A MULTIVARIATE-ANALYSIS OF PHYTOPLANKTON AND FOOD-WEB CHANGES IN A SHALLOW BIOMANIPULATED LAKE

1. Phytoplankton dynamics, food chain changes and resilience in Lake Z wemlust, a shallow lake in The Netherlands, are described for the peri od 1986-94. 2. After biomanipulation in 1987, the lake moved through t wo alternative states, while the external nutrient loadings were maint ained. A clear-water phase, mostly dominated by macrophytes, persisted from 1987 to 1991, and a rather turbid state, dominated by algae, occ urred in the summers of 1992-94, after several consecutive and sustain ed perturbations affecting different parts of the food web in the lake . These two periods were characterized by different community structur es. 3. The phytoplankton assemblage gradually changed in a pattern tha t reverted in later years towards that of the pre-biomanipulation stag e, although the same species composition was not regained. This agrees with some mathematical models. During the clear-water phase, nutrient shortage, light climate and zooplankton feeding selected in favour of small, high surface :volume ratio and rapidly reproducing algae. Howe ver, in mid-summer of 1992-94, nutrient availability and cladoceran gr azing on edible algae favoured cyanophyte. 4. Nutrients were transferr ed to higher trophic levels or lost from the system at relatively high rates when the lake was in a piscivore-macrophyte-dominated state, wh ile they tended to accumulate in the algae in a planktivore-dominated chain without macrophytes. The role of weed beds was central for nutri ent competition (mostly nitrogen) with algae, as well as a refuge and a base for alternative food sources to grazers. Weed beds seemed to ha ve a strong effect in increasing connectedness, resilience and stabili ty of the lake community. 5. The complete return of Zwemlust to a turb id state dominated by phytoplankton seems to have depended upon turnov er of the limiting nutrient, which was retarded by macrophytes and sti mulated by planktivorous fish and waterfowl.