EFFECTS OF FOOD LIMITATION BY ZEBRA MUSSELS (DREISSENA POLYMORPHA)
  AND FISH PREDATION ON LIFE HISTORY CHARACTERISTICS OF DAPHNIA
  PULEX.
  
  Michael Kinsella1, Mark B. Sandheinrich1, and William B. Richardson2
  1River Studies Center, Department of Biology & Microbiology, University of Wisconsin-La
  Crosse, La Crosse, WI  54601. 2 U.S. Geological Survey, Upper Mississippi Science Center,
  La Crosse, WI  54602
  
  Life history strategies of organisms have evolved to maximize the fitness of the individual and
  may be influenced by the environment of the species.  Competition and predation are
  interspecific interactions that may alter life history characteristics.  Zooplankton (e.g.,
  Daphnia spp.) subject to intense fish predation, generally shunt energetic resources into
  reproduction rather than growth and produce large clutches of small eggs at an early age.  In
  contrast, Daphnia subject to invertebrate predation and competing for food with other filter
  feeders grow quickly, delay reproduction, and produce small clutches with large eggs.  Since
  its introduction into the Laurentian Great Lakes in 1986, the zebra mussel (Dreissena
  polymorpha) has spread throughout the Great Lakes region and now occurs within the upper
  Mississippi River.  Although several studies have demonstrated that zebra mussels effectively
  filter phytoplankton and compete with zooplankton, no studies have assessed the effects of this
  exotic mussel on the life history characteristics of native zooplankton.  The objectives of this
  mesocosm study were to determine the effect of resource depression due to filter-feeding
  zebra mussels and the effect of fish predation on life history characteristics of Daphnia pulex. 
  A two factor cross-classified experimental design was used to assess the main and interactive
  effecs of the presence or absence of zebra musssels and fish.  There were four treatment
  combinations, with three units per treatment.  Intrinsic rate of increase, instantaneous birth and
  death rates, clutch size, size at first reproduction, and population size of the Daphnia were
  estimated weekly during the eight-week study.  Preliminary results suggest that zebra mussels
  increased clutch size and increased death rates.  Fish increased death rates and decreased
  population size and size at first reproduction of Daphnia.  When co-occurring, zebra mussels
  and fish decreased population size and increased the average clutch size of Daphnia.  These
  results suggest that zebra mussels may alter zooplankton life history characteristics and
  depress population growth in Daphnia.
  
  Keywords: zebra mussel, Daphnia, life histories, resource depression, predation
  
  
  
  1. Michael Kinsella
     Upper Mississippi Science Center
     2630 Fanta Reed Rd.
     La Crosse, WI 54601
  
     Phone: (608) 783-6451
     FAX: (608) 783-6066
     email: michael_kinsella@nbs.gov
  2. Poster Presentation only
  3. Student Author