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Partitioning the influence of anthropogenic and oceanic nutrient fluxes in the coastal ocean, and the question of cultural eutrophication

A multi-disciplinary study was implemented in the mid 1990’s in order to address concerns about apparent increases in the frequency and magnitude of marine phytoplankton blooms along the eastern Australian coast off New South Wales. This coastal region is frequently impacted by the western boundary flow of the East Australian Current (EAC), and anthropogenic discharges from major estuaries, and sewage outfalls within the Newcastle-Sydney-Wollongong conurbation. These coastal waters are typically nitrogen limited with respect to idealised Redfield ratios for optimal phytoplankton growth, indicating plankton sensitivity to nitrogen loading. Three deepwater outfalls servicing the city of Sydney represented the principal (~80%), continuous, anthropogenic source of nutrients to NSW coastal waters, predominantly in the form of ammonia nitrogen. The discharged effluent typically dilutes to 1:1000 to form plumes that remain trapped (~90%) by thermal stratification (≥1°C), and are dispersed by the prevailing alongshore currents. Of the eleven coastal catchments in this study region, the large Hunter and Hawkesbury catchments account for ~80% of the total catchment area and account for the majority of catchment nutrient loads discharged into the ocean. These discharges only produced significant increases to coastal waters (mainly as silica) when major flooding breached strong salinity gradients in the lower estuary. Episodic slope water intrusions were identified as the principal source of nitrogen in the form of nitrate to coastal waters. These shelf encroachments occurred seasonally associated with strong thermal dynamics observed during spring and summer. An understanding of mechanisms of slope water intrusion emerged from the deployment extensive mooring arrays, modelling exercises and responsive sampling transects triggered by synoptic monitoring of telemetered moorings and satellite imagery. Slope water intrusions operated over (alongshore) length scales of hundreds of kilometres and over time scales of a few days to a few weeks. Intrusions were related to coherent alongshore wind induced upwellings and EAC shelf encroachments, and localised baroclinic instabilities on the shoreward edge of EAC formed by perturbations in shelf topography. Phytoplankton blooms were correlated to these slope water intrusions irrespective of proximity to other major anthropogenic nutrient sources. However, the high frequency of occurrence of both the chain-forming diatom Thalassiosira spp. and the ‘red tide’ heterotrophic dinoflagellate Noctiluca scintillans during the study period was unprecedented in these waters. Long-term monitoring data indicated these anomalous conditions were associated with the strong El Niño episode of 1997/98, where warmer conditions prevailed. Corresponding studies along the New Zealand east-coast indicated a similarly unusual phytoplankton responses to El Niño episodes.
Type of thesis
Lee, R. (2002). Partitioning the influence of anthropogenic and oceanic nutrient fluxes in the coastal ocean, and the question of cultural eutrophication (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14168
The University of Waikato
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