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Adult females of the marine copepod Acartia hudsonica Pinhey were fed the unicellular diatom Thalassiosira constricta (10.4 pm equivalent spherical diameter) a t 4, 8, 12, and 16 "C, to determine whether there was a threshold food concentration below which feeding ceased or was sharply reduced. Field-collected copepods were acclimated for 24 h at 2000 cells ml-' T. constricta, and then transferred to a range of low food concentrations (15 to 876 cells ml-l). Gut pigments were determined fluonmetrically after a 5 h period of feeding, which was calculated to be sufficient for gut contents to equil~brate to the new food concentration. Experiments were carried out during the same phase of the diel cycle (05:OO to 10:OO h) and were terminated near midday, a time when feeding rate was relatively stable, in order to standardize the effects of the die1 feeding rhythm. Lower feeding thresholds of 280, 127, 282, and 214 cells ml-' respectively at 4, 8, 12, and 16 "C were indicated by segmented linear regression analysis of the relationship between gut pigments and phytoplankton concentration. These thresholds corresponded to carbon concentrations of 30.5, 11.6, 20.7, and 16.4 pg C 1-l, and were not significantly different at the 4 temperatures. Die1 changes in gut pigment content were measured at 8 and 12 "C, at a T. constricta concentration of 2000 cells ml-l. Highest gut contents occurred at night, and were ca 4-fold higher than the lowest measured gut contents. Total daily ingestion rates were respectively l l 460 and 14 430 cells d-l, or 1.34 and 2.40 ~g C and 0.24 and 0.42 pg N d-l, and 33.4 and 56 5 % body C and 23.3 and 38.5 % body N d-l. The instantaneous gut evacuation rates were also determined at the 4 temperatures, with mean values of 0.0239, 0.0494, 0.0518, and 0.0645 min-' respectively at 4, 8, 12, and 16 "C, yielding a Q," of 1.88.

Effect of temperature on lower feeding thresholds, gut evacuation rate, and diel feeding behavior in the copepod Acartia hudsonica