ENUMERATION OF ZOOPLANKTON SAMPLES: EFFECTS OF MESH SIZE 
ON DENSITY ESTIMATES OF COPEPODS, CLADOCERANS, AND ROTIFERS

Alex P. Levchuk1, John H. Chick1, Kim A. Medley2, John E. Havel2, Jeff D. Jack3
1Illinois Natural History Survey, Great Rivers Field Station, 8450 Montclair Avenue, 
Brighton, IL  62012,2Department of Biology, Missouri State University, Springfield, MO 
65897, 3Department of Biology, University of Louisville, Louisville, KY 40292

      Zooplankton are critical components of many aquatic ecosystems, linking primary 
production, microbial, and detrital resources to higher trophic levels.  They form a critical 
food source for young of the year fishes of many species.   Zooplankton support large 
biomasses of recreationally, commercially, and ecologically important zooplanktivorous 
fish such as bluegill (Lepomis macrochirus), bigmouth buffalo (Ictiobus cyprinellus), 
gizzard shad (Dorsoma cepedianum), and invasive bighead and silver carp (Aristichthys 
nobilis, Hypophthalmichthys molitrix).  As part of the United States Environmental 
Protection Agency’s (USEPA) Environmental Monitoring and Assessment Program 
(EMAP), 56 zooplankton samples were collected from the Upper Mississippi River 
during the summer of 2004.  We divided zooplankton into two major groups: 
microzooplankton, which includes rotifers and copepod nauplii, and macrozooplankton, 
which includes cladocerans, adult, and juvenile copepods. Rotifers are links to both the 
primary production and to microbial and detrital resources, and tend to dominate the 
zooplankton communities in great rivers.  Crustacean zooplankton (copepods and 
cladocerans) are more directly linked to primary production.  Separate sampling 
procedures were used for the two groups: 18 L of water filtered through a 20 ?m mesh for 
microzooplankton and 180 L of water filtered through a 63 ?m mesh for 
macrozooplankton.  Microzooplankton were enumerated and identified using a 
compound microscope and Sedgewick-Rafter cells.  Macrozooplankton were enumerated 
and identified using a dissecting scope and a Ward-Whipple counting wheel.  Both 
groups of zooplankton were counted in all samples.  Our analysis strongly supports our 
methodological decisions.  These results suggest that the majority of zooplankton studies 
grossly underestimate the abundance of rotifers, possibly by 2-3 orders of magnitude.  
Many studies of zooplankton community structure and the diet of zooplanktivorous fishes 
are strongly biased in favor of crustaceans relative to rotifers.    
      
Keywords:  zooplankton, sampling methodology, mesh size, great rivers