This study provides a detailed account of the population dynamics and ecology of the heterotrophic dinoflagellate, Noctiluca scintillans, along the south-east coast of Australia. The motivation for this study was the apparent increase in the frequency and intensity of red tides formed by Noctiluca in recent years. Anecdotal reports suggested that the red tides were a result of chronic sewage discharge from ocean outfalls in the coastal waters surrounding Sydney. The present study showed that upwelling was the dominant mechanism that stimulated population growth of Noctiluca along the south-east coast. Peaks in abundance of Noctiluca followed diatom blooms that resulted from upwelling during the austral spring and summer. A high proportion of Noctiluca cells contained diatoms in their vacuoles during the upwelling events indicating that population growth of Noctiluca was associated with its feeding activity. Stable isotope analyses also indicated that the nutrient source for the diatom prey of Noctiluca was most likely derived from upwelled slope water.

The temperature of the water column appeared to influence the extent of population growth of Noctiluca regardless of the amount of prey available. Noctiluca was found to be most abundant in areas where water temperatures were between 19-24 degrees C. Based on this temperature range, it was hypothesised that growth of Noctiluca is likely to be optimal in areas between 32.5 and 35 degrees-S during the austral spring and summer. Spatial abundance patterns of Noctiluca were also influenced by the immediate physical flow field. Noctiluca cells were advected southward with the East Australian Current. Cell size distributions of Noctiluca were used to trace the path of advected cells, and identify the areas where population growth was initiated. Small cells were considered to be capable of population growth, in contrast to red tide cells which were large and senescent. Small cells were considered to be located closer to the region in which growth was stimulated. A high proportion of small cells was positively correlated with phytoplankton biomass. This relationship was used to show that some red tides appearing in Sydney originated from northern upwelling regions. Further seeding stocks of Noctiluca were found to originate from estuaries with relatively high ambient standing stocks of phytoplankton, capable of sustaining a low but stable standing stock of Noctiluca during non-upwelling periods.

The inter-annual variability in the abundance of Noctiluca between March 1997 and January 2002 was predominantly attributed to the inter-annual variability in water temperature. Anomalously high numbers of Noctiluca were observed during 1997, when hydrological conditions were dominated by the presence of a strong El Nino. Minimum sea surface temperatures in the winter of 1997 were warmer, while maximum SSTs in the summer were cooler than other years. It was hypothesised that this unusual temperature regime extended the active growing season of Noctiluca to the winter months and reduced the likelihood of death of Noctiluca in the summer months. In addition, nearly all phytoplankton blooms that occurred during the 1997 sampling year were dominated by the diatom, Thalassiosira partheneia, most likely an optimal prey source for Noctiluca. The unusual dominance of T. partheneia was similarly attributed to the hydrological conditions induced by the El Nino, suggesting that the long-term increase in the abundance of Noctiluca off the south-east coast of Australia may be climatically forced.