Mainstem dams and zooplankton in the Missouri River

The persistence of plankton in flowing water presents an enigma: how can populations be sustained while constantly losing individuals downriver?  We examined the distribution and abundance of zooplankton from 146 sites on the Missouri River (USA) and found large shifts in the dominance of major taxa between management zones of this regulated river.  Crustacean zooplankton were dominant in the inter-reservoir zone of the river and their taxonomic composition was similar to regional lakes and reservoirs.  The exponential decline of cladocerans and copepods with distance from main-stem dams suggests that conditions within the river are adverse to population growth and that reservoirs are the main source of these crustaceans in the river.  Rotifers dominated in the channelized zone of the river.  High algal biomass and rapid population growth rates likely allow persistence of rotifers in segments of the river that do not receive direct reservoir inputs.  Rotifers were less abundant in the inter-reservoir zone, suggesting that their numbers are limited by internal processes, such as food or predators.  Since zooplankton are known to be an important food for larval fishes in rivers, this shift of major taxa in regulated rivers has implications for river food webs.