The effects of velocity and competition on riverine zooplankton food selection

Zooplankton typically feed on phytoplankton in aquatic environments.  This study investigated the effect of velocity and competition on food selection by zooplankton on phytoplankton collected from the Kentucky River, U.S.  We sorted the zooplankton by size classes to create three grazer treatments of microzooplankton (less than 153 micrometers, rotifers), macrozooplankton (greater than 153 micrometers, copepods and cladocera), and the entire community.  Each grazing treatment was then placed under high and low velocity on shaker tables and the difference between initial and final algal densities and types were measured to determine grazing impact.  Our findings indicate that microzooplankton impacted diatoms more under high velocity while macrozooplankton impacted diatoms more under low velocity when only diatoms were present.  When presented a choice between diatoms and cryptophyceae, micro and macrozooplankton grazed the latter, but microzooplankton ate more under high velocity and macrozooplantkon under low velocity.  Velocity appears to affect food selection of riverine zooplankton and may be an important factor in structuring riverine phytoplankton and zooplankton communities in the Kentucky River.