Guilty by association: Are all benthivorous fish created equal?

Common carp Cyprinus carpio has long been identified as a contributor of water quality deterioration, disrupted ecosystem processes, and shifts in biological assemblages. In contrast, there is a paucity of information regarding the effects of native benthivorous species on aquatic systems despite sympatry with common carp and the potential to benefit from degraded habitat conditions. The effects of the non-native common carp and the native black bullhead Ameiurus melas on sediment and nutrient resuspension, submerged macrophyte biomass, and zooplankton and benthic macroinvertebrate assemblage structure were evaluated in experimental mesocosms. We found that common carp had negative impacts on water clarity (i.e, increased turbidity), nutrient resuspension (i.e., increased ammonium, nitrate, and phosphorus concentrations), macrophytes (i.e., decreased biomass), and benthic macroinvertebrates (i.e., decreased total abundance and biomass). In contrast, black bullhead had little or no effect on measured water quality and biological variables when compared to control conditions. Overall, results from this experiment suggest that although black bullhead tend be tolerant of degraded ecosystems, they are not causing physical changes to the environment known to be responsible for perpetuating degraded water quality and shifts in biological assemblages. Therefore, increases of native benthivorous species abundance may serve as an indicator of ecological conditions.