Changes to chronic nitrogen loading from sewage discharges modify standing stocks of coastal phytoplankton

•Vulnerability of subtropical coastal systems to wastewater (STP) input was assessed.•STP nutrient load affects mean dissolved inorganic nitrogen and chl a concentration.•STP nutrient load affects chl a concentration during all seasons.•Effects are spatially variable and strongest in the lower flushed nearshore zones.•Decline in chronic TN loads may improve resilience to record high-flow events.

Nutrient delivery in subtropical coastal systems is predominantly via acute episodic high flow events. However, continuous nutrient discharges from point sources alter these natural fluctuations in nutrient delivery, and are therefore likely to lead to different ecosystem responses. The aim of this study was to assess how a reduction in chronic sewage nutrient inputs affected chlorophyll a (chl a) concentrations in a subtropical bay, in the context of seasonal fluctuations in riverine nutrient inflows. Reduced nutrient inputs from a large sewage treatment plant (STP) resulted in lower mean dissolved inorganic nitrogen and phytoplankton chl a concentrations during both the austral summer wet and winter dry season. This was measurable within 10 y of nutrient reductions and despite the confounding effects of nutrient inflow events. Our study demonstrates that reductions in STP inputs can have significant effects on phytoplankton biomass despite confounding factors over relatively short time frames.