Item

Abstract

We tested the hypothesis that species (clones) of Daphnia, originating from lakes with very different
cyanobacterial abundances, use strategies to optimize their performance in the presence of toxic Microcystis
aeruginosa by distributing differently in vertical gradients of valuable food, toxigenic cyanobacteria, and
temperature. A laboratory tube system with different combinations of food items in the temperature gradient was
used to determine the vertical distribution and performance (growth and lipid index) of Daphnia from three
contrasting environments: (1) Daphnia carinata from a eutrophic lake with dense populations of cyanobacteria;
(2) Daphnia galeata adapted to low cyanobacteria densities in a deep, mesotrophic lake; and (3) arctic Daphnia
pulex assumed to be naı¨ve with few adaptations against pelagic cyanobacteria. When confronted with toxic
Microcystis in the epilimnion, Daphnia can respond by avoidance behavior (i.e., suffer metabolic costs from low
temperature), reduction of their overall feeding rate in order to avoid the ingestion of toxic cells, metabolizing the
toxin biochemically, or not responding if they were never confronted with toxic cyanobacteria. The experiments
suggest that D. carinata was sensitive to toxigenic Microcystis and responded by avoidance, D. galeata was less
sensitive and preferred to stay in the warm epilimnion, and D. pulex was naı¨ve as expected. Thus, the behavioral
strategies of the three Daphnia species appear to reflect interplay between evolutionary history, sensitivity to
Microcystis, and the environmental conditions.