The difficulty of knowing exactly how much water is filtered by a net when its meshes are becoming clogged by the organisms sampled, has been got over by an ingenious device invented by Dr. Harvey of Plymouth (1934). At the mouth of the tow-net he has fixed a little propeller which is turned by the water flowing into it; the number of revolutions it makes are recorded on little dials which measure the amount of water actually passed through the net. There is still, however, the difficulty of forming a true quantitative estimate of the plant life present. We can calculate, as we have just seen, the number of plant-cells in the sample; but these vary so enormously in size it is difficult to convert such an estimate into a measure of the total bulk of planktonic vegetation. Measurements of the volume of the sample can be made after all the plants have been killed by the addition of formalin and allowed to settle for several days in the bottom of a measuring jar; but this too is a very misleading estimate, because the various kinds, having different shapes, may pack together very differently: for example spiny forms take up more space than round or flat ones. However, these various methods do enable us to say broadly that one area is relatively so much richer in phytoplankton than another—always excluding the small flagellates which escape the net and must be estimated with the centrifuge. A more recent method of estimating the quantity of plant life caught in a plankton sample is to extract the plant pigment by acetone and measure the quantity present by matching up the samples obtained with a standard colour scale and expressing it in so many pigment units.
The late Dr. E. J. Allen (1919), when Director of the Plymouth Laboratory, made a simple but important experiment that gives us some idea of the vast numbers of little plants there are in the sea which are not caught by our ordinary methods. He had first perfected a method of growing them in bottles in a special culture solution, i.e. in sea water enriched with the addition of certain beneficial chemicals. He then took a sterilised quart-sized bottle and filled it with sea water from just below the surface about half a mile outside the Plymouth breakwater. This water he treated in two ways. The procedure may seem a little involved but it is worth following. Firstly he took four 10 cc samples of it and centrifuged them each twice with the result that he obtained an average of 14.45 organisms per 1 cc of water which gives us an estimate of 14,450 per litre. Secondly he took just ½ cc of the water he had collected and added it to 1,500 cc of his culture solution which he had previously sterilized; then after it had been thoroughly shaken up he divided this between 70 small flasks—a little over 20 cc in each—and placed them against a north window. After 10 days signs of growth were apparent. When they were finally examined there was not a flask that had not had some growth in it. He now recorded the different kinds of organisms in each. In two flasks there was only one species; in all the others there were from two to seven different species present, giving an average of 3.3 different kinds per flask. Thus at least 70 × 3.3 or 231 separate plants must have been taken up in the ½ cc originally added to the culture solution; that makes 464,000 per litre as compared with the 14,450 estimated by using the centrifuge! For comparison with the larger plant forms caught by the net in the former example we must express the number as per cubic metre: i.e. 464 million, or about 12½ million per cubic foot. Now this must be regarded as an absolute minimal estimate, for it is made by assuming that only one individual of each kind of plant recorded in a flask went into that flask at the beginning; this is most unlikely.
We begin to have some idea of the great wealth of plant life there is in the sea. Can we make it still richer by adding fertilizers in the same way as we increase our crops on land? Experiments have been made in that direction, but a discussion of them will come better in the next chapter, where we will deal with the various factors which govern phytoplankton production. For a more detailed and fuller account of the pelagic plants in general I would recommend for further reading the splendid chapter by Professor H. H. Gran in Murray and Hjort’s Depths of the Ocean (1912).
1 They were subsequently well described by G. Murray and V. H. Blackman 1898).
2 Subjecting to a force greater than gravity by spinning in a rotary apparatus: the centrifuge.
CHAPTER 4 SEASONS IN THE SEA
THE NATURALIST with a tow-net, if he can sample the plankton at different times of the year, will find contrasts between spring, summer, autumn and winter in our seas almost as striking as those in the vegetation on the land. These seasonal changes in the plankton have a profound effect on the lives of many fish. Just as we can tell the age of a felled tree by the number of concentric rings in its trunk representing summer and winter growth-zones, so we can tell the age of a herring by similar rings on its scales; these mark summer growth-periods, when its planktonic food was abundant, separated by lines showing where the scale, and the fish, had ceased to grow during winter when the plankton was scarce.
There is not, however, a simple and gradual increase in the plankton as spring advances into summer followed by a gradual decline in the autumn. Our naturalist with a tow-net will find some of the changes very puzzling at first sight. In British waters in the winter there is a general paucity of both animals and plants in the plankton; then as the sunlight grows stronger (the date varying in different years, but usually in March) there is a sudden outburst of plant activity. The little diatoms start dividing at a prodigious rate: in a week they may have multiplied a hundred-fold and by a fortnight perhaps ten-thousand-fold. The meshes of the tow-net are clogged by them and the little jar at its end is filled with a brown-green slime, a slime which under the microscope resolves itself into a myriad forms of beautiful design. Then as spring advances into summer the number of little floating plants steadily declines until by late summer there are surprisingly few. Some reduction in their numbers might indeed be expected, for the little animals in the plankton which feed on them are also multiplying as the season advances and the waters are warming up; with the increasing sunlight, however, we might have thought that the plants’ remarkable power of increase could largely keep pace with the grazing of the animals. Something seems to be preventing the diatoms from keeping up that rapid multiplication. In the autumn comes another surprise. As the days begin to shorten and the sunlight is getting less intense, when in fact we might least expect a renewal of plant activity, there comes a second phytoplankton outburst; it is not as spectacular as the spring maximum and not in every year is it of an equal intensity, but there it is—a definite surging up again of reproductive power. From this second peak of production, as winter approaches, the numbers fall again to the lowest level of the year.
This sequence of events was known for a long time before it was properly understood; it was only after much more had been discovered about the physics and chemistry of the sea that it was possible to see at all clearly the chain of cause and effect throughout the year. So important are these events that we must devote a little space to considering some of the more important elements in the physical and chemical background that will help us to explain them.
Let us first consider some of the physical properties of the water. At the very beginning, in the introductory chapter, we referred to the limited transparency of the sea and some figures were given to show how quickly light is actually absorbed on its passage below the surface. As might be expected absorption of light will be found to vary considerably according to the amount of suspended matter, either sediment or plankton, in the water. Far out from the land the water is usually much clearer than in the shallower regions against the coast where detritus and mud may continually