Mesoscale coupling between planktonic cnidarian distribution and water masses during a temporal transition between active upwelling and abatement in the northern Benguela system

Abstract

The composition and spatio-temporal distribution of planktonic cnidarians in the northern Benguela system are described from plankton samples collected on four cruises between December 1981 and March 1982. The sampling period was characterized by initially active coastal upwelling, followed by abatement of upwelling. In all, 20 species of medusae and 24 species of siphonophores were identified. The most abundant medusae were Chrysaora hysoscella, Aequorea aequorea and Liriope tetraphylla, and the most abundant siphonophores were Muggiaea atlantica, Abylopsis tetragona and Bassia bassensis. Three assemblages of planktonic cnidarians were differentiated. The first was associated with coastal upwelling and consisted of the medusae Proboscidactyla menoni, Aglauropsis edwardsii and juvenile C. hysoscella, along with the siphonophore M. atlantica. The second assemblage was associated with oceanic water and the component species were indicative of the offshore influence of the Benguela system; L. tetraphylla and most of the siphonophores belonged to this assemblage. The third assemblage consisted of species typical of shelf fauna, namely A. aequorea, Mitrocomella grandis and adult C. hysoscella, all also typical of the northernmost waters of the Benguela system. Overall, there was coupling between mesoscale hydrographic features and the pattern of distribution of cnidarian populations, with a distinct cross-shelf gradient that was more pronounced during active upwelling