Lake Hayes is an important ecological, historical and cultural asset to the Otago Region. The lake and its catchment have been substantially altered from their natural condition, including conversion of the catchment from forest to pasture, invasion of the lake by non-native macrophytes, blooms of cyanobacteria (blue-green algae) and lake Trophic Level Index has increased over recent years (Figure A). Since 2010 the lake has been characterised by very poor water quality and pronounced summer algal blooms. This has attracted public attention and provided impetus from community and stakeholders to assess available management options for restoring water quality in the lake. This report examines the drivers of poor water quality and associated algal blooms in the lake, and assesses various management options that have been either specifically proposed for Hayes, or have proven effective for other lakes in New Zealand and globally. Here we report on monitoring undertaken by Otago Regional Council in the catchment (surface inflows) and in the lake. Primarily, we apply state-of-the-art computer models to simulate physical, chemical, and biological processes within the lake. These simulations are then used to evaluate a range of management scenarios to provide guidance on the likely effects of management options.