Plankton assemblages across a coastal to oceanic gradient of Northern Australia and responses to a storm perturbation in the Argo Basin

Plankton sampling was undertaken across the 5000 m deep Argo Abyssal Plain (hereafter Argo Basin), the only known spawning region for Southern Bluefin Tuna (SBT). The Continuous Plankton Recorder was towed from Darwin across the Northern Australian shelf to the Argo Basin and bottle and net samples collected during experimental studies in the southern Argo Basin. Larger diatoms (> 15 μm) were the main phytoplankton group that distinguished Coastal shelf from Slope and Argo Basin provinces, with higher diatom and total cell abundances, higher diatom diversity and a higher ratio of diatoms to dinoflagellates in Coastal waters. We confirm the descriptions of the phytoplankton provinces defined from historical data and show that zooplankton distributions follow a similar pattern in terms of abundance but not diversity. The shelf-slope province boundary also occurred further east than in historical records. Whilst this region of the Indian Ocean is characterised by low productivity compared to other Eastern Boundary Current regions, tropical storms during the SBT spawning season are shown to be important for stimulating productivity and increasing abundances of zooplankton prey for tuna larvae. Results of this study solidify prior knowledge from this poorly studied area and highlight the need to better understand the ephemeral dynamics of plankton communities of this region.