Seasonal analysis of Daphnia pulicaria swimming behavior

Our study documents individual swimming behavior of Daphnia pulicaria over a yearly cycle in a temperate lake. We collected D. pulicaria, a common fre shwater zooplankton, from Lake Mendota on 10 dates between July 1994 and Ju ne 1995 from two depths, 2 m and 10 m. The Daphnia were rushed to the labor atory and video-taped as they swam in lake water under lake-ambient tempera ture and light conditions. Five-second swimming tracks of individual Daphni a were filmed and digitized using a motion analysis system. We measured ave rage turning angle, swimming speed and sinking rate for each track. D. puli caria swimming behavior varied over the annual cycle. We found significant differences in turning angle between depths and among months. Sinking rate and swimming speed were significantly different among months but not depths . Sinking rate and swimming speed were not significantly correlated with wa ter temperature. Our results were contrary to Stokes' Law predictions, in t hat D. pulicaria had the slowest sinking speed in June, not in the winter w hen water temperatures were lowest and viscosity was highest. Body length w as significantly correlated with all three swimming variables. We also stud ied swimming behavior in clonal populations of D. pulicaria in different co ncentrations of the alga, Chlamydomonas reinhardtii. D. pulicaria did not c hange swimming speed, turning angle or sinking rate over a range of food co ncentrations. Finally, swimming behavior in a D. pulicaria clone, tested at two temperatures in the laboratory, confirmed the results from our seasona l study; Daphnia did not sink as predicted by changes in viscosity.