Ecological impacts of an exotic benthivorous fish in wetlands: A comparison between common carp (Cyprinus carpio L.) additions in large experimental wetlands and small mesocosms in Delta Marsh, Manitoba

I examined the impacts of an exotic benthivorous fish, the common carp (Cyprinus carpio L.), in large (5-7 ha) experimental wetlands and in small (5 x 5 m) mesocosms located in Delta Marsh, Manitoba, Canada. In addition to following the impacts of common carp on water quality, sedimentation, and submerged macrophyte biomass, the impacts of common carp on phytoplankton, zooplankton, benthic invertebrates, and forage fish populations in the small mesocosms were intensively studied. As carp are often the dominant fish species found in degraded aquatic systems, the interaction between nutrient loading and carp, and the resulting impacts on important biotic and abiotic components were investigated through the small mesocosm experiments.; In both the large experimental wetland cells and the small mesocosms common carp significantly increased turbidity and suspended solid concentrations, sedimentation rates, chlorophyll a concentrations, and reduced submerged macrophyte biomass and light penetration. These results indicate that carp can significantly affect water quality and nutrient cycling in wetlands and lakes of Manitoba, even at relatively conservative densities. However, carp only induced a shift to the turbid state in the small mesocosm experiments. Failure to shift the large wetland cells to the phytoplankton dominated state was likely due to high DOC concentrations and submerged macrophyte biomass in these large ponds, which reduced the ability of the common carp to increase phytoplankton biomass.; At high densities of common carp in the fertilized enclosures, phytoplankton biomass was greatly reduced relative to the no carp and low carp treatments. My results contradict the belief that carp interact synergistically with nutrient loading to enhance phytoplankton biomass, and imply that there is likely a threshold carp density above which phytoplankton biomass is reduced regardless of nutrient supply due to light limitation resulting from carp induced turbidity.; Although phytoplankton biomass was reduced in fertilized enclosures at high carp biomass, the overall effect of carp was to increase phytoplankton biomass. Carp also altered the composition of the phytoplankton community. Additionally, carp significantly increased zooplankton density, reduced benthic invertebrate and forage fish populations, and dependent on the treatment altered the community structure of the zooplankton and benthic invertebrates.; Scaling up my results from the large experimental wetland cells indicated that at 400 kg.ha, a density likely to in Delta Marsh, nutrient loading from carp would be equivalent to 66% of internal phosphorus loading in the marsh. Furthermore, a common carp biomass of 200 kg.ha in Lake Manitoba, which is similar to the biomass found in other large shallow lakes, would contribute approximately the same amount of nitrogen, and double the amount of phosphorus to Lake Manitoba, relative to the combined loading from the major tributaries entering the lake.