Biophysical dynamics of western transition zones: a preliminary synthesis

The nature of the western portions of the biogeographic temperate or transi tion zones in the North Pacific and North Atlantic is reviewed. The physica l transport of nutrients and biomass into them from the Kuroshio and Gulf S tream as well as from the poleward sides are estimated. The conclusion is t hat the upwelling in the two western boundary currents makes the largest co ntribution to the nutrient and biomass fluxes into these transition zones. A conservative estimate of the amount of upwelled fluid is derived from abs olute velocity sections in the Gulf Stream. The estimate suggests that upwe lling into the euphoric zone exceeds 2 x 10(6) m(3) s(-1). This implies tha t upwelling in these western boundary currents matches or exceeds that in e astern boundary cur-rents such as the California Current. The two western b oundary regimes have very different poleward situations. The Oyashio extens ion flows parallel to the Kuroshio and is a deep current. The North Atlanti c Shelf Front flow is to the west where it is ultimately entrained into the edge of the Gulf Stream. There does not seem to be any tendency for this t o occur in the Kuroshio. Despite these differences in the northern and west ern boundaries, the two transition zones are similar with large amplitude m eanders, anticyclonic rings and streamers dominating their physical structu re. The physical features responsible for the transfer of materials from th e boundary current extensions into the transition zones are similar in both systems. Ring formation contributes only similar to 10% of the transfer, w hile ring-induced streamers contribute 30%. The rest of the transport is co ntributed by branching of the boundary current front. Both currents have we ll developed secondary fronts consisting of subtropical surface water pulle d into the transition zone. Biologically, the upwelling in both western boundary currents leads to a bi omass maximum along the boundary in both secondary producers (copepods) and in small pelagic fish. In the Kuroshio, the latter are the Japanese sardin e, Sardinops melanostictus, that spawn in the Kuroshio and then enter the t ransition zone for the summer and fall months. In the Gulf Stream, the domi nate species are menhaden, Brevoortia tyrannus and B, smithi. These species make use of the coastal environments of North America and although the adu lts spawn in the Gulf Stream, they are not thought to play a major role in the Slope Water, transition zone. The similar differences in the use of the Kuroshio and the Gulf Stream ecosystems occurs in the behaviour of bluefin tuna, squid and other large pelagics. The Gulf Stream system also lacks an equivalent to Pacific saury, Cololabis saira. The biology therefore is at least subtly different, with saury and sardines being replaced by mid-water fish in the North Atlantic. A fuller comparison of the biology with quanti tative methods in both systems should be encouraged.