Diel feeding and respiration rhythms in zooplankton1

Diel rhythms in respiration and feeding of zooplankton from three British Columbia lakes were examined in laboratory experiments conducted at constant temperature and in the absence of light. Endogenous rhythms in respiration and feeding were demonstrated at temperatures from 10–22°C and were typically bimodal with maximum rates found at dawn and dusk and lowest values near midday. Dawn and dusk respiration rates were an average of 2.3 times higher than during midday; the corresponding diel difference in feeding was 6.6 times.