Seasonal changes in maximum ingestion rate of Acartia tonsa in Narragansett Bay, Rhode Island, USA

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Maximum ingestion rate (Imax) in Acartia tonsa females from Narragansett Bay, Rhode Island, USA, when measured under standardized conditions of temperature (20°C) and food (the diatom Thalassiosira weissflogii), varied by a factor of 2 to 3 (22 000 to 50320 cells copepod-1 d-1, 6.00 to 18.6 µg C copepod-1 d-1, and 1.33 to 3.32 µg N copepod-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 µg C copepod-1 d-1, 1.96 µg N copepod-1 d-1, 203% final body C d-1, and 146% final body N d-1. Copepods gained weight during laboratory incubations, and consequently Imax averaged 25.6 % and 19.9 % higher as a percentage of initial, than of final, body C and N. Imax was most strongly related to the residual effects of field temperature, and secondarily to in situ food level, and initial body weight and condition factor (CF: weight per unit length). Weight and CF were strongly affected by the degree of food limitation. Imax was highest in copepods with low initial body weight and CF, and from the poorest food conditions in the field. This compensatory increase in Imax resembles the hunger response described for other copepods, and would enable food-limited A. tonsa to more effectively exploit transient plankton blooms. Copepods increased significantly in both weight and CF during the 24 h laboratory incubations, demonstrating that body size was food limited even 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 growth efficiency, were inversely related to initial CF. Thus copepods that were most severely food limited in the field not only exhibited higher Imax and higher growth rates, but also allotted a greater fraction of ingested energy to growth, when provided with excess food in the laboratory.

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Marine Ecology Progress Series