Lake Horowhenua is a shallow (±2m maximum depth) dune lake (300 ha) in New Zealand with a long history of eutrophication. It is among the most eutrophic lakes in the country (nitrogen inputs about 10 g/m3). Macrophytes grow rapidly during spring, resulting in high pH (10-11) and decomposition of decaying macrophyte material starting during summer results in low oxygen concentrations near the bottom. High denitrification rates and phosphorus (P) loading from the sediment during summer result in low nitrogen (N):P ratios. During summer the N:P ratio decreases about 2 orders of magnitude from the tributaries to the lake’s water column. High internal loading of P from the sediment during summer, as a result of low oxygen concentrations and high pH, stimulates blooms of cyanobacteria. However, despite low N:P ratios the dominant cyanobacteria species are not N fixers. A water budget and nutrient budgets were constructed and used to estimate rates of denitrification and internal loading, in combination with analysis of sediment core contents. Denitrification and internal loading rates were found to be not related to external loading but to temperatures and dissolved oxygen concentrations. Three decades after nutrient input reduction the P legacy in the sediment affects primary production, P cycling and lake water quality, with P concentrations in the lake about 4 times the average in the tributaries.