

Second Advisor

Dr Albertus J. Smith

Abstract

Recent work indicates that nitrogen stable isotopic signatures of aquatic biota can be used to link nutrient inputs in the marine environment to terrestrial sources. While this technique of tracing nutrients has been shown to be effective in environments that are not nutrient limited, it has not been proved that this method is effective in oligotrophic environments. This study examined whether nitrogen stable isotopic signatures of marine biota can be used to trace the distribution of sewage effluent in highly mixed, nitrogen limited waters, by examining the δ15N values of marine biota situated close to and distant from a secondary treated sewage outlet. The study was conducted in the vicinity of the Marmion Marine Park, off the coast of Ocean Reef, Western Australia. The study comprised two experimental components. The first component tested whether the δ15N value of three macroalgal species (Ulva australis, Vidalia sp. and Eklorzia radiata) with varying nutrient uptake capacities (high, medium and low respectively) were representative of the distribution of sewage dissolved inorganic nitrogen (DIN) over different time periods. Also, whether the δ15N values of two filter feeding species (Clathria sp. and Pyura australis) were representative of the distribution of sewage particulate organic matter (POM). The second component was a laboratory experiment designed to confirm that this same set of species were actually able to develop an isotopic signature representative of sewage, when exposed to low concentrations of sewage in seawater. Results from the laboratory experiment confirmed that all species, with the exception of E.radiata, developed higher isotopic signatures when exposed to very low concentrations of sewage nitrogen (1:500 dilution in seawater). In the field experiment, the three species of macroalgae sampled north and south of the sewage outlet generally had higher δ15N values than organisms sampled west of the outlet and at the reference site. This is consistent with our current knowledge of effluent dispersal in the area. The results also suggest the plume moves further north than previously expected. Thus it was confirmed that the δ15N signature of macroalgae can be used to trace sewage disposed in well mixed waters. It also appears that macroalgae are able to log a unique time-integrated record of ambient sewage, which can be used to provide data on the short and medium-term accumulation of sewage N into the benthic system. It is less likely that the low nutrient uptake species selected for this study (E.radiata) is capable of representing long term pattern in effluent dispersal. δ15N values of benthic filter feeders did not provide comprehensive evidence to suggest that they can be used to represent the distribution of sewage POM, however trends found in the field experiment for Chlathria sp. warrants further investigation. These results demonstrate that the δ15N values of marine biota, specifically macroalgae, can be used to map the distribution of terrestrial nutrient sources affecting marine ecosystems in an oligotrophic environment and on a limited range of time scales.

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