The ability to predict reservoir vulnerability to summer blooms and to detect thresholds of change in phytoplankton cell densities in response to environmental factors would provide information critical for decision making, hazard prevention and management. We used a new method to detect synchronous change points in densities of phytoplankton taxa along the gradient of percentage grazing land cover in catchments, based on data collected from 15 reservoirs in subtropical Australia during summer 2009. In addition, we propose a predictive index of vulnerability that is based on simple measures of reservoir and catchment characteristics, including percentage grazing land cover. Our findings suggest that land use in the catchments of the studied reservoirs has a strong impact on phytoplankton composition and densities, and hence summer bloom phenomena.