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

Since the early seventies, Lake Wolderwijd (2650 ha, mean depth 1.5 m) suffered from cyanobacterial blooms, turbid water and a poor submerged vegetation as a result of eutrophication. From 1981 onwards the lake was flushed with water low in phosphorus and high in calcium bicarbonate. Total-P and chlorophyll a in the lake more than halved, but Secchi depth in summer increased from 0.20 to 0.30 m only. In the hope of triggering a shift from the algae-dominated turbid water state to a macrophyte-dominated clear water 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 May 1991, 575 000 (217 ind ha(-1)) pike fingerlings (Esox lucius) were introduced. In spring 1991 the lake cleared as a result of grazing by Daphnia galeata. The clear water phase lasted for only six weeks. Macrophytes did not respond as expected and most of the young pike died. However, from 1991 to 1993 the submerged vegetation is gradually changing. Characeae began to spread over the lake (from 28 ha in 1991 to 438 ha in 1993). The water over the Chara meadows was clear, probably as a result of increased net sedimentation in these areas. It is hypothesized that expansion of the Chara meadows might ultimately result in a shift of the whole lake to a lasting clear water state. In order to promote the Chara, the fish stock reductions aimed at a spring clear water phase should be continued.