Influence of physical processes and wastewater effluent on phytoplankton dynamics in the Perth coastal margin, Western Australia

Daniel Machado Vegas

    Research output: ThesisDoctoral Thesis

    334 Downloads (Pure)

    Abstract

    [Truncated abstract] The availability of nutrients, light, and physical properties of the water, including small-scale fluid motion, influence phytoplankton dynamics. The Western Australian (WA) coast is characterized by low nutrient concentrations, nitrogen limitation, low primary production (oligotrophic) and lack of large-scale upwelling. The Perth coastal margin is semi-enclosed from the open ocean by chains of submerged reefs and islands. Alongshore coastal currents, driven by southerly winds, prevail and contribute to a highly dispersive environment. This thesis investigated how physical processes, occurring at a range of spatial and temporal scales, influence phytoplankton dynamics in the temperate coastal margin of Perth. The thesis is formed by three main components. First, the relative importance of the main sources of nutrients, including treated wastewater effluent, was assessed. Analysis of a 14-year field monitoring record revealed seasonal variations in nutrients and chlorophyll-a (Chl-a) concentrations. Dissolved inorganic nitrogen (DIN) and Chl-a concentrations were higher in the winter period than in summer. Remote-sensing information was used to place the seasonal variations of Chl-a into an oceanographic context. Three-dimensional hydrodynamic-ecological simulations were used to determine the drivers of seasonal variations and reconcile the major inputs of DIN: superficial runoff, groundwater, wastewater effluent, atmospheric deposition and exchange with surrounding coastal waters. The results showed that the increase of DIN concentration during winter was driven by enhanced exchange with offshore waters, caused by changes in the wind field. This suggested that additional wastewater nutrient removal is not likely to affect these dynamics. Second, the hydrodynamic-ecological model was used to assess the effect of an alternative scenario in which the wastewater effluent was considered as a potential resource instead of as a waste. This involved running scenario simulations corresponding to a less-stringent wastewater nutrient removal during summer, the season of lowest nutrient and Chl-a levels. The simulation results indicated a moderate Chl-a increase, within the level of historical variability observed in the monitoring data, suggesting that such a scenario could enhance the ecological services provided by the coastal ecosystem while preserving its oligotrophic state...
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2013

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