Species-specific responses of aquatic macrophytes to fish exclusion in a prairie marsh: A manipulative experiment

An exclosure experiment was carried out at two sites in Delta Marsh, Manitoba, Canada to investigate the role of fish in limiting the growth of submersed macrophytes. The experiment consisted of three treatments: (1) fine-mesh exclosures designed to exclude both planktivorous fish and carp, (2) coarsemesh exclosures designed to exclude adult carp but admit smaller fish, and (3) reference plots marked with corner stakes but without sides. Treatments were established in mid-May, and macrophyte biomass was sampled from within the exclosures in late August to assess treatment effects. Exclosure treatments had strong effects on the macroalgae Chara, with biomass 11.9-fold greater in full-exclosure plots than in reference plots, and 3-fold greater in carp exclosures than in reference plots. Exclosure treatments had no effect on above-ground or below-ground biomass of Stuckenia pectinata, the most widespread and abundant macrophyte in Delta Marsh. Thus, fish appear to limit macrophyte growth in Delta Marsh, but the effect of fish exclusion was dependent on species composition of the macrophyte assemblage.