Alternation between clear, high-vegetation and turbid, low-vegetation states in a shallow lake: the role of birds

External phosphorus load to a wetland with two shallow lakes in the Botshol Nature Reserve, the Netherlands, was reduced since 1989, resulting in a rapid reduction of phosphorus levels, phytoplankton biomass and turbidity, and after 4 years, explosive growth of Characeae. The clear water state was unstable, however, and the ecosystem subsequently alternated between clear, high-vegetation and turbid, low-vegetation states. The switch from clear to turbid states occurred at a higher phosphorus level than the switch from turbid to clear states and area covered by macrophytes was significantly correlated to Secchi disc depth (r = 0.86, p < 0.001). Beginning in 1997, phosphorus input from droppings of greylag geese and black-headed gulls increased. The hypothesis that grazing prevented the return of Characeae populations was tested during 1996–1997 using large-scale exclosure experiments. The biomass and cover of Characeae were monitored in 17 exclosures for 2 years. The experiments showed no significant increase in Characeae in the absence of grazers compared with the controls. During the course of the experiments, the entire lake switched from turbid water to clear water and high Chara biomass. These ecosystem developments suggest that light limitation was the main factor controlling the collapse and return of Characeae in Botshol.