AbstractWaajen, G, Lürling M, van de Sande R. The unfulfilled promise of urban Lake Kleine Melanen (The Netherlands): diagnostics, experiment on reduction of sediment P-release and in-lake restoration. Lake Reserv Manage. 35:8–24.Lake Kleine Melanen (The Netherlands) experienced cyanobacterial blooms during the summertime, causing public health risks, turbid water, and lack of macrophytes. To improve the situation, we determined the underlying causes of the poor water quality. Our diagnosis included the water and phosphorus (P) budget of the lake and showed the need for the reduction of both the external and internal P loads. The external P load (7.5 mg P/m2/d) exceeded the transition between the clear water state and the turbid state by 44–121 mainly due to the discharges from the separated stormwater sewer system in the adjacent neighborhood. Due to political and financial considerations of the managing authorities, the restoration (2010–2012) was restricted to internal management. To reduce the high internal P load (3.8 mg P/m2/d), we tested 3 sediment capping options in an enclosure experiment, showing the best results when we combined sand capping with the P fixative Phoslock® and flocculant polyaluminumchloride. A 0.6–1.3 m thick layer of soft sediment was dredged from the lake. Thereafter, the sediment was capped with a 0.2 m thick layer of sand to which Phoslock® was added. Additional in-lake measures including fish removal and shoreline reconstruction were also conducted to support improvement. We compared water quality before (2008–2010) and after (2012–2014) interventions. Although the mean concentration of total P was reduced from 0.30 to 0.11 mg P/L, and cyanobacterial chlorophyll a from 52 to 6 μg/L, cyanobacterial blooms still occurred. For further water quality improvement, the reduction of the external P load caused by stormwater runoff is required.