The boundary circulation along the European continental slope as transport vehicle for two calanid copepods in the Bay of Biscay

Abstract

The copepod fauna south-west of the British Isles includes a number of species which have their centre of distribution in the subtropical or tropical Atlantic. Among these, some species appear to be faunistically linked with the Iberian continental slope or slope waters even farther south along the NE Atlantic margin. It seems that these species reach their north-eastern range by means of poleward-flowing currents. For example, Calanoides carinatus has deep-living resting stages and ascends to surface waters only during its reproductive period, which is short at northern latitudes. Its main dwelling layers along the west Iberian continental slope cover the poleward-flowing undercurrent and the Mediterranean outflow water underneath. Part of the poleward flow regime tends to follow the slope bathymetry, i.e. it bends eastwards into the Bay of Biscay along northern Spain. Another part separates from the continental slope at Cape Finisterre and crosses the Bay of Biscay, approaching the continental slope at about 47° N, where it joins the north-westward flowing slope currents again. Because of its preference for the deeper layers during the long resting phase it is assumed that resting C. carinatus drifts mainly with the northward branch of the Mediterranean outflow water and penetrates only slightly into the inner Bay of Biscay. In contrast, Calanus helgolandicus has, on average, a shallower vertical distribution. Reproductively active individuals occur in the surface layers off western Iberia throughout the year. Here it drifts presumably northwards during late autumn and winter, but is recirculated southwards during late spring and summer. Due to advection with both the intermediate undercurrent and the variable near-surface currents, Calanus helgolandicus occurs more commonly in the inner Bay of Biscay than C. carinatus.