Development of Indicators for Assessing and Monitoring Nutrient Influences in Coastal Waters

With increasing human pressures on coastal ecosystems, there is a need to develop better approaches to assess and monitor anthropogenic influences in these systems. The aims of this thesis were to a) develop indicators that describe and predict nutrient input effects, b) synthesise and interpret these indicators in assessment programs, and c) provide cost-effective methods for use in regular monitoring programs. Three study regions in tropical/sub-tropical coastal environments (Queensland, Australia) were chosen to represent a variety of nutrient inputs: aquaculture (land-based shrimp farming), sewage effluent (urban centre) and agriculture (sugar cane farming). Field sampling programs were conducted to assess the spatial and temporal variabilities associated with these nutrient inputs (i.e. discharge periodicity, run-off events and seasonality). In addition to standard water quality parameters, a variety of novel biological indicators were developed and implemented to characterise zones of biotic responses to the various nutrient inputs. Passive biological indicators employed in this study included nitrogen stable isotope ratios (delta 15N) of natural marine flora communities (macroalgae, seagrasses and mangroves), which were used to infer nitrogen source and fate. In regions devoid of macrophytes, an active biological indicator was employed using nutrient deplete macroalgae incubated in the water column, enabling intensive spatial resolution. A phytoplankton response index was developed in order to ascertain a measure of the susceptibility of a system to nutrient additions. This technique was based on responses of 7-d phytoplankton bioassays to increased light and nutrient availability. delta 15N signatures of marine flora proved successful in distinguishing nitrogen sources in each study region, often revealing trends not discernible using physical/chemical parameters. delta 15N signatures were characteristic of the nitrogen source - sewage effluent (~10 C°); shrimp effluent (~6 C°); and cane farming (2-4 C°). Incubated macroalgae rapidly changed their delta 15N signature within 4-d when exposed to 15N enriched nitrogen sources (e.g. sewage). Analysis of a variety of marine flora provided a more comprehensive understanding of nitrogen sources and fates. Mangroves integrated longer temporal variations and were indicative of sediment processes, whereas incubated macroalgae reflected short term conditions in the water column. Phytoplankton bioassays highlighted regions susceptible to nutrient enrichment, not predictable from water quality parameters alone. The nature of the different anthropogenic nutrient inputs strongly influenced water quality and biotic responses in receiving waters. Variable release rates of shrimp pond effluent and runoff variability from agriculture were detected with the methods described. The continuous input of sewage effluent resulted in large detectable plumes, which were influenced by environmental factors such as temperature and water currents. Statistical and spatial analyses were used to enhance data interpretation and presentation, which facilitated communication with resource managers and the wider community. Communication of these results led to successful integration and application into environmental management and monitoring programs.

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