Environmental Management of Aquaculture Effluent: Development of Biological Indicators and Biological Filters
Rapid global expansion of the aquaculture industry has prompted the need for development of techniques for effective environmental management. In intensively farmed regions, aquaculture effluent has resulted in environmental degradation of receiving waters. The issues to be addressed include analysis of effluent water quality, determination of the ecological impact of effluent on the ecosystem, and development of remediation strategies to reduce these impacts. Physical and chemical water quality analyses can identify elevated concentrations of suspended solids, chlorophyll _a_, water column nutrients and other components of aquaculture effluent, however, additional biological sampling is required to provide meaningful information about the ecological impacts of effluent discharge on receiving waters. Analyses of the amino acid composition, tissue nitrogen content and stable isotope ratio of nitrogen ([delta][super:15]N) in seagrasses, mangroves and macroalgae were developed as biological indicators to determine the influence of shrimp farm effluent on a coastal ecosystem. Different responses in these biological parameters revealed that the impacts of aquaculture effluent on receiving waters were qualitatively different to the impacts of sewage effluent. The impacts were also spatially more extensive than identified by water quality analyses, which revealed no elevation in the concentration of water column nutrients, chlorophyll _a_ concentration or total suspended solids further than 400 m from the mouths of the creeks receiving the sewage and aquaculture effluent. The maximum [delta][super:15]N of the mangroves, seagrass and macroalgae associated with the treated sewage discharge was 19.6[permil] , which was significantly higher than the influence of the shrimp effluent (7.6[permil] ). A [delta][super:15]N value of 4.5[permil] , which is elevated relative to unimpacted sites, indicated that the impacts extended up to 4 km from the mouths of the creeks. Differences in the concentrations of the amino acids proline, serine, glutamine and alanine in the seagrass and macroalgae were suggested to reflect the source (aquaculture or sewage) of the nutrients taken up by the plants. To reduce the environmental impacts, effluent treatment techniques using biological filters were investigated. Filtration by oysters (_Saccostrea commercialis_) significantly reduced the concentrations of chlorophyll _a_ (phytoplankton), bacteria, total nitrogen, total phosphorus and total suspended solids to 5%, 32%, 67%, 63% and 11% of the initial concentrations, respectively. However, oyster excretion increased the concentrations of the dissolved nutrients, ammonium (from 18 to 51 [mu]M), nitrate / nitrite (from 1.0 to 13 [mu]M), and phosphate (from 0.5 to 3.3 [mu]M), however macroalgal (_Gracilaria edulis_) absorption significantly reduced these concentrations to 2.3%, 2.2% and 4.8%, respectively. The ratio of ammonium to nitrate / nitrite in the effluent was also significantly reduced, which has positive implications for recycling of wastewater back into shrimp production ponds, and reducing impacts on receiving waters. The efficiency and condition of the oysters and macroalgae was reduced by fouling from the high concentration of suspended particulates in the effluent. Several novel techniques such as dissolved free amino acid composition, pigment concentrations, PAM fluorescence, tissue nitrogen and d15N were used to assess the condition of the macroalgae. It was observed that an intermediate reduction in the concentration of suspended particulates resulted in the best growth and condition of the biofilters. The concentration of particulates in this treatment (11 nephelometric turbidity units) provided sufficient particulates for oysters to filter, and a source for regeneration of nutrients for macroalgal uptake, as well as reducing the effects of photoinhibition which can occur in Gracilaria spp. at relatively low light intensities. The problems associated with fouling were successfully mitigated by incorporating natural sedimentation prior to oyster filtration, and subsequent macroalgal absorption. This combined system of treatment proved effective at optimising the performance of the biological filters to improve the water quality of the effluent. Using this combination of polyculture, it was estimated that up to 18 kg N ha[super:-1] d[super:-1] and 15 kg P ha[super:-1] d[super:-1] could be removed from commercial shrimp ponds. The water quality of aquaculture effluent and its impact on the receiving waters will vary due with differing environmental conditions, as well as the type of aquaculture being conducted. Regardless, this thesis has demonstrated that filtration / absorption by various marine organisms can be effective tools for monitoring and reducing the environmental impacts of aquaculture effluent.
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