Phytoplankton Dynamics in Contrasting Coastal Embayments of South Australia: Implications for Bivalve Aquaculture

This study investigated seasonal phytoplankton communities in two contrasting oyster-growing embayments, Coffin Bay and Franklin Harbour (Eyre Peninsula, South Australia), to assess how environmental conditions influence food availability across the full phytoplankton size spectrum. In both embayments, nutrient availability was a major driver of phytoplankton dynamics during summer, while seasonal changes in physical conditions coincided with a shift in the phytoplankton community from oligotrophic to mesotrophic assemblages. In Coffin Bay, the community was dominated by picoplankton (<2 µm), accounting for more than 50% of total biomass for over 8 months of the year. In contrast, Franklin Harbour was dominated by larger phytoplankton (>20 µm). Productivity in outer Coffin Bay was primarily supported by inflowing shelf waters, which sustained peak picoeukaryote biomass (44.27 µg C L−1), and was further enhanced by a late-summer upwelling event in the eastern Great Australian Bight. However, productivity in Franklin Harbour was primarily generated internally, with the highest picoeukaryote biomass (68.15 µg C L−1) in the southern end. This study provides new insights into phytoplankton dynamics across the full spectrum in two contrasting embayments. The findings demonstrate how variations across the phytoplankton size spectrum can inform assessments of coastal productivity and carrying capacity, with implications for the sustainable management of bivalve aquaculture in temperate coastal regions.