Abstract
Upstream-moving Gammarus pulex pulex L. were continuously trapped from March 1968 through May 1969, in 2-hr catches, yielding approx. 150,000 individuals > 3 mm in size. Upstream movement activity reached a maximum in June and July and a minimum in March. Significant correlations were found between the seasonal variations in activity and temperature, precipitation, and water level. The activity response to precipitation and water level is probably released by a sensitivity to water current fluctuations which causes positive rheotaxis at increasing water velocities. The diel activity pattern was affected by light and temperature. In winter the activity was restricted to darkness with modifications due to temperature. In agreement with the circadian rule the activity in summer was displaced to dawn or later, being in early August restricted to the day-time. This occurred during a spell of constant cloudless weather when also the maximum circadian period, about 25 hr, was registered. In spring, low temperature prevented the transition to light activity.
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