Complex Fish-Snail-Epiphyton Interactions and Their Effects on Submerged Freshwater Macrophytes

Abstract

Eutrophication of shallow freshwater and marine ecosystems has often resulted in a drastic decline in the areal extension and biomass of submerged macrophytes and a concomitant increase in the biomass of phytoplankton (e.g., Phillips et al., 1978; Cambridge et al., 1986; Hough et al., 1989; Shepherd et al., 1989). Light availability is usually the most important factor determining the distribution pattern, biomass, and production of submerged macrophytes (e.g., Chambers and Kalff, 1985; Duarte et al., 1986), and it has been suggested that increasing phytoplankton biomass due to higher nutrient input results in a reduction of available light to a level at which net photosynthesis by submerged macrophytes is impossible (e.g., Jupp and Spence, 1977; Jones et al., 1983). However, Phillips et al. (1978) suggested that macrophytes may disappear even when the bottom is within the euphotic zone where light availability is adequate for photosynthesis. Instead of shading by phytoplankton, they argued that increasing nutrient levels stimulate epiphyton growth, which has a negative effect on the macrophyte host through shading and competition for nutrients. Recent modifications of the model of Phillips et al. (1978) also invoke epiphyton as a key factor in the decline of submerged macrophytes (e.g., Silberstein et al., 1986; Moss, 1989; Brönmark and Weisner, 1992; Van Vierssen et al., 1994).