Biomass-dependent effects of age-0 common carp on aquatic ecosystems

Abstract

Fishes play a functional role in structuring aquatic ecosystems through top-down and bottom-up processes. Adult common carp (Cyprinus carpio )a re well recognized for their middle-out effects on aquatic ecosystems that can shift shallow lakes from the clear- to turbid-water stable state through benthic foraging activities. However, less is known about ecosystem effects of age-0 common carp. Age-0 common carp are planktivorous and can be highly abundant, suggesting that large year classes may also produce undesirable ecosystem effects. We evaluated the effects of four age- 0 common carp densities (0, 175, 475, and 812 kg/ha) on water quality (ammonium, total phosphorus, total Kjeldahl nitrogen, and turbidity) and primary (macro- phytes and phytoplankton) and secondary (zooplankton and benthic invertebrates) production. Common carp increased nutrient availability and phytoplankton pro- duction and decreased water transparency with effects increasing with carp biomass. Common carp also reduced macrophyte coverage and cladocera density (through effects on chydorus and ceriodaphnia) and body size, but effects were not density dependent. In contrast, common carp did not appear to affect copepod, rotifer, chironomid, or gastropod densities. These results suggest that even relatively low age-0 common carp densities ((175 kg/ha) may have many comparable ecosystem effects as adult carp, although effects may be accrued through different pathways.