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-degrees, 8-degrees, 12-degrees, and 16-degrees-C with the solitary diatom Thalassiosira constricta as food. Narrapnsett 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-mu-g 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 (mu-g copepod C)-1d-1 between 4.5-degrees-C and 16-degrees-C (Q10 = 1.8). Feeding rates at 4.5-degrees-C and 8-degrees-C were similar; the seemingly low ingestion rates at 8-degrees-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 approximately 15% C and 12% N d-1 at 4.5-degrees-C to 25% C and 17% N d-1 at 16-degrees-C.