Modelling the effects of changing loads of nitrogen discharged from a sewage treatment plant on plankton dynamics in a large, urbanised bay

Abstract

The input of nutrients to the coastal environment is essential to support biological productivity but an excess of nutrients can lead to eutrophication. Here we use a pelagic ecosystem model to investigate the effects of nutrient inputs from a sewage treatment plant on the plankton dynamics of Port Phillip Bay, southern Australia. The effects of a 50% increase or decrease in nitrogen concentration in the Western Treatment Plant (WTP) outfall was examined relative to the 2016–17 base case. Increasing the nitrogen concentration in the WTP discharge had only a small effect on the summer diatom bloom. This can be explained by the fact that summer diatom blooms are typically silicate rather than nitrogen limited and WTP discharges are dominated more by N than Si compared with flows from the catchment. Consequently, increasing N had a relatively greater effect on flagellates, that in turn led to an increase in abundance of zooplankton (Paracalanus) at the beginning of the spawning window for snapper, Chrysophrys auratus. Paracalanus are the preferred prey for snapper larvae and the abundance of this zooplankter can strongly influence the larval survival and recruitment of snapper. Decreasing the nitrogen load from the WTP led to a marked reduction in the planktonic production in the bay, reflecting the potential effects of factors such as drought that can lead to a reduction in nitrogen load. Overall, the effect of changing nutrient load was sensitive to differences in relative nutrient concentrations from the treatment plant versus catchment sources.