Direct and Indirect Effects of Size-Selective Predation By Bighead Carp on Zooplankton Communities

Populations of Asian carp (Hypophthalmichthys nobilis) have increased substantially in the past two decades.  These invasive planktivores pose a substantial threat to zooplankton communities, which are important prey for native species, and functionally important because they recycle nutrients within pelagic habitats.  When prey contribute greatly to the recycling of nutrients or the processing of particulate matter, disruptions to these organisms may have propagating impacts throughout the food web.  To test whether bighead carp alter body size, biomass, and secondary production of zooplankton, we conducted an additive experiment.  Ponds were stocked with native fishes or native fishes plus bighead carp.  Conditions represent pre- and post-invasion conditions, holding fish assemblages constant, with the bighead carp being the differing factor.  Our experiment indicated that bighead carp exerted strong size-selective predation, which altered the body size of Sididae and Daphnia, reduced the size structure, biomass, and production of whole communities, and indirectly reduced P recycling by the zooplankton assemblage.  The strength of top-down regulation by bighead carp generally diminished with decreasing body size of zooplankton taxa, with the exception of rotifers.  Predation by bighead carp exerted strong direct effects, with attendant indirect effects on nutrient recycling.