The Open Sea: The World of Plankton. Alister Hardy

      FIG. 8

      The varying saltness (31.0 0/00 to 35.4 0/00) of the surface waters and the main circulation typical of the North Sea and English Channel in winter. Drawn from a chart kindly provided by Commander J. R. Lumby, of the Fisheries Laboratory, Lowestoft.

      Just as the pendulum is deflected in relation to the objects in the hall, so any body of water in motion tends to be deflected to the right in the northern hemisphere and to the left in the south in relation to the surrounding land masses and the ocean floor; account has to be taken of it in every practical treatment of tides, wind drifts and ocean currents. Whenever a water-mass meets an obstruction, either a mass of land or an opposing water-mass, it will, other things being equal, turn to the right in the north rather than to the left, and vice versa in the south. There is another important effect. We have seen how through differences in temperature and saltness the water varies in density; the lighter water will naturally be on top. In a current system the water of a particular density—say the lightest water at the top—is not lying in a layer of uniform depth; owing to the earth’s rotation the lighter water is pushed more to the right-hand side of the current stream than the heavier water, so that imaginary surfaces separating waters of different density are not horizontal but tilted. It is from a consideration of the deflection of waters of different densities that the speed and direction of ocean currents can be mathematically worked out as mentioned earlier in the chapter.

With this slight introduction, intended merely to give an idea of the kind offerees at work to produce the circulatory ocean-systems, we may now briefly review the main streams of water in the seas around our coasts. Fig. 8, is based on the account by Comd. J. R. Lumby (1932), hydrologist at the Fisheries Laboratory, Lowestoft, with a small revision which he has kindly made in the drawing for this figure. The water in the North Sea and English Channel is slightly less salt than the Atlantic Ocean water; it is typically coastal water diluted by freshwater drainage from the land. The Baltic has a much lower salinity still. A stream of Atlantic water flows into the North Sea from the north, mainly passing round to the east of the Shetland Islands to flow due south and not usually entering between the Orkneys and the Shetlands as was originally thought; a less powerful stream flows up the English Channel and enters it from the south. The northern influx is generally thought to flow on a broad front down the middle of the North Sea forming, as it goes, swirls off the coast of Scotland especially in the Moray Firth and in the region of the Firth of Forth. Dr. J. B. Tait of the Scottish Fishery Department has in recent years, however, put forward the view (1952) that the main streams are much narrower than formerly supposed—more like rivers flowing in the sea. Which is the correct view is at present by no means certain; some evidence from plankton distribution appears to support one view and some the other. Just before reaching the Dogger Bank the main stream, whether broad or narrow, appears generally to divide into three branches: one running south-westerly, another south-easterly and a third turning east to enter the Skagerrak. The south-westerly and south-easterly branches form large swirls in the southern North Sea as they meet the stream of water entering from the Channel. Another smaller swirl is formed outside the Skagerrak as the stream entering on the southern side meets the stream flowing out of the Baltic on the northern side.¹ The stream entering the North Sea from the Channel flows north-eastwards past the Dutch and Danish coast and some of it joins the stream going into the Baltic. Most of the North Sea is shallow, but there is a deep hollow running up the western coast of Norway to the north; it is along this Norwegian trough that the water leaves the North Sea—the less saline water from the Baltic on the top and the bulk of the North Sea water proper in the deep channel below.

The extent of the inflow of Atlantic water varies from year to year; such variations affect the distribution of the plankton and are likely to influence the distribution of the herring shoals which depend upon the plankton for food. In some years of exceptional influx numbers of plankton animals usually only found in the more open ocean make their appearance in the northern North Sea. There is some evidence to support the view that it is the pressure of this water from the north (produced by the main wind systems) which, apart from the occasional effects of local winds already referred to, controls the inflow of Channel water into the southern North Sea. If the pressure from the north is high it seems that the Channel flow is reduced; if it is weak then a larger influx from the Channel seems to take place. It is in the study of this inflow into the northern North Sea that the charting of the relative movements of certain Atlantic plankton animals in different years can be most helpful. We shall see in a later chapter (see here) how, by the use of plankton-recording machines towed at monthly intervals by commercial steamships on regular routes, we can compare the areas of invasion of these more oceanic forms in different months and years. We shall find that not only does the extent of the Atlantic inflow vary from year to year, but the time of the advance of typical invading organisms will vary: in some years it may be a month earlier or later than in other years. There is an interesting suggestion now being investigated that the time of the appearance of the shoals of herring at different points down the east coast of Scotland and England, and consequently the time of the different fisheries, may be earlier or later in different years depending on whether this Atlantic inflow is earlier or later.² Whether this indication—it is no more at the moment—will be proved correct or not, there can be no doubt that the fluctuations that are found to occur in the water movements round our islands must have a profound effect upon the fish and other life inhabiting our seas.

A more definitely established connection between water changes and fisheries has been demonstrated at the western entrance to the English Channel. The water of the greater part of the Channel is like that of the southern North Sea—coastal water which is less saline and less rich in plankton than the Atlantic water that flows into it. This more coastal water can readily be distinguished from the more oceanic water by the presence of certain of these indicator plankton species—particularly two species of Sagitta, the slender transparent arrow worm shown in Plate IX; Sagitta setosa being found in the coastal water and Sagitta elegans in the more oceanic water.³ The boundary between the two waters formerly used to lie somewhere in the region of Plymouth where sometimes the plankton would have elegans predominating in it and sometimes setosa) during the investigation up to 1929 it was more usually elegans, indicating Atlantic water richer in phosphates and other nutrient salts. The importance of these salts in the economy of the sea will be discussed in Chapter 4. Since 1929 the boundary between elegans and setosa water has lain much further to the west so that the water off Plymouth has been of the coastal type and much poorer in plankton. Since this date there has also been a marked reduction in the number of young fish of many kinds present in the plankton as well as a change in the herring fishing; since that time the herring which used to visit the Plymouth area around Christmas have not turned up in their usual numbers so that this winter fishery, once quite a prosperous one, now no longer takes place. An excellent account of this trend was given by the late Dr. Stanley Kemp in his presidential address to the Zoology Section of the British Association in 1938. More recently some other