Wingecarribee Reservoir experienced a detrimental water quality regime shift following the structural collapse of an adjacent peat wetland in 1998. The collapse was triggered by a high inflow event during a period of low reservoir water levels, combined with long-term destabilisation from peat mining. After the collapse, lake turbidity and dissolved nutrients peaked before returning to pre-collapse levels, and Microcystis blooms developed that persist to the present day. This study analysed 20 years of post-collapse monitoring data to inform ongoing management of the Wingecarribee Reservoir and Swamp for drinking water supply and biodiversity values. A combination of graphical analysis, univariate regressions, and GLM modelling was used to compare the relative influence of a) water transfers from an upstream reservoir, b) storage level fluctuations and c) catchment inflows on water quality and cyanobacteria dynamics.
Our results suggest that cyanobacteria growth is primarily driven by storage level fluctuations that trigger nitrogen release from the degraded peat. This process interacts with seasonal temperature variations that produce cyanobacterial growth peaks in late summer and autumn. Catchment inflows could not be directly linked to cyanobacteria dynamics, but their nitrogen loading likely contributes to long-term cyanobacterial proliferation. Water transfers from an upstream reservoir had a net neutral or even beneficial effect on cyanobacterial growth, potentially through reducing lake water residence time. This study demonstrates how careful interrogation of a long-term dataset can be used to unravel the complex and interactive drivers of aquatic ecosystems impacted by multiple human activities.