Annual phytoplankton dynamics in the Gulf Saint Vincent, South Australia, in 2011

Abstract

Phytoplankton communities are the basis of many marine and freshwater food webs. Their composition fluctuates depending on hydrochemical conditions, such as light, temperature, salinity, pH, nutrients and turbulence. This study investigates the effect of changing environmental conditions on the coastal phytoplankton community of the Gulf St Vincent in South Australia. This is the first study simultaneously investigating the phytoplankton communities and their environment in this area and is essential to set up the baseline of future studies. In total, 179 phytoplankton species were identified and enumerated between January and December 2011. Phytoplankton communities were numerically dominated by chlorophytes during 6months of the survey and an intense bloom (representing 62% of the overall phytoplankton community) of the diatom Cylindrotheca closterium was observed in February. Our results suggest that in the coastal waters of the Gulf St Vincent, the variability in environmental conditions is driven by temperature, wind speed/direction and the changing levels of phosphorus. However, the variability observed during autumn and winter months seems to be driven by changing levels of nitrogen and silica. In this shallow environment, the wind speed is proportional to the stress at the ocean floor and should directly influence the resuspension of sediment and associated nutrients. Nutrient ratios were observed to investigate potential phytoplankton nutrient limitation patterns. These ratios indicated that nitrogen was usually the limiting nutrient, which is typical of marine systems. Since nutrient enrichment is generally the main factor driving the succession and composition of phytoplankton communities in coastal waters, further work is now needed to identify the sources of nutrients in this region where river runoff is limited and evaporation is high relative to precipitation.