Flow events drive patterns of phytoplankton distribution along a river-estuary-bay continuum

Freshwater flow events drive phytoplanktonproductivity in subtropicalcoastal river systems. However, few studies have the necessary temporal and spatial resolution to fully characterise the effect of events on the distribution of phytoplanktonacross the full river-estuary-bay continuum. The present study characterised the response of phytoplank- ton to high-flow events in an Australian subtropicalsystem; and identified the primary drivers of this response. During high-flow events, the concentration of phytoplanktonchlorophyll a (Chl a) initially declined in the estuary, a response primarily driven by the shortened water-residence time.In the bay, phytoplanktongrowth in the near-shore zone was light limited;however,nutrientsstimulatedphytoplanktongrowthontheseawardedgeoftheriverplume.Duringthepost-high- flow phase, the concentration of Chl a in the freshwater reaches peaked downstream, where catchment-derived nutrients accumulated. In the estuary, elevated nutrient loads stimulated phytoplanktongrowth upstream and downstream of the light-limited zone. In the bay, nitrogen availability declined, and Chl a declined with an increasing distance offshore. The phytoplanktonresponse to events documented in the present study can be used to identify when and where phytoplanktonin subtropicalsystems may be strongly influenced by changes in the magnitude of nutrient, sediment and freshwater loads associated with high-flow events which result from anthropogenic pressures within the catchment.