Original paper

Effects of common carp on nutrient dynamics and littoral community composition: roles of excretion and bioturbation

Matsuzaki, Shin-ichiro S.; Usio, Nisikawa; Takamura, Noriko; Washitani, Izumi

Abstract

We manipulated through an enclosure experiment the presence and absence of small common carp (Cyprinus carpio, L) and their sediment access using a mesh net. The aims of the experiment were to determine carp effects on nutrient dynamics and community composition, and partially separate bioturbation effects from the remaining carp effects such as excretion and predation. During the two months, carp influenced water quality and nutrient dynamics, altered phytoplankton biomass and composition and decreased submerged macrophyte biomass independently of sediment access, indicating that nutrient excretion was the primary mechanism for the carp effects. However, Secchi depth decreased while the concentration of suspended solids increased when carp access to the sediments was allowed, suggesting that bioturbation by carp influenced water transparency. Increase in phytoplankton (dominated by cyanobateria) and suspended solids resulted in reduced submerged macrophyte biomass through reduction of light availability. Zooplankton, numerically dominated by rotifers, increased in carp enclosures. In addition, benthic macroinvertebrates decreased in carp enclosures, regardless of the presence and absence of the netting. Because direct predation effects were not evident, carp probably affected benthic macroinvertebrates through reduction of submerged macrophytes. These results indicate that carp can have dramatic direct and indirect impacts on nutrient dynamics and littoral community structure through excretion and bioturbation. Ecosystem engineering by carp may therefore trigger a shift from a clear water state dominated by submerged macrophytes to a turbid water state dominated by phytoplankton.

Keywords

enclosure experimentphytoplanktonsubmerged macrophytesecosystem engineerscatastrophic regime shifts