Anthropogenic and Oceanic Nutrients in NSW's Dynamic Coastal Waters and Their Effect on Phytoplankton Populations

A three-year, multi-disciplinary study was conducted by the NSW EPA to assess the relative importance of natural and anthropogenic nutrients on the development of phytoplankton blooms. Sydney's three deepwater outfalls represented the principal, continuous, anthropogenic source of nutrients to NSW coastal waters. Effluent plumes generally remained submerged and sewage derived nitrogen was mainly in the form of ammonia. Episodic slope water intrusions were the principal source of nitrogen in the form of nitrate to coastal waters especially during spring and summer. An understanding of mechanisms of slope water intrusion emerged from modelling exercises commissioned for this project and from direct observations. Slope water intrusions operated over (alongshore) length scales of hundreds of kilometres and over time scales of a few days to a few weeks. Phytoplankton blooms appeared to occur in response to slope water intrusions irrespective of proximity to other major 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. Despite this, general patterns of spring phytoplankton bloom successions were otherwise consistent with previous studies with no discernible difference in phytoplankton biomass or species diversity.