The Open Sea: The World of Plankton. Alister Hardy

the seasons. Water, above 4°C, expands when warmed and contracts when cooled; so its density is altered: a given volume of cold water weighing more than the same volume of warm water. In winter the atmosphere is colder than the sea so that the surface waters are cooled and therefore sink beneath the warmer and less dense layers which were below; this is repeated again and again until after a time there is an almost uniform low temperature from top to bottom. The winter gales help in the process of mixing up the layers too. The sea, of course, is rarely so cold in winter or so warm in summer as is the atmosphere; as we have already noted, it gains and loses heat much more slowly. As spring passes into summer the air warms up and the radiant heat of the sun gets stronger, so we find the upper layers of the sea becoming warmer too; as they heat up they become increasingly lighter than the layers below and thus tend more and more to remain separated on the top because less and less are they likely to be mixed with the heavier waters beneath. This division between the upper and lower waters is called a discontinuity layer (or thermocline in still more technical language) and is usually set up at a depth of round about 15 metres. Let us take an actual example from the summer temperatures in the English Channel in July as found by the hydrologists of the Plymouth Laboratory. At depths from just below the surface down to 15 metres the temperature only varied from 16.5° to 15.82°C; but at 17½ metres it had dropped to 12.09°C and then, as it was sampled deeper and deeper, it remained practically constant to read 12.03°C at 60 metres. The upper layer was effectively cut off from the lower by this sudden drop in temperature of nearly 4°. A strong summer gale may destroy this discontinuity layer, but if it is not too late in the season it will soon form again. It is in the autumn that the air cools again and so the surface water loses heat; also the equinoctial gales stir up the sea and the more uniform temperatures of winter again become established from top to bottom. It will be noted that this warm summer upper layer corresponds very closely to the region (sometimes called the photic zone) in which the little plants get sufficient light to carry out effective photosynthesis. Two bits of the puzzle seem as if they would fit together; we require, however, yet another piece to go with them before we can see the explanation of the seasonal changes in the plankton. This last link concerns certain salts in the sea, and to them we must now turn.

      First we must consider the general saltness of the sea; this, of course, is mainly due to the abundant sodium chloride which accounts for almost 77.8% of the total salt content. However there are many other salt constituents, of which the next more important, in order of descending quantity, are magnesium chloride (10.9%, magnesium sulphate (4.7%), calcium sulphate (3.6%, potassium sulphate (2.5%), calcium carbonate (0.3%) and magnesium bromide (0.2%). These proportions are actually those in which these different salts would be recovered from the sea on evaporation; their molecules as dissolved in the sea, however, would largely—some nine out of ten—be split up into their respective parts or ions: sodium and chlorine or magnesium and sulphate ions as the case may be. It is better to think of the salt constituents of sea water, as they mostly are in the sea itself, in terms of separate ions. We can tabulate the percentage proportions as follows, based upon a mean of 77 samples collected from different localities by the Challenger Expedition:

      In addition there are minor constituents, for example iron, strontium, silicates, phosphates and nitrates, which constitute together only 0.06%. The degree of saltness of the sea, or its salinity, is usually expressed in terms of the total weight of salts in grams per thousand (°/oo) grams of sea-water; it varies in the open ocean from 34°/00 in polar waters, where it is low on account of additions of fresh-water from melting ice, to 37°/00 near the equator where it is high because of excessive evaporation of water. The North Atlantic surface water as it flows round our islands has a salinity of about 35°/00, but in the southern North Sea it is diluted to some 34.5°/00 by water-drainage from the land.

      Now the important salts for our little plants are those which have only been mentioned among the minor constituents: they are the phosphates and the nitrates. Because they are present in such small quantities, it was a long time before accurate methods for their estimation could be devised; these were developed largely through the work of Drs. Atkins and Harvey at the Plymouth Laboratory just after the first world war. It had been realised that the plants of the sea must be limited, as are the plants of the land, according to Liebeg’s Minimum Law; i.e. so long as any really essential nutritive substance occurs in minimum quantities, plant production will be proportionate to the available quantities of it, even though there is a super-abundance of all other essentials. This seemed obvious enough but could not be proved until we had these more refined methods. It now became possible to measure the amounts of phosphates and nitrates taken up from the water by the little plants; it was shown that in our waters these salts could and did in fact limit their growth. The reproductive rate of these little plants grown in culture solutions was seen to fall off as the phosphates and nitrates were depleted and finally growth would stop altogether when they were entirely used up.

It is now possible to explain the seasonal cycle of events. In the winter, as we have seen, the waters from top to bottom are well mixed and their temperature is almost uniform. As the length of the days and the intensity of the light increases there comes a point at which the little plants can begin to multiply and they find a comparatively rich supply of phosphates available—about 40 milligrams per cubic metre of water. We have seen how rapidly they undergo fission when once they start. They are multiplying only in the upper, well illuminated zone; in the early spring these upper waters are being well mixed up with the lower layers by the equinoctial gales and there is a general reduction of the free phosphates as they pass into the plants. Presently, however, there develops an upper warm layer which becomes more pronounced as spring advances into summer; this is also the photic zone, in which alone the plants can multiply. The phosphates and nitrates are now being used up by the plants in this upper zone and are not being replaced by any mixing with the lower waters because of the difference in density between them. In fact the phosphates and nitrates are continually passing from the upper to the lower layers. The plants, which have taken up the salts, may either eventually form resting spores and sink, or may just die and sink, or more likely be eaten by the animal members of the plankton; these may themselves just die and sink or in turn be eaten by other larger animals and so on. The nutritive salts which were once present in the upper zones are now by late summer reduced to a minimum; they are carried in the falling bodies to the bottom or still locked up in animal life. Actually a good deal may be excreted back into the water by the animals,² but as most of the animals only make comparatively short visits from the lower into the upper layers to feed on the plants at night, most of the excreted phosphates and nitrates will pass into the lower waters. Thus we see that so long as the discontinuity layer lasts, the plants, such as have not been eaten by the animals, are cut off from the richer phosphates and nitrates below. That is why their numbers decline so markedly as the summer advances and why they cannot reproduce at a rate sufficient to counterbalance the inroads made upon their population by the grazing animals. Down below, the supplies of phosphates and nitrates are being to some extent built up again by their return from dead animals broken down by bacterial action. Now, as the summer wanes, the upper layers are cooled again and the autumn equinoctial gales assist in a general mixing; the water richer in phosphates and nitrates is brought up from below towards the surface where once again we have a fertile layer while the sunlight is still strong enough to encourage photosynthesis.

      Here at last we have the explanation of that autumnal outburst of phytoplankton which had for so long been such a puzzle. The time of its appearance and the quantity produced vary markedly in different years; it is usually not very long-lived and eventually the population of plants dwindles to a winter minimum as the light gets too weak to allow much active reproduction. The winter gales stir up the water and the nutrient salts are once again more or less evenly spread through the different layers of water. The temperature, too, is more or less uniform; the cycle is complete.

      This brief account of the events throughout the year has dealt with the phytoplankton as a whole. If it suggests, as well it might, that all the different kinds of little plankton plants are increasing and declining together, as the seasons come and go, it would be giving a very false picture. There is in truth a succession