SS4.05 Tribute to Thomas Frost
Date: Monday, June 10, 2002
Time: 2:30:00 PM
Location: Sidney
 
HavelJE, Southwest Missouri State University, Springfield, USA, johnhavel@smsu.edu
Lampert, W, , Max-Planck-Institut fuer Limnologie, Ploen, Germany, lampert@mpil-ploen.mpg.de
 
NICHE PARTITIONING IN NATIVE AND EXOTIC DAPHNIA: VERTICAL MIGRATION IN PLANKTON TOWER EXPERIMENTS
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The tropical exotic Daphnia lumholtzi co-occurs with several native Daphnia in US reservoirs and, based upon its large size and growth rate, has the potential to out-compete the natives. We explored vertical niche partitioning of D. lumholtzi and the native Daphnia mendotae, using controlled experiments in plankton towers (11m tall, 6500 L), which mimicked the warm summer conditions of Missouri lakes (29C epilimnion, 20C metalimnion, 15C hypolimnion). Although food density was manipulated each day (highest in epilimnion and lowest in metalimnion), Daphnia densities became high enough to control food concentration (daily range 0.07-1.9 mgC/L). Results indicate that D. lumholtzi migrated up to the warm epilimnion each night, whereas D. mendotae avoided these surface waters. These behaviors appear to follow temperature cues and were unaffected by the presence or absence of D. lumholtzi. This vertical niche partitioning leads to a fitness cost for D. mendotae, suggesting a competitive disadvantage under such warm conditions. These experimental data are consistent with the range expansion of D. lumholtzi and its dominance during summer in reservoirs of the south-central US.