Effects of biomanipulation in the large and shallow Lake Wolderwijd, The Netherlands

Since the early seventies, Lake Wolderwijd (2650 ha, mean depth1.5 m) suffered from cyanobacterial blooms, turbid water and a poorsubmerged vegetation as a result of eutrophication. From 1981onwards the lake was flushed with water low in phosphorus and highin calcium bicarbonate. Total-P and chlorophyll a in the lakemore than halved, but Secchi depth in summer increased from 0.20 to0.30 m only. In the hope of triggering a shift from thealgae-dominated turbid water state to a macrophyte-dominated clearwater state, the lake was biomanipulated during winter 1990/1991.The fish stock, mainly bream (Abramis brama) and roach (Rutilus rutilus),was reduced from 205 to 45 kg ha−1. In May1991, 575 000 (217 ind ha−1) pike fingerlings (Esoxlucius) were introduced. In spring 1991 the lake cleared as aresult of grazing by Daphnia galeata. The clear water phaselasted for only six weeks. Macrophytes did not respond as expectedand most of the young pike died. However, from 1991 to 1993 thesubmerged vegetation is gradually changing. Characeae began tospread over the lake (from 28 ha in 1991 to 438 ha in 1993). Thewater over the Chara meadows was clear, probably as a resultof increased net sedimentation in these areas. It is hypothesizedthat expansion of the Chara meadows might ultimately resultin a shift of the whole lake to a lasting clear water state. Inorder to promote the Chara, the fish stock reductions aimedat a spring clear water phase should be continued.