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Diel feeding behavior and gut evacuation rates were investigated in adult female Acartia tonsa, in laboratory experiments at 20 "C using different concentrations of cultured Thalassiosira weissflogii as food, and in large mesocosms at -13°C using natural plankton as food. Diel feeding pattern was remarkably stable under the different food conditions and temperatures, maintaining about a 3-fold amplitude in gut contents and feeding rate over the diel period. The diel rhythm was retained even when the copepods were food limited and lost about 20 % body carbon during the course of the 24 h experiment. The persistence of the diel feeding rhythm in A. tonsa under starvation implies a strong selective pressure, possibly related to predator avoidance. Daily ingestion rates of copepods fed a high concentration of T. weissflogii at 20°C were about 90 000 cells d-l, or -148 % body carbon and 104 % body nitrogen, compared with 74 and 85 % body carbon and 40 and 45 % body nitrogen in mesocosm experiments at -13 "C with natural food. Daytime and night-time gut evacuation rates at 20 "C were not significantly different, indicating that the diel change in gut contents reflects a real change in feeding rate, rather than a change in the instantaneous gut clearance rate. Instantaneous gut evacuation rates were 0.0907 and 0.0894 min-l in 2 experiments at 20°C and 0.0415 and 0.0432 minp' at 12.7 and 13.3"C respectively. Day-night comparison of gut contents measured in individual A. tonsa, fed T. constricts at 8"C, revealed considerable individual variability. Range in gut contents was about 50-fold during the day (0.17 to 8.97 ng pigment copepod-'), and 100-fold at night (0.0 to 13.66 ng copepod-'). Distribution of gut content values was skewed towards lower values, especially during daytime. However all but 1 copepod in the day/night sequence had a measurable amount of food in the gut, indicating fairly recent feeding.

Diel feeding behavior in the marine copepod Acartia tonsa in relation to food availability