Gulf Stream;
2 S. setosa: a shallow-seas form, usually the dominant in most of the
nearshore or shelf waters of Western Europe;
3 S. elegans: occurs in the zone of mixing between the two. It is closely
related (the same or called by the same name) to a species of the oceanic
sector of the Atlantic (well north of the West Wind Drift) of the Arctic
Ocean, the Bering Sea, and subarctic Pacific.(^) At any given site, say Plymouth, UK, the species present allow identification of the
advective sources of water resident at any given time. Oceanographic conditions
correlate with appearances of a given indicator species. Appearance of S.
serratodentata off the British Isles usually corresponds to warmer than usual coastal
temperatures, low phytoplankton nutrients, low primary productivity and poor fishing.
(^) This idea can be applied to any coastal region, and it has been applied along the US
West Coast. The California Current is not a simple, one-way flow from north to south.
Rather it has a complex and varying structure involving a northward undercurrent and
seasonal oscillations of the direction and speed of nearshore flow. In spring and
summer there is southward and seaward surface flow, the offshore component
producing nearshore upwelling. The intensity and timing vary with latitude. Satellite
pictures show large onshore-to-offshore jets as a feature of upwelling. Winter flow is
northward and onshore. During El Niño events the winter pattern is more extreme and
can persist into summer, often characterized as a difference between warm years and
cold years (Fig. 10.24).
Fig. 10.24 Comparison of 10 m isotherm patterns in the California Current area
between (a) a typical year, 1962, and (b) an El Niño or warm year, 1958. Offshore, at
130°W, say, the positions of the 14° to 18° isotherms are not very different. But
inshore they are strongly changed between these two situations.
(^) (After Anonymous 1963; Wyllie & Lynn 1971.)