SEASONAL-CHANGES IN MAXIMUM INGESTION RATE OF ACARTIA-TONSA IN NARRAGANSETT BAY, RHODE-ISLAND, USA

Maximum ingestion rate (I(max)) in Acartia tonsa females from Narragan sett Bay, Rhode Island, USA, when measured under standardized conditio ns of temperature (20-degrees-C) and food (the diatom Thalassiosira we issflogii), varied by a factor of 2 to 3 (22 000 to 50320 cells copepo d-1 d-1, 6.00 to 18.6 mug C copepod-1 d-1, and 1.33 to 3.32 mug N cope pod-1 d-1, or 121 to 376% final body C d-1, and 90 to 245% final body N d-1). Overall mean values were 38200 cells copepod-1 d-1, 10.3 mug C cope-pod-1 d-1, 1.96 mug N copepod-1 d-1, 203% final body C d-1, and 146% final body N d-1. Copepods gained weight during laboratory incuba tions, and consequently I(max) averaged 25.6 % and 19.9 % higher as a percentage of initial, than of final, body C and N. I(max) was most st rongly related to the residual effects of field temperature, and secon darily to in situ food level, and initial body weight and condition fa ctor (CF: weight per unit length). Weight and CF were strongly affecte d by the degree of food limitation. I(max) was highest in copepods wit h low initial body weight and CF, and from the poorest food conditions in the field. This compensatory increase in I(max) resembles the hung er response described for other copepods, and would enable food-limite d A. tonsa to more effectively exploit transient plankton blooms. Cope pods increased significantly in both weight and CF during the 24 h lab oratory incubations, demonstrating that body size was food limited eve n during plankton blooms. Mean weight increments over 24 h were 27.8 % C, 20.3 % N, and 22.6 % dry weight. The amount of growth, and the gro wth efficiency, were inversely related to initial CF. Thus copepods th at were most severely food limited in the field not only exhibited hig her I(max) and higher growth rates, but also allotted a greater fracti on of ingested energy to growth, when provided with excess food in the laboratory.