American Geophysical Union Fall Meeting Washington DC 10-14 December 2018 2018 https://agu.confex.com/agu/fm18/meetingapp.cgi/Paper/453327 Nitrogen (N) budgets and dynamics in Rotorua, a 80 km2 lake with a 500 km2 catchment, have been the subject of intensive research for 50 years, and have also been the focus of large-scale restoration interventions involving both policy and management. Algal growth in the lake has historically been N limited, but has become P limited through a programme of alum dosing that has successfully controlled cyanobacterial and algal blooms since ~2010. This presentation reviews and synthesizes science developed on the nitrogen budgets and dynamics within the catchment and the lake as part of a 5-yearly science review process. The main source of N inputs to the catchment occurs via clover N fixation in pastoral agriculture, which is estimated to be greater than N fertiliser application. Substantial reduction in the N load to the lake, and associated impacts, was achieved in 1991 through urban wastewater treatment, but was later negated through increases in agricultural intensity and associated N losses to the lake via streams. By 2005, land use and other sources of new inputs to the lake were capped at ~2001 levels. Legislation has proposed to reduce the N load entering the lake to 435 Mg y-1 by 2032, which is a substantial reduction from from the 755 Mg y-1 estimated as a steady-state post-2001 load. Application of the lake hydrodynamics-ecosystem model DYRESM-CAEDYM successfully predicts outcomes of different N and P nutrient loading to support policy. Review of monitoring data associated with key areas of model testing and improvement has revealed useful lessons. First, changing internal N dynamics within the lake following the initiation of the alum dosing programme and associated P limitation reversed a trend of decreasing N retention that coincided with increasing eutrophication. Second, improving temporal and spatial detail for N loads to the lake in major streams was the most important factor to improve model performance. A separate trend evaluation found unexpected flat and decreasing inflow nitrate concentrations in catchments with long tritium-derived mean residence times. As a result, future benefits appear likely from understanding in-lake N dynamics, rapid responses of catchments with old groundwater, and extending monitoring to better detect the effects of management changes in subcatchments.