The Open Sea: The World of Plankton

The Dinoflagellates are the most striking members of the phytoplankton after the diatoms and are usually present in large numbers; for a full account of them another excellent volume by Dr. Marie Lebour (1925) should be consulted. They have a cell wall made up of a number of plates of cellulose fitting together to form a mosaic and are characterised by possessing two flagella: one working transversely in a prominent groove which almost completely encircles the body like a girdle, the other projecting behind from out of a small longitudinal groove running backwards from the girdle. This latter groove is often protected by curtain-like membranes so that the flagellum may be withdrawn spirally into a sheath. Typically, as in Peridinium, they have a single spine pointing forward in front and two spines projecting backwards from the half of the body behind the girdle. The flagellum working in the groove sets them waltzing round as they are at the same time driven forwards by the other flagellum behind (Fig. 15); they screw themselves through the water. There are a great many species of Peridinium and closely allied genera with very much the same general appearance (Plate IIIb); one related genus, however, Ceratium, is most striking in having the spines drawn out into long horns, with the two posterior ones usually curving forwards to give the whole body the shape of a little anchor. Sometimes, particularly in late summer and autumn, plankton samples may be full of Ceratium tripos or perhaps another of the many species of the genus distinguished by only small differences (Plate Ib). Two species which are very common in our waters stand out in contrast to the rest: Ceratium furca (Plate 1) in which the posterior spines are rather short and point straight backwards, and C. fusus (Fig. 15) in which there is only one posterior spine, long and only very slightly curved, just like the anterior one. There is a remarkable range of colour in Ceratium from a bright green to a yellow-brown.

Dinoflagellates, like other flagellates, multiply by simple fission into two and, like diatoms, each daughter-cell retains one half of the old cell-wall and forms another half anew; but unlike the diatoms the new halves are not formed within the old cell wall and so there is no gradual diminution in size. Occasionally recently-divided individuals of Ceratium may remain adhering together to form little chains. The cell-wall grows in thickness but when the little plates of armour become too heavy they fall off to be replaced by new and extremely thin ones. The long horns of Ceratium are certainly organs to assist in suspension. Species in the warmer waters generally have much longer ones than those in colder waters. Warmer water is more fluid—or less viscid—than colder water, for the molecules move over one another more freely with greater heat-motion; in the tropics an object will sink twice as fast as will one of similar density and shape in the polar seas. In general plankton organisms, both plants and animals, are more spiny in the tropics to give them a greater surface resistance. It has been observed that species of Ceratium have the power of adjusting the length of their horns to the varying viscosity of the water; on being carried by currents into warmer water they grow longer horns, conversely if carried into a colder area they can shed parts of them. (Gran 1912).

Several species of Dinoflagellates can produce a brilliant phosphorescence. Many plankton animals are luminous and produce the sparks of light we often see in the water at night. In addition to such displays, however, we may also see a more general ghostly light or sometimes when out in a rowing boat our oar as it cuts the water may leave a trail of blue-green flame behind it; and even from the shore we may see the waves breaking in a flash of light. Such displays are caused by countless millions of dinoflagellates each glowing by an oxidation process as it is agitated in the water. Noctiluca is the most celebrated for this, but although a dinoflagellate it is curiously modified to be entirely animal in its mode of life and so will be described in a later chapter (see here); other members of the group, however, particularly Ceratium, give almost as good a display. Once on a fisheries research trawler, having stopped at night to make some observations in the Channel, I looked over the side to see a small shoal of fish, most likely mackerel, lit up by each individual being covered by a coat of fire; they were being chased this way and that by some much larger fish similarly aflame. On putting over a tow-net, which came up brilliantly illuminated, the sea was seen to be full of a very small Peridinium-like dinoflagellate of the genus Goniaulax.

Two other genera of dinoflagellates occuring in our waters, and shown in Fig. 15, will just be mentioned. Dinophysis has the part of the body in front of the girdle reduced to a minimum so that the girdle itself, with very pronounced margins to its groove, appears like a band round its very front or top; the posterior part bears a marked keel at one side as if designed to prevent the rotation which is normal to the group. Instead of spinning round it is thought to set up a vortex current by which it draws into the groove still smaller organisms as food. Polykrikos is a remarkable genus having a number of girdles, usually four or eight, placed in regular succession down the body; it is often spoken of as a ‘colonial form’ as if made up of several individuals which have failed to separate on division, but this can hardly be the correct view since the number of nuclei is always smaller than the number of girdles present. It appears to be an individual with a repetition of organs similar to the segments of an animal like an annelid worm. They are also said to feed like animals as well as like plants; they possess remarkable little capsules containing coiled threads which can be shot out like those found in the stinging cells of sea anemones and jelly-fish, and may possibly be used for a similar purpose—the capture of prey. In addition to all these forms with their different characteristic patterns of armour plating and spines, there are a great many so called ‘naked’ dinoflagellates which lack all such coverings; many of these are, for part of their lives, internal parasites in a number of different marine animals.

Among the small shells of Globigerina first brought up from the ooze of the ocean bed were found numbers of still smaller calcareous bodies, little plates, some oval and perforated, others round and bearing stout blunt spines; they were called coccoliths and rabdoliths respectively, and presented naturalists with a puzzle as to what they were. It was Sir John Murray who discovered their real nature by showing them to be plates which had covered the bodies of other little plank-tonic flagellates which were given the name of Coccolithophores.1 Coccosphaera and Coccolithus (Fig. 15) occur in our seas. They are commoner in the tropics, although in the Atlantic water coming into the northern North Sea they may occasionally be so numerous as to give a milky appearance to the water and cause a chalky deposit to be left on the fishing nets as they dry. This is what the herring fishermen call ‘white water’ and generally believe to be a good sign for the presence of herring. A well-known herring skipper, Mr. Ronald Balls, who is also a keen naturalist, has recently written, under the pen-name of “Peko”, an excellent article on this white water in World Fishing (July 1954). He describes how this water gives ‘the queer impression of whiteness coming upwards: as if the light was below the sea instead of above it’. He then refers to recent views that the coccoliths are shields reflecting light from their owners which normally live in tropical seas where the illumination is too strong; ‘and here’, he writes, ‘was the perfect explanation of the fairy glow or white reflection that I had experienced long ago, and wrote about before I knew even that this organism existed’. As with the cell walls of diatoms, the electron microscope is showing that each little plate or coccolith has a much more complicated structure than was originally supposed; its base consists of radiating ribs like the spokes of a wheel and its rim is decorated with a frill like that with which a chef may decorate a ham. There are other similar little creatures, the Silicoflagellates, which form a delicate siliceous skeleton with radiating spines (Fig. 15, i and j).

Herring nets, although they hang in the water near the surface, may often come out of it in a very slimy condition; this may be due to an excessive number of diatoms; more usually, however, it is due to globules of jelly large enough to be seen quite easily by the unaided eye. These slimy blobs are produced by aggregates of microscopic flagellates called Phaeocystis which colour the surface of the jelly in green patches (Fig. 15k). All the meshes of a tow-net may be blocked with them. Dense concentrations of Phaeocystis, like those of diatoms, which cover wide areas of sea, have also been thought to have a deleterious effect on the shoaling of herring and at times to have led to a poor or delayed fishery. We shall refer to this again in Chapter 15.

This completes the review of those planktonic plants we shall mention by name; but we have so far left out of account a vast number of still much smaller flagellates which have escaped capture by passing through the meshes of the finest net we can use. It is only comparatively recently that their influence in the economy of the sea has been realised. Their prominence was first demonstrated by the German naturalist Lohmann who examined the remarkably fine filtering mechanism, far finer than any gauze that man can make, used for their capture by some little plankton animals, the Larvacea, to be described here. They may, however, be extracted from a sample of sea water by centrifuging2 small quantities of it in tapering tubes. If after such treatment the greater part of the water is carefully decanted, a drop of the remaining fluid may be taken up in a pipette and examined on a slide under the high power of a microscope; then they will be seen as tiny yellow specks jigging in the water Today a great many of these minute flagellates are being successfully cultured in the laboratory, notably by Dr. Mary Parke at Plymouth (1949). Five examples, sketched in line from her beautiful coloured drawings, are shown in Fig. 15 n–r.

Smaller still, of course, are the bacteria which really lie outside the scope of this book; at present, very little is known about their occurrence in the plankton. Dr. H. W. Harvey (1945) states that their population density decreases on passing from inshore waters to the open sea and that in the ocean the greatest numbers are found where phytoplankton is abundant and in the water immediately above the sea-floor. They are found particularly in dense phytoplankton regions because of the undigested organic matter passed out by the animals which are eating more of the plants than they really require.

It is exceedingly difficult to get an accurate measure of the amount of plant life in a given quantity of sea water, even of the larger forms which are captured by a net. Although we can calculate the filtering efficiency of the net and know the quantity of water it should filter, we cannot be sure that it actually does filter this amount; in fact it rarely does, for to a varying extent under different conditions the meshes of the net become clogged by the organisms themselves and the filtering is much reduced. However we can get an approximate idea of the number of larger forms—the diatoms and dinoflagellates—in a given volume of water by using a net. Let us take an example. In 1907 Sir William Herdman and his co-workers began an intensive study of the plankton of Port Erin Bay in the Isle of Man which they continued until the end of 1920. Usually six times a week, every week for fourteen years, two standard nets of coarse and fine mesh were towed in exactly the same way over the same distance—half a mile—across the bay. Johnstone, Scott and Chadwick, who describe the results in their book The Marine Plankton (1924), estimate that for each such double haul “taking the two nets we shall not be very much in error (when all the conditions are considered), in assuming that 8 cubic metres of water were filtered through both nets.” The following figures, taken from their book, give the average number of the principal plant forms taken in such a catch during the month of April, i.e. the average of all the April hauls made over fourteen years:

That in round figures is 727,000 per cubic metre or about 20,000 per cubic foot. The number of plankton animals taken at the same time is given in Chapter 5 where the zooplankton is considered and may be compared here. The actual numbers present are estimated by using a specially calibrated pipette which takes up a known fraction of the sample; the fraction is spread out on a glass slide ruled in squares so that the number of plant cells can be counted below the microscope just as the corpuscles are counted in a sample of blood. We must remember two important things about the figures just given. Firstly they are for the larger microscopic plants; the very small ones are present in far greater numbers as we shall see in a moment. Secondly they are average figures for April over fourteen years; those for one year may be very different from those of another and the average figures for other months of the year will show still greater differences. There are marked seasonal changes in the plankton; but that is the subject of our next chapter.

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 be stirred up by the tides or brought in by drainage from the land. If we compare measurements of light made at different depths below the surface in the waters near Plymouth we find the penetration in inshore waters at Cawsand Bay to be only half what it is at a point some 10 miles S.W. of the Eddystone Lighthouse (Poole and Atkins, 1926). Taking the light entering the sea, i.e. just below the surface, as our standard, we find in Cawsand Bay that half of it has been absorbed at 2 metres depth (i.e. 1 fathom), some 75% at 4½ metres and 90% at about 8 metres depth; whereas at 10 miles out the same percentage reductions in light intensity are found at depths of about 4¼, 9½ and 17 metres respectively. At points in the open sub-tropical or tropical Atlantic, where the phytoplankton is very sparse, as in the Sargasso Sea, the corresponding depths might be increased four or five times. A very simple bit of apparatus known as the Secchi disc, which can easily be home-made, will enable you to compare the transparency of the sea at different points; you may well be surprised at some of the results you will get with it. Take a white painted metal disc, say two feet in diameter, and drill three equidistant holes near its margin; now take three cords each about 6 feet long, tie them to a weight, say a 7–1b lead, and then tie a knot in each at 3 feet from the lead; next pass the cords through the holes in the disc and bring them together as supporting bridles to be tied to a loop or eye at the end of the line which will suspend the whole device in the water as shown in Fig. 16. If a metal disc cannot easily be obtained, a white dinner plate, with wire clips behind it to take the cords will serve the purpose quite well. To compare the transparency of the sea all you have to do is to stop your boat and lower the disc over the side at different places and find how deep it must go before you can no longer see it. The weight not only carries the disc down but, if the cords are properly adjusted, ensures that it is always kept horizontal. The line can be knotted at metre intervals to facilitate measuring the depth. It is well to raise and lower it about the disappearance point several times in order to make quite sure just at what depth it goes out of sight; at some places it may vanish in only 5 metres, at others it may be seen for as much as 12 metres or more.