Seasonal feeding patterns of the dominant Antarctic copepods Calanus propinquus and Calanoides acutus in the Weddell Sea

Abstract.

The diets and feeding activity patterns of two dominant Antarctic copepods, Calanus propinquus and Calanoides acutus, were studied throughout the seasonal cycle on material from several "Polarstern" cruises to the southeastern Weddell Sea. The observed differences in feeding patterns were closely linked with the peculiarities of the life-cycle strategies and changed with ambient food concentration. Calanoides acutus underwent diapause at depth and was never found feeding in winter. The winter descent of the population of Calanus propinquus was less pronounced, as some of the older copepodids stayed in the upper layer and contained food. The period of active feeding in Calanoides acutus was shorter than in Calanus propinquus. The former did not start feeding before late in spring. Feeding activity at the main study region (south of 67°S) was compared with that at the northern (60–67°S) stations with higher food abundance, where feeding activity of Calanus propinquus was high even in winter. Calanoides acutus contained a higher proportion of diatoms and unidentified mass in the gut, while the Calanus propinquus diet included more dinoflagellates, proto- and metazoan prey. The proportion of unidentified mass decreased in the older developmental stages and from winter towards autumn; it was higher in the deep layer. Gut contents of the two species were compared with seasonal distribution of the food items in the environment. The differences in feeding of the two species are directly connected to a higher degree of carnivory in Calanus propinquus, especially when phytoplankton food was scarce, and reflect the specificity in the life-cycle strategies, which is rigid in Calanoides acutus, and more flexible in Calanus propinquus.