Macrophytes and Turbidity in Brackish Lakes with Special Emphasis on the Role of Top-Down Control

Abstract

Evidence from both empirical studies (Canfield et al., 1984; Jeppesen et al., 1990; Faafeng and Mjelde, this volume, Chapter 27) and numerous experimental field studies (see e.g., Gulati et al., 1990; van Donk et al., 1990; Mortensen et al., 1994) indicates that in freshwater lakes extensive growth of submerged macrophytes may lead to clearwater conditions, even at high nutrient concentrations. Several factors seem to be involved, including both increased Zooplankton grazer control and nutrient constraint on phytoplankton, alterations in the physical environment that result in less wind-induced and fish-induced resuspension, and possibly also allelophatic effects (Jeppesen et al., 1990; Moss, 1990; Scheffer et al., 1993). A cross-analysis of survey data from 35 Danish brackish lakes revealed a significant decrease in Secchi depth with increasing concentrations of total phosphorus (TP); in contrast to freshwater lakes, however, transparency was independent of whether submerged macrophytes were present at high density (Jeppesen et al., 1994). Similarly, Moss (1994) found that nutrient-rich brackish lakes with extensive growth of submerged macrophytes tend to be in a turbid state. By using both empirical data and field experiments conducted in several brackish and freshwater shallow Danish lakes, we examine here how differences in top-down control may influence the turbidity of freshwater and brackish lakes in the macrophyte state.