Transition of a lake to turbid state six years after biomanipulation: mechanisms and pathways

Six years after application of biomanipulation in 1987, Lake Zwemlust (The Netherlands) returned during the summer from a clear water state dominated by aquatic vegetation to a turbid state characterized by high algal biomass. Herbivory and growth of epiphytes on macrophytes were the main factors triggering the switch. Selective herbivory by coots (Fulica atra) and rudd (Scardinius erythrophthalmus) caused a change in macrophyte species composition from a dominance of Elodea nuttallii in 1988/1989 to Ceratophyllum demersum in 1990/1991, and finally to Potamogeton berchtholdii in 1992/1993/1994. Observations revealed a general lack of epiphytes associated with Elodea and Ceratophyllum, while Potamogeton showed a progressive coverage with epiphytes, causing Potamogeton to decline markedly during late summer. Phytoplankton blooms, dominated by cyanobacteria, appeared again during three consecutive autumns, 1992, 1993 and 1994, with chlorophyll-α concentrations reaching 60-240 μg 1⁻¹.