The effect of grazing by the snail, Goniobasis clavaeformis Lea, on aufwuchs in artificial streams

The effects of grazing by snails on the biomass, primary productivity, chlorophyll a concentration, and species composition of aufwuchs were studied in once-through artificial stream channels. The snail, Goniobasis clavaeformis Lea, was stocked in three channels -2 at densities of 1.4, 2.8, and 5.6 g AFDM • m^-2 and a fourth channel was left ungrazed. These densities span the range of snail densities encountered in streams in the vicinity. Allochthonous detritus and other macroconsumers were excluded from the experimental system. Over the six week experiment, grazing reduced aufwuchs biomass, chlorophyll a standing stock, and primary productivity per unit area and per unit chlorophyll. Chlorophyll a concentrations (μg chlorophyll a • mg AFDM^-1) however, tended to be higher in the grazed systems than in the ungrazed channel in the last 3 weeks of the experiment, indicating that grazing may have caused a change in the species composition of the aufwuchs community. This was supported by a decrease in the proportion of large diatoms observed in samples from the ungrazed system with a corresponding increase in the relative numbers of small diatoms. A similar change in the community composition of the grazed treatments was not observed.

Primary productivity per unit biomass was higher in the ungrazed treatment than in the grazed treatments throughout the experiment, although biomass-specific productivity actually increased relative to its initial value at the low grazing pressure. This suggests that the potential for grazer stimulation of aufwuch activity exists.

Analytical studies of a five-compartment non-linear aufwuchs-grazer stream model supplemented the empirical work. These studies imply that the stability of stream systems is determined by the growth rates of aufwuchs and fine particulate organic matter. This result, however, may be an artifact of 1) the inclusion of dissolved inorganic and organic carbon within a single compartment and 2) the lack of consideration of advection and diffusion effects. Additional modeling and experimental investigation are suggested in order to better clarify the role of grazers in stream systems.