Measurement of photosynthetically available radiation in freshwater: comparative tests of some current instruments used in studies of primary production

Abstract

The avoidance reaction of Calanus finmarchicus copepodids was observed in the presence of quantifiable fluid mechanical signals. The experimental apparatus was a rotating cylinder of sea water with fixed obstacles (diameters 6 mm and 10 mm) which disturbed the flow and stimulated avoidance. The experimental conditions prevented unambiguous discrimination of signals; the amount and rate of fluid deformation appear to be the effective stimuli. Avoidance reactions began at a mean distance of 7 mm from the small obstacle and 6 mm from the large obstacle. Mean values for fluid deformation for the small and large obstacles were 0.12 and 0.24 mm/mm; fluid deformation rate values were 0.80 and 1.0 sec⁻¹. There was less scatter for deformation values. The copepods preferentially avoided obstacles at angles of about 60° with respect to the approach streamline. Avoidance speeds averaged 16 cm.sec⁻¹; mean distance of avoidance flights was 15 mm.