Effects of temperature and food abundance on grazing and short‐term weight change in the marine copepod Acartia hudsonica

Ingestion and short‐term weight change in adult female Acartia hudsonica were investigated at 4.5°, 8°, 12°, and 16°C with the solitary diatom Thalassiosira constricta as food. Narragansett Bay copepods were preadapted to the desired temperature and saturating food level for 3 d to standardize feeding history before the experiments. Maximal ingestion rates at the four temperatures were 16,460, 14,120, 21,470, and 29,960 cells copepod‒1d‒1 or 43.9, 37.2, 67.9, and 92.9% body C(Q10 = 2.3) and 34.2, 30.7, 42.6, and 74.7% body N d‒1 (Q10 = 2.4). The critical concentration varied between 840 and 1,900 cells ml‒1 (0.17–0.23 µg C ml‒1) and was not significantly related to temperature. Maximal clearance rate was similar at all temperatures (20.8–23.9 ml copepod‒1d‒1), but on a weight‐specific basis increased from 3.3 to 6.0 ml (µg copepod C)‒1d‒1 between 4.5° and 16°C (Q10 = 1.8). Feeding rates at 4.5° and 8°C were similar; the seemingly low ingestion rates at 8°C were interpreted as evidence for a senescent population of adult copepods in the bay from late April to early May. During preadaptation at high food, A. hudsonica body C and N either stayed constant (indicating saturating food in the field) or increased (indicating food limitation in situ). Body weight was sensitive to variation in food supply; during the 24‐h feeding experiments weight remained stable at the two highest food levels, but declined significantly at lower levels. Maximal observed weight loss was temperature‐dependent, increasing from ∼ 15% C and 12% N d‒1 at 4.5°C to 25% C and 17% N d‒1 at 16°C.