Invasive planktivores as mediators of organic matter exchanges within and across ecosystems

Bighead carp (Hypophthalmichthys nobilis) are an invasive planktivore that can greatly deplete planktonic resources. Due to the inefficient conversion of food into fish tissue, large portions of consumed materials are egested and shunted to benthic habitats. We explored how bighead carp alter pools of organic matter between planktonic and benthic habitats, and across ecosystem boundaries. Here, we report evidence from a manipulative experiment demonstrating that bighead carp greatly reapportion pools of organic matter from planktonic to benthic habitats to such a degree that additional effects propagated across ecological boundaries into terrestrial ecosystems. Strong direct consumption by bighead carp reduced filamentous algae, biomass and production of zooplankton, and production of a native planktivorous fish within planktonic habitats. Reduced herbivory indirectly increased phytoplankton (chlorophyll a). Direct consumption of organic matter by bighead carp supported high carp production and concomitant losses of materials due to egestion. Perhaps in response to organic matter subsidies provided by fish egestion, ponds having bighead carp had higher standing crop biomass of Chironomidae larvae, as well as cross-boundary fluxes of their adult life stage. In contrast, we detected reduced cross-boundary fluxes of adult Chaoboridae midges in ponds having bighead carp. Consideration of bighead carp as mediators of organic matter exchanges provides a clearer framework for predicting the direct and extended impacts of these invasive planktivores in freshwater ecosystems. The perception of bighead carp must evolve beyond competitors for planktonic resources, to mediators and processors of nutrients and energy within and across ecosystems.