Phytoplankton composition under contrasting oceanographic conditions: upwelling and downwelling (Eastern Australia)

Phytoplankton abundance and distribution along the east coast of Australia are driven primarily by the southward flowing East Australian Current (EAC), which transports tropical water masses to temperate latitudes. The Solitary Islands Marine Park (SIMP, ~30°S) is located north of the EAC separation point (~32°S) in this tropical–temperate transition zone. In this study, we describe the oceanographic context (wind, current and nutrient load) during a wind-driven downwelling and a current-driven upwelling event, both sampled in austral winter only ten days apart. We investigate the effect of these contrasting oceanographic conditions on phytoplankton abundance, composition and distribution along a cross-shelf transect. During downwelling we find a cross-shelf transition in microphytoplankton composition from an offshore- to an inshore-community associated with nutrient gradients (nitrate and silicate). Strong vertical mixing leads to increased occurrences of benthic diatoms in near-shore surface waters. During upwelling conditions, elevated nutrient availability results in maximum microphytoplankton abundances (mainly oceanic diatoms) and increased species richness on the mid-shelf. An increase in dinoflagellates and silicoflagellates (mid-shelf) and the appearance of tropical phytoplankton (especially picoplankton and dinoflagellates, offshore) signals a strong impact of the EAC across all shelf communities. Nanoplankton are a major part of the winter phytoplankton community during both oceanographic regimes (~40–50% of TChl a). Our findings provide evidence of EAC-driven, nutrient-rich, slope water intrusion in the SIMP as expressed by cross-shelf phytoplankton variability. We suggest that rapid (~weekly) changes in phytoplankton composition along the east Australian coast are likely to be enhanced by the climate change-induced warming/strengthening of the EAC.