New evidence links changing shelf phytoplankton communities to boundary currents in southeast Tasmania

Southern Tasmanian shelf waters are host to the seasonal interplay of Australia’s two poleward boundary currents; the East Australian Current (EAC) and the Leeuwin Current (LC). While the behaviour and properties of the LC remain underexplored, strong research focus has allowed insight into how an intensifying EAC has created greater subtropical influence, leading to changes in the physical and biological oceanography of the region. In this cool temperate setting seven species of dinoflagellates, all in the genus Ceratium, which are more typically associated with warm waters of eastern Australia, were observed. This coincided with the seasonal increase in the EAC’s southward penetration beginning in October. Despite the seasonal peak in EAC activity, temperature-salinity plots, nutrient, chlorophyll a and phytoplankton concentrations all indicate the presence of subantarctic waters on the shelf and in coastal waters in summer. Our results are consistent with the description of the EAC as an erratic, eddy-driven current; this itself allowing the periodic influx of subantarctic waters across the shelf. In winter, temperature-salinity plots and nutrient concentrations indicate that the LC was present in southern shelf waters. In addition to its high nitrate signature, the LC displayed low silicate properties in southern Tasmania. Chlorophyll a concentrations revealed a distinct spring bloom event and an extended, productive summer, typical of temperate and subantarctic systems, respectively. This suggests the region is a transitional state between classic seasonal primary production cycles for temperate and subantarctic waters. This paper links changes in southern Tasmanian microphytoplankton communities to shelf ventilation by the EAC, the LC and subantarctic waters, and provides new insight into the oceanography of the region. Consequently, this study provides an awareness of potential phytoplankton perturbations that may be applied to other coastal cool temperate marine environments.