Effects of common carp on submerged macrophytes Bellrichard, S. J. 1994. Effects of common carp (Cyprinus carpio) on submerged macrophytes and water quality in a backwater lake on the Upper Mississippi River. M.S. thesis submitted to the faculty of the graduate school of the University of Wisconsin-La Crosse. Reprinted by the National Biological Service, Environmental Management Technical Center, Onalaska, Wisconsin, July 1996. LTRMP 96-R008. 44 pp. (NTIS # PB96-202734) ABSTRACT Macrophyte populations have recently decreased in the Upper Mississippi River (UMR). This decline may be due to abiotic factors, such as a reduction in nutrients; however, biotic factors are also suspect. The common carp Cyprinus carpio has been reported to affect submerged macrophyte populations in other systems but not in the UMR. This study was conducted to determine if common carp can directly or indirectly reduce submerged macrophyte biomass. Twelve enclosures (25 m2) and four reference sites were constructed in Lawrence Lake, a backwater in the UMR, and stocked with one of three densities (0, 1, 10) of common carp. High densities of common carp (10/enclosure or approximately 7000 kg/ha) significantly reduced the biomass of submerged macrophytes, apparently by uprooting the plants. Total suspended solids, total volatile solids, turbidity, dissolved oxygen, temperature, total phosphorus, and chlorophyll-a were similar among the enclosures and reference sites during the 7-week study. KEYWORDS Common carp, ecology, fish, interactions, macrophytes, Mississippi River