The objective of this study was to combine a catchment model with a one–dimensional lake water quality model to simulate the trophic state of a eutrophic shallow lake in response to nutrient load reductions and climate change. The catchment and lake models gave satisfactory performance in simulating observed data, indicating that the key processes that affect nutrient loads and lake trophic status were adequately represented. Simulating removal of nutrients by reducing fertiliser applied to farmland or irrigated wastewater had minor effects on nutrient concentrations in the lake, but simulations using a projected climate for 2090 showed a major impact on nutrients and water quality. This overarching effect indicated that polymictic lakes may be particularly vulnerable to eutrophication associated with climate change due to increased internal nutrient loading, which will lead to a biological response of increased algal biomass, while changes in external loads will have lesser relative impact.