Effects of common carp (Cyprinus carpio) on sediment mixing depth and mobile phosphorus mass in the active sediment layer of a shallow lake

The common carp (Cyprinus carpio) is a globally distributed, benthivorous fish that is capable of substantially altering lake ecology. Although dozens of studies have demonstrated that it has the potential to negatively affect submersed vegetation and water quality, little attention has been paid to effects of carp on sediment mixing and sediment phosphorus (P) in lakes. This study examined carp effects on sediment mixing and mobile (mainly iron bound and pore water) sediment P in a shallow, Midwestern lake (Kohlman Lake, MN, USA) undergoing restoration with aluminum (Al) to reduce internal P loading. Using Al as a tracer, we determined that the sediment mixing depth was at least 2.5 times greater in areas with carp (13.0 ± 3.7 cm) than in areas from which carp had been excluded (5.0 ± 1.2 cm) using exclosures. Vertical sediment profiles of P mass suggested that the increase in sediment mixing depth caused by carp increased the amount of mobile P potentially available for release by 55–92%, depending on location within the lake. The increase in sediment mixing depth is likely to have a negative effect on the efficacy of management methods designed to reduce mobile sediment P availability in lakes.