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Daphnia lumholtzi; plankton towers; vertical migration
Abstract:
1. Invasion of tropical zooplankton into temperate lakes provides an interesting opportunity to explore habitat segregation in a thermal gradient.
2. We explored differing vertical positioning of native and exotic Daphnia (Daphnia mendotae and Daphnia lumholtzi) in a large indoor mesocosm system (Plon plankton towers) during 2 month-long experiments. The two towers were manipulated to provide gradients of both temperature (15-29 degrees C) and algal food (0.05-0.58 mg C L-1) and a day-night cycle.
3. Both juvenile and adult D. lumholtzi showed a 'typical' vertical migration pattern, with higher densities in the epilimnion at night than during the day. They avoided the food-poor middle layer. In contrast, D. mendotae adults showed little tendency to migrate into the epilimnion at night, remaining in the cooler hypolimnion while juveniles migrated. The vertical distribution of D. mendotae appeared unaffected by the presence of D. lumholtzi.
4. The strong migration behaviour of D. lumholtzi in the absence of fish cues suggests that this behaviour may be a constitutive trait. Habitat partitioning of the two species is probably the result of different thermal tolerances, with D. mendotae constrained to remaining in deeper water by high temperatures in the epilimnion and the tropical D. lumholtzi able to use the warm epilimnion at night.
