Top-down versus bottom-up control in planktonic systems: some case studies from the English Lake District

In this review, I use examples drawn from long-term studies in the English Lakes to highlight the limitations of top-down control in planktonic systems. These limitations are most obvious at the base of the food chain where there are large differences in the potential growth rates of the phytoplankton and zooplankton. Three of the examples are taken from a 40-year study of the impact of perch onDaphniain the North Basin of Windermere, one on the rates of change measured in an experimental enclosure and one from an 18-year study ofDaphniadynamics in Esthwaite Water. The Windermere examples show that the commercial perch fishery established in 1941 had little effect on the abundance ofDaphniabut the enrichment of the lake by war-time ploughing led to a short-lived increase in their reproductive rate. The other examples demonstrate that time plays an important part in all trophic interactions and shows that established patterns of phytoplankton succession are frequently disrupted by changes in the intensity of wind-induced mixing. The results are discussed in relation to recent reviews of the efficacy of biomanipulation and the challenges of analysing complex interactions in dynamic systems.