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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 (#litres_trial_promo)); 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 (#litres_trial_promo), 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.
(#ulink_915325a6-6fdf-5ea6-8c39-8c85b71158f3)Coccosphaera and Coccolithus (Fig. 15 (#litres_trial_promo)) 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 (#litres_trial_promo), 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 (#litres_trial_promo)). 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 (#litres_trial_promo).
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 (#litres_trial_promo). They may, however, be extracted from a sample of sea water by centrifuging
(#ulink_a62ac299-94ae-50d9-a773-49ac31ea8893) 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 (#litres_trial_promo)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 (#ub9d1ef9b-f5a9-51ce-96c7-37d3c4e2a8de) where the zooplankton is considered and may be compared here (#litres_trial_promo). 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).
(#ulink_2b4c27ff-fcc8-5b2f-a31d-36f3f0c6dd4b) They were subsequently well described by G. Murray and V. H. Blackman 1898).
(#ulink_3aa26175-8e8a-51fc-a388-e5531838abd4) Subjecting to a force greater than gravity by spinning in a rotary apparatus: the centrifuge.
CHAPTER 4 (#ulink_a3346a5f-9582-5e90-b849-8ef1e018c7ee) 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 (#ubcb89914-6a12-5f9a-961d-a32f7a64e31a), 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 (#litres_trial_promo). 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.
FIG. 16
The Secchi disc for measuring the transparency of the sea.
In passing I may say that the Secchi disc is also valuable in helping us to compare the varying colour of the water. Here, of course, I am not thinking of the often striking and delightful changes of hue which we may see as light of different quality is reflected from a changing sky: when for example dark grey clouds give place to an open space of blue or when cumulus clouds dapple the sea with purple shadows. Reference was made in Chapter 2 (#u3397ab9e-de11-5382-badf-93137316dee4) to the contrast between the green water of the Channel and the deep blue of the Atlantic; such differences are due to the nature of the contents of the sea itself and are examples of what I mean by the varying colour of the water. A great variety of shades and hues may be found at different times; these are mainly due to the presence of different kinds of very small plankton organisms in exceptional numbers. The light reflected back through the water from the white background of the disc enables us to judge and compare these differences more easily than by just looking into the depths. Dense concentrations of diatoms such as Rhizosolenia and Biddulphia, or of the colonial flagellate Phaeocystis, may give a brown appearance to the water over large areas; the North Sea fishermen often call such patches of water “Dutchman’s baccy juice”. Some dinoflagellates may make the water almost red, coccolithophores may give it the white milky appearance referred to in the last chapter, and other small flagellates may occasionally make it a vivid green.
But let us return to the sunlight and these little plants of the sea; in order to flourish and grow they must produce more oxygen in the process of photosynthesis (see here (#ulink_6442f009-7e57-5b3f-89d0-645624049396)) than they use up in respiration. Plants, of course, breathe as well as animals. Some very significant experiments were performed in the Clyde sea-area by Drs. Marshall and Orr (1928) of the marine biological station at Millport on the Island of Cumbrae. They grew cultures of diatoms in glass bottles in the sea at different depths; these they suspended on strings from a long thin rod between two buoys at the surface and so kept them free of shadow. All their bottles were in pairs; one of each pair was exposed to the light and the other covered with a black cloth. In each bottle the oxygen-content of the water was measured at the beginning of the experiment and again at the end of twenty-four hours. An increase in the oxygen in the uncovered bottles showed the amount produced by photosynthesis less that used up in respiration; a fall in oxygen-content in the ‘blacked-out’ bottles measured respiration alone. By adding this oxygen-loss to the oxygen measured in the uncovered bottles the total oxygen-production as a record of photosynthesis could be estimated. The experiments were repeated as the spring passed into summer, and were also made on days which were overcast and on others which were sunny. As the sun went higher in the sky and the light became more intense the depth at which diatoms could produce more oxygen than they used in respiration increased from a depth of less than 10 metres on an overcast day in March to nearly 30 metres on a sunny day at midsummer. By far the greatest photosynthetic activity—on which their growth depends—took place, however, in the top 5 metres. In the waters round Great Britain we may now say that practically all the plant-production that matters takes place in the top 10 or 15 metres. This is one important clue in the puzzle of the seasons; we must now turn to temperature.
FIG. 17
The Nansen-Pettersen water sampling bottle: shown open and closed.
The water round our coasts varies in temperature from about 8°C in winter to sometimes as much as 17°C in the Channel in a warm summer. It is, of course, because the sea loses and gains heat so much more slowly than the land that we in Britain have so equitable a climate compared to that of an area in the middle of a continent. Two methods are used in taking the temperature of the sea. Down to moderate depths, say to 50 metres, the insulated Nansen-Petterson water-bottle, which is shown in Fig. 17 (#litres_trial_promo) is used; it is of metal and is sent down suspended on a wire to obtain samples of water both for chemical analysis and for temperature determination. It goes down with the bottom and top open so that water can circulate through it; then at the required depth a small ‘messenger’ weight is sent sliding down the wire to hit a trigger which releases springs to close it. Projecting through the top, in a protective casing, is the stem of a thermometer whose bulb is in the centre of the sampling bottle; its scale and mercury thread are visible through a slit in the upper casing so that it can be read as soon as the bottle is brought back to the surface. There are actually three walls to the cylindrical bottle, one inside the other, with a little space between; when the top and bottom are firmly closed there are thus two water jackets outside the bottle proper and these act as insulating chambers preventing loss or gain of heat in the water sample while it is coming up and the thermometer is being read. As soon as the temperature has been noted the water is run out from a cock at the bottom to be stored for later analysis and the bottle is opened ready to be sent down to another level. From much greater depths the bottle would take so long being drawn up that the insulation just described would not be adequate to prevent a change of temperature in the process. To get over this, special so-called reversing thermometers and bottles have been devised. The mercury tube of the thermometer, just above the bulb, has a loop and a kink in it, so that when it is swung rapidly upside down the thread of mercury breaks; as soon as this happens all the mercury that before was above the kink now runs to the opposite, and now lower, end of the tube. When it is brought up the height of this inverted column of mercury is seen against a scale which can only be read when the thermometer is upside down; it tells us the temperature that the thermometer was recording at the moment it was turned over. The bottle and thermometers (there are usually two to give check readings) are mounted in a frame which rotates when a trigger is hit by a messenger weight; the bottle, which before was open, is closed as it swings over.
(#ulink_4b8d8474-868f-5ef4-9b22-459132cbd589)
After this digression on thermometers, let us return to consider the temperatures of our seas with the passing of 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,
(#ulink_01fdcb72-0ed5-537d-abcc-c78b4d4e5927) 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 of different forms which wax and wane in turn within this larger framework. As the summer advances and the quantity of the phytoplankton is declining, the dinoflagellates come to occupy a much more prominent part in the community; in late August species of Ceratium and Peridinium may be much more evident in the fine net samples than the diatoms. At the second autumn outburst the diatoms will swing back into prominence again. Then within these spring and autumn periods of production there is usually a fairly definite order of appearance of different species of diatoms as the weeks go by; not that one kind disappears entirely of course, but after a period of abundance the reproduction falls to a low ebb and the stock is maintained by only a few individuals or by the resting spores already referred to. The intensive work, already referred to (see here (#ulink_c8c1a1a8-3a7c-5c0f-9c97-cff4a75db940)), carried on week by week for fourteen years at Port Erin in the Isle of Man, has furnished us with a mine of information about these detailed seasonal changes at one place; and now the monthly plankton recorder surveys which will be described in the last chapter (see here (#litres_trial_promo)) are giving us similar information for a very wide area.
What makes one species give place to another? Why for example should Chaetoceros decipiens give way to Ch. debilis and socialis as the season advances or Rhizosolenia semispina be replaced by Rh. shrubsolei which in turn may leave the stage to Rh. stolterfothii? Whilst the grazing of the little plankton animals coupled with the reduction of phosphates and nitrates in the upper layers is bringing about the general decline in the planktonic vegetation it can hardly be controlling the rise and fall of the different species. Johnstone, Scott and Chadwick (1924) in discussing this seasonal sequence of species which they found in their long series of tow-nettings at Port Erin, made an important suggestion as to its cause.
“It is known that some bacteria are incapable of producing their typical effects (say in fixing elementary nitrogen from its solution in sea water) if they are present in pure culture. In order to function effectively they must be associated with some other organism which, by itself, cannot produce the effect in question. Probably such symbiotic relationships may exist on the great scale in the sea. The work of Allen and Nelson (1910) on the artificial culture of diatoms suggests this. In mixed cultures there is always a certain succession of species, one attaining its maximum when another has ceased actively to reproduce. The succession of diatom species during the period of the spring growth suggests that something of the same kind occurs in the sea.”
A similar effect has, of course, now been demonstrated by Sir Alexander Fleming’s great discovery that moulds such as Penicillium produce substances which inhibit the growth of bacteria. In my hypothesis of animal exclusion (in Hardy and Gunther, 1935), which will be referred to again in a later chapter (see here (#litres_trial_promo)), I have suggested that dense concentrations of planktonic plants may produce an effect in the water which is uncongenial to animal life and so account for the fact that animals are usually scarce in regions of great phytoplankton abundance. Dr. C. E. Lucas, my former pupil and colleague, now Director of the Scottish Fishery Laboratory at Aberdeen, has developed much further the idea of chemical interaction between organisms and stressed the possible importance of various substances given out into the water by different plants and animals as a result of their internal activities. Just as cells inside the body of an animal produce those various substances called hormones (or endocrines) which circulate in the blood stream to have profound effects on other parts of the body, so also may substances (ectocrines) be liberated from the body to have their effects on other organisms in an aquatic environment. The changed conditions set up in the water by one species may perhaps become both injurious to itself and at the same time more suitable to another kind which will follow it. Thus, among other interesting ideas, he gives strong support to this idea of seasonal succession: a chain of action, a conditioning and reconditioning of the water, as the year advances (Lucas, 1938, 1947 and 1956a).
Among the animals of the plankton there are also successive changes; particularly noticeable are the various broods of different species which follow one another, giving us in one month mainly adults and in another the young developing stages. The seasons are marked too by the throwing up into the plankton of the young larval stages of various bottom-living invertebrates, and also by the eggs and fry of different species of fish, all of which have their own distinct breeding times.
To return to more general considerations, it is interesting to compare the conditions as found in our waters at mid-summer with those in the surface waters of the tropics; we have the same heating up of the surface to form a discontinuity layer, but there it tends to be a permanent feature. In the open ocean in the tropics the phosphates and nitrates in the upper layers are thus reduced to a minimum all the year round and as a consequence the plankton of those regions is extraordinarily sparse compared with the more temperate or polar seas. This relative poverty of the tropical seas compared to our own was one of the surprising discoveries made by Victor Hensen’s German Plankton Expedition in 1889 and was at first disbelieved by many who thought, on false a priori grounds, that the warmer tropical seas must be richer in life than our own or the cold polar regions. A tow-netting in the tropical ocean may yield many more species than one in our own waters, but the total quantity of life is very much less; when we glance at a tropical sample at first sight nearly every specimen seems a different kind, whereas in one from our own waters there will be thousands of representatives of the same species. Plankton at certain places in the tropics, however, can be remarkably rich; this happens when deeper water with a supply of nutritive salts comes welling up into the sun-lit zones as against the coast or where a submarine bank comes near the surface and gives rise to disturbed water conditions.
The upwelling of water rich in phosphates and nitrates may well be very important in producing a more prolific phytoplankton in our own waters. There can be no doubt that the abundance of fish-food on the sea-bed which makes the Dogger Bank so renowned as a fishing ground, is due to the heavy rain of plankton showered upon it from above; there can also be little doubt that this rich phytoplankton in the surface layers is in turn produced by the upwelling of the richer phosphates and nitrates from below as the Atlantic inflow into the North Sea meets this large submarine bank set across its path (Graham, 1938).
A prolonged off-shore wind may have the effect of producing a heavy crop of plankton near the coast: it pushes the surface water away from the land so that its place has to be taken by water from below which wells up near the coast and thus again brings the desired nutritive salts up into the sunlit upper layers.
There are indications that at times other minor constituents of sea-water may have an effect upon phytoplankton. There is some experimental evidence that organic salts of iron and manganese will stimulate phytoplankton production; and it is suggested that such salts carried out by the drainage from the land may lead to an earlier outburst of reproductive activity among planktonic diatoms in coastal waters than among those further out to sea. Much information about these minor constituents will be found summarised by Dr. H. W. Harvey (1942, 1955) who has himself done so much of the experimental work.
For a long time there has been evidence that suggests that there is in the sea some trace substance at present unknown which is necessary before life can exist—some substance rather like the vitamins in our diet. Artificial sea-water has been made up to contain precisely the same proportions of chemicals that are known to be present in natural sea-water by the most exact analysis; yet planktonic plants will not grow in it unless about 1% of natural sea-water has been added to it (Allen, 1914). Quite recently the vitamin B
(Cobalamin) has been shown to be necessary for the growth of several marine flagellates and a diatom (Droop, 1954 and ’55) and its presence in natural sea water has now been demonstrated.
In addition to ‘trace substances’ which affect plant production, it now appears that there are some which are essential for the healthy development of delicate young animals. D. P. Wilson (1951) has recently shown a remarkable difference in this respect between the two types of water which we discussed in Chapter. 2 (#u3397ab9e-de11-5382-badf-93137316dee4): the more oceanic water characterised by one species of arrow-worm Sagitta elegans and the coastal water by another species S. setosa. It will be remembered that in the 1920’s there was usually elegans water off Plymouth; at that time Dr. Wilson had no difficulty in rearing the planktonic young of some of the bottom-living worms he was interested in. In the 1930’s, however, when the setosa water was over the area, he experienced much frustration in doing so. Thinking that there might be some subtle difference between the two waters, he made experiments to test his suspicions. He took the fertilised eggs of two different kinds of worm and of a sea urchin, and then divided each lot into a number of smaller batches; some he put into elegans water collected out in the Celtic Sea to the west of the Channel and the others into setosa water taken off Plymouth near the Eddystone. In the former water most of the larvae developed well, but in the latter they were abnormal or in poor health. “The experiments indicated,” writes Wilson, “that the Channel water lacked some unknown constituent, essential for the healthy development of these species, present in the Celtic Sea.”
Not infrequently a particularly rich outburst of phytoplankton is reported at a place where the waters of two current systems meet and mix. I have seen it particularly in the region of South Georgia in the sub-Antarctic where waters from the Weddell Sea and the Belling-hausen Sea meet in eddies on each side of the island. It is said to be a feature too of the boundary separating the North Atlantic current from the arctic water and it is not uncommon generally where oceanic and coastal waters meet and mix. Perhaps, on account of different plankton communities, each water has become deficient in some different but vitally important minor constituent; then on their coming together each will fertilize the other with the missing ingredients and so release an outburst of reproductive activity.
A good deal of interest was aroused in experiments performed during the war by the late Dr. Fabius Gross and his co-workers (1944) to see to what extent the growth offish could be accelerated by increasing the quantity of plankton in an enclosed sea loch by the addition of nutritive salts. The plankton was certainly enriched and an increased growth of the fish was recorded. It was in fact doing in a confined part of the sea what had already been successfully done in fresh-water fish-ponds; it is indeed a practice dating from ancient Chinese days. It has been suggested that it might be possible to add fertilizer to parts of the more open sea to increase the plankton in a limited area to provide a better chance of survival for the hosts of young fish that are expected to be developing there. But the open sea is a very big place and to do anything at all effective would need the provision of fertilizers on a scale perhaps too vast to be contemplated as a feasible proposition. Yet it seems that man does unwittingly influence the production of phytoplankton in the sea and consequently the yield of fish. In the southern North Sea opposite the opening of the Thames estuary there is frequently developed an area of a particularly rich growth of phytoplankton and here Mr. Michael Graham (1938) has shown an abundant source of phosphates and nitrates derived from the sewage of London. Dr. K. Kalle of the Oceanographic Institute at Hamburg has recently written a paper on the influence of this drainage from the Thames upon the fish population of the southern North Sea and this has been conveniently summarised in English by Dr. J. N. Carruthers (1954). He points out that the water from the continental rivers is carried quickly to the north-east by the current from up the channel; whereas that from the Thames is held up, wedged between two streams of oceanic water of higher salt content: i.e. that just mentioned and the Atlantic influx from the north. He estimates that 2,900 tons of phosphorus a year are carried from the rivers and when spread through the 171 cubic miles of English coastal waters south of the Humber amounts to an increase of 4 milligrams (0.004 grams) of phosphorus per cubic metre. Dr. Kalle then shows that the catch of fish in this region is per unit area ‘about double the corresponding catch made in the rest of the North Sea, in the English Channel and in the Kattegat/Skagerak region … and is about 25 times the catch reckoned for the Baltic Sea as a whole.’ He holds that two-thirds of this higher average catch may be attributed to the rich supply of nutrients from the population of our metropolis.
For a comprehensive treatment of the physics and chemistry of the sea in relation to plankton production the important books by Dr. H. W. Harvey (1945 and 1955) should be studied.
(#ulink_9b1c9b25-ca70-5b21-b304-2f2392cfbb39) When taking samples from a series of levels in very deep water several of these reversing bottles are generally used together, one above the other, on one wire at intervals of perhaps a hundred metres or more; as the messenger weight hits the trigger to reverse the first bottle, it also releases from below it another messenger which now slides down the wire to operate the second bottle and this again liberates a third messenger and so on to the bottom.
(#ulink_bc69a687-9dee-5558-8517-219d990aa9f2) See Gardiner (1937).
CHAPTER 5 (#ulink_6520a5dc-2d55-5488-9666-9b2074100333) INTRODUCING THE ZOOPLANKTON
THE ANIMALS of the plankton are by definition those which are passively carried along drifting with the moving waters; those other inhabitants of the open sea which are powerful enough to swim in any direction—the fish, whales, porpoises and the squids or cuttlefish—are in contrast referred to as the nekton (see here (#ulink_90dbbd09-52bf-5298-a614-64d2ae53d4e0)). The vertebrates therefore, except for certain primitive relations, will only be represented in the plankton by the floating eggs offish and the young fish themselves up to the time when they become strong enough to migrate at will instead of being just helplessly transported.
In spite of this limitation, and the absence of insects, I believe it is no exaggeration to say that in the plankton we may find an assemblage of animals more diverse and more comprehensive than is to be seen in any other realm of life. Every major phylum of the animal kingdom is represented, if not as adults, then as larval stages with the partial exception of the sponges; the sponges do indeed send up free-swimming larvae but they are in the plankton for so short a time that they can only be claimed as very temporary components of it. In no other field can a naturalist get so wide a zoological education and in few others will he find a more fascinating array of adaptational devices.
It is this great variety of forms, and the unexpected finds which are always turning up, which make hunting in the plankton such an exciting occupation. Except for the jelly-fish and some of the larger crustacea, it is of course hunting with a lens. Nearly every member of the zoo-plankton can be seen with a ×6 hand-lens or a simple dissecting microscope, and the most effective searching can be done with these. Before transferring any specimen to a slide for examination under the more powerful compound microscope, it is well to watch it for a time swimming in its own characteristic way in a small glass dish under the simple magnifier. To anyone who has never seen this life before it is difficult to convey in words a picture of the delights in store for him. I am indeed lucky to have the privilege of having my account illustrated and enriched by the beautiful photographs of living plankton animals taken through the microscope by my friend Dr. Douglas Wilson of the Plymouth Laboratory; they are quite unique and many of them have been taken by that remarkable new device, the electronic flash, which has for the first time made the photomicrography of such small and rapidly moving creatures possible. The naturalist will soon forget the absence of the insects in the wealth of variously shaped and often beautifully coloured crustaceans which are to be seen swimming rapidly in all directions. Tiny pulsating medusae—miniature jellyfish—swim into view; and here and there can be seen the transparent arrow-worms Sagitta which remain poised motionless for a time and then dart forward at lightning speed to capture some small crustacean. Then there may be delicate comb-jellies propelling themselves by rows of beating iridiscent comb-like plates and trailing long tentacles behind them. These comb-jellies and the arrow-worms belong to two phyla—i.e. major groups of the animal kingdom—which are found nowhere else but in the marine plankton. There are many different kinds of Protozoa, among which one order (the Radiolaria) is also entirely planktonic. The segmented worms may be represented by beautiful pelagic polychaetes and the molluscs by the so-called sea-butterflies (pteropods) which are really small snail-like animals with the foot drawn out into wing-like extensions to assist in their swimming and support. Most of these animals are permanent members of the plankton, spending all the stages of their life-histories drifting in the open sea; in addition there are, however, a vast number of the young, or larvae, of the bottom-living invertebrates which ascend to live for a time in the plankton and so distribute the species far and wide. These temporary members present us with some of the most striking adaptations to this floating life. Some of them are nearly always to be found in a tow-net sample from our surrounding seas which have such a rich fauna on their floor. Group by group—flatworms, segmented worms, different kinds of polyzoa, starfish, sea-urchins and, of course, the bottom-living crustacea—each has its own characteristic way of solving the problems of pelagic life. The plankton indeed presents a paradise for the student of invertebrate development; we shall devote a special chapter (Chapter 10 (#litres_trial_promo)) to a consideration of these larval forms.
Hitherto only a small minority of amateur naturalists have shared the delights of exploring the living plankton. Preserved samples, such as are often obtainable from marine laboratories for examination, are certainly full of interest; they can, however, never give the observer the same satisfaction as seeing this teeming world all alive. The professional marine biologist, engaged in investigating the relationship between plankton distribution and the fisheries, finds it very tantalizing to be able only very rarely to find time to stop and look at his captures before he must kill them; he travels to and fro across the sea taking as many samples at intervals as he can, in order to get the most comprehensive picture of conditions in the time available. Usually he only just has time to deal with the concentration, labelling and preservation of one set of collections before the ship arrives at the next position where another set must be taken; for the sake of understanding the fisheries he must always hurry on. In the past the amateur has often had an even more disappointing experience: having obtained a tow-net and hired a boat to take him out in the bay, he has returned home only to find that the wonderful sample of plankton he collected is now just a mass of dead or dying creatures crowded together at the bottom of the jar. Two modern inventions have altered all this: the Thermos flask and the refrigerator. If you have a Thermos flask, or preferably two, you can go to the sea, travel back by train for several hours and still have your plankton alive; if you have a refrigerator, or know a kind neighbour who will allow you to keep one or two 4 lb or 7 lb preserving jars in his, then you can keep your animals healthy for several days to be studied at your leisure.
I believe there are a great many people—and not only those who would call themselves naturalists—who would like to see something of this strange planktonic world, or show it to their children, if only they knew how. Anyone who goes to the sea can catch plankton quite simply. Those who can take a yachting cruise are particularly fortunate; they can study the changes in the plankton as they move from one area to another, can see the difference between the animals at the surface at night and in the daytime, and can try and find out just what organisms are making the flashing lights around their vessel in the darkness. Those, however, who can only take out a rowing boat may make very good collections, especially if there is water from the open ocean bathing their coast. If there is a pier sticking out into the sea and sufficient tidal current, as there usually is at some time of the day, quite good samples may be obtained by streaming out a net on a line and allowing it to fish for a quarter of an hour or so. Some may even think this preferable to a boat if the sea is a bit choppy! If you can only collect from a pier, or from a confined area in a rowing boat, you need not be too envious of your friends in the yacht, for fortunately the water is always on the move; a sample taken at the pier today may be very different from one taken only a few days ago and quite different again from one you may get next week. I have taken very good samples from some of the many piers built out to receive the steamers plying in the Firth of Clyde area.
To help those who do not know how to proceed I will give a few instructions. It is a good thing to have at least two Thermos flasks, so that you can keep at least two different plankton samples separate from one another. If you can manage it, it will be an advantage to start out with one of your flasks filled with sea-water that has stood in a jar in the refrigerator over night. Half of this you can pour into the other thermos just before you add the plankton sample collected. Thus in each flask the animals will be added to sea-water that has been chilled; it will keep them cool, inactive and in good condition whilst they are brought home. Details of how to make and use a tow-net have already been given in Chapter 3 (#uf28d2542-dfd2-59a4-8652-470397ba0dae). The net of very fine gauze suitable for collecting the small plants will also at the same time catch the very small animals, particularly the protozoa and small larval forms. For the capture of most of the zooplankton a coarser net having some 60 meshes to the inch is the most useful. If more of some of the larger animals are required, for example the larger crustacea and medusae, a still wider mesh net, say 25 meshes to the inch, should be used; this will filter much more water but let nearly all the smaller animals escape. The three nets of 200, 60 and 25 meshes to the inch will provide a very good equipment. Remember, as stressed in Chapter 3 (#uf28d2542-dfd2-59a4-8652-470397ba0dae), to tow slowly, at a speed of not more than 1½ knots. It is best to tow only for short periods—not more than five minutes at a time—which can be repeated if too small a sample has been collected. If the plankton is very abundant a longer haul will give you much too much so that all the little animals will be far too crowded together to live healthily for more than a very short time. If you have too thick a sample, pour a lot of it away and only take home in your flask a small part of it, diluted as much as possible with more sea-water. It seems hard to pour most of it back, but you will be sure to have sufficient of the commoner kinds and a few kept in good shape will be better than a great many in poor condition.
I must now give some idea of the actual numbers of animals you may expect to get. Here (#litres_trial_promo) I gave the figures for the diatoms and dinoflagellates taken in two 14-inch diameter tow-nets hauled for half a mile across the bay at Port Erin in the Isle of Man; they were averages for several hauls a week during the month of April over a period of fourteen years. For comparison I now give in the accompanying table the corresponding figures from the same source (Johnston, Scott and Chadwick, 1924) for the more important elements of the zooplankton in the same series of hauls.
The corresponding average totals for the months of June, August and October were 39,105, 38,812 and 35,631 respectively. Since it was calculated that approximately 8 cubic metres of water were filtered by the nets during towing, this gives an average of about 4,500 animals per cubic metre or some 120 per cubic foot of sea-water during the summer months. It must be remembered that these figures are averages and that individual samples may vary enormously from week to week. For comparison it may be interesting to give the average figures for the total plants of the plankton—the diatoms and dinoflagellates—recorded from the same series of net hauls for the four months April, June, August and October; they are in round figures 5,815,000, 6,674,000, 107,000 and 485,000 respectively. It must be remembered, however, that there will have been much larger numbers of the still smaller plants, the tiny flagellates referred to here (#ulink_21cacdb4-e2f9-576c-86e5-ce66f10ad48e), which will have passed through the meshes of the net and so not been recorded. To give the number per cubic metre we must again divide by 8.
If you have time, and the sea is calm enough, you should pour your plankton haul into a dish and examine it with a pocket lens as soon as it comes up; then with a wide-mouthed pipette you can pick out from it into another jar some of the rarer animals that you particularly want to study. After that you can more light-heartedly pour away most of the sample before putting the remainder, together with the rarities you have picked out, into your Thermos for transport home. The most useful dish from which to pick out specimens is one of the large oblong photographic dishes made of white porcelain and used for washing whole-plate negatives; half of the bottom of this can be covered with black paper so that you have a contrast of backgrounds to enable you to see both the darker and lighter forms more easily. All the jars, dishes and pipettes you use for living plankton must be kept thoroughly clean and never be used for samples that have been preserved with formalin or other chemical fixatives. These small animals are delicate in constitution as well as in form.
The majority of plankton animals tend to come up towards the surface at night and sink down into the deeper layers during the day (Chapter 11 (#litres_trial_promo)). Very rich samples of plankton may be collected by simply towing the net just below the surface at night; in the daytime, however, if you are over deep water you may have to send your net down to 15 to 20 fathoms to get a good haul. To reach this depth you will require a good length of line—50 to 60 fathoms—and you will also require a much heavier weight, say a 20 lb lead, to take your net and all this line down. Care must be taken, of course, to know just how deep the sea is at the point where you are working so as not to run the risk of trawling the bottom with your net and either bursting it by filling it with mud or tearing it to ribbons by dragging it over a rough bottom. If you have not a chart you can consult, you should take a sounding with your lead and line before starting.
It is often very interesting to take a series of samples from different depths at the same place as near together in time as possible to enable you to study the depth distribution of the various animals; if you repeat the series again at night you may be very surprised at the different results you will get. As you let the net run out on its line to a deeper level it will fish very little on its way down, for it is moving backwards with the water as it runs out and sinks; when you haul it up, however, at the end of a tow, it will of course fish all the way up. This difficulty is got over in modern oceanographic practice by having in front of the net a special closing mechanism which is operated by a brass messenger weight sent sliding down the cable; this releases the bridles when a trigger is struck and the net falls back to be closed and held by a throttling noose which passes round it behind the mouth. The net is thus hauled up to the surface closed like a sponge-bag with the strings drawn tight and you know that all the animals in it must have been caught at the actual level at which it was fishing. A simple example of this arrangement is shown opposite in Fig. 18 (#litres_trial_promo). These devices, however, are perhaps rather elaborate to be practised by the amateur, especially as a smooth steel cable is required for the messenger and this means the use of a winch; they will not be further described but full information about them will be found in the descriptions of the equipment used on the Discovery Expeditions (Kemp and Hardy, 1929). To minimise the effect of catching whilst hauling up an open net, it is well to make rather longer hauls with it down; the time taken in coming up will then only be a small fraction of that during which the net was fishing at its proper level.
If you are going to take a number of such hauls for study you will soon accumulate far more material than you can hope to keep alive successfully; in this case it will be best to keep only a small part of one or two samples fresh and preserve the rest for study dead. The living plankton will give you the greatest pleasure in studying the swimming movements and behaviour of the animals; the dead samples may nevertheless give you interesting information about the depth distribution of the same animals, which you could not otherwise obtain. The best general preservative for plankton, and the easiest to use, is formalin, i.e. a 40% solution of formaldehyde in water; this, which can easily be obtained at any chemist, can be added to the sample in quite small quantities to give a mixture (about 5% formaldehyde) strong enough to keep it indefinitely in good condition. Remember always to reserve separate jars for preserved samples—never mix them with those used for fresh; a good plan is to stick a red label on them for danger! The dead formalined samples can of course be concentrated into a smaller space; 1lb honey jars with screw-on tops are convenient for their storage. If you are going to keep the samples for any length of time it is well to use what is called neutral formalin, i.e. that to which just sufficient borax—from 5 to 10 grams per litre—has been added to neutralise its acidity; ordinary formalin nearly always contains formic acid which if not so neutralised will very soon dissolve away the calcareous shells and skeletons of many of the animals.
FIG. 18
Sketches of a simple release mechanism for closing the mouth of a tow-net before hauling it to the surface. A, the rig of the net when towed; B, enlarged view of release gear about to be struck by messenger weight; C, the towing bridles released and the net closed by throttling rope.
What has just been said will have been sufficient to have corrected that very common misconception that the plankton exists almost entirely near the surface of the sea. Some people seem to have thought of it as existing as a kind of scum on the very surface itself; this is no doubt due to a misunderstanding of the expression often used that the plankton is the ‘floating life’ of the sea. The plants, as we have already seen, do in fact only flourish for a little way below the surface; but animal members may be found at all depths. Later on—in Chapter 12 (#litres_trial_promo)—we shall describe the plankton and nekton that is to be found at various levels in the ocean between the surface and the bottom, thousands of fathoms deep beyond the continental shelf. There is another erroneous impression about the plankton that is frequently held: the idea that it is more or less evenly distributed over quite wide stretches of the sea; it is often thought that if we used a tow-net in one place and another two or three miles away on the same day, the two samples would be almost exactly alike. This indeed may occur, but it is by no means always so.
A great many surveys have been made in the past, often in relation to some fishery problem, attempting to give some idea of the varying quantities of the major plankton organisms over a particular area. I have already described how a research ship will proceed in such a survey to traverse the area, stopping or slowing down to take tow-net samples at regular intervals. If the area to be covered is a big one, the stations—as the different points of observation are termed—cannot be very close together or the survey would take much too long; they are frequently spaced twenty miles apart. It has usually been assumed that a sample at one point, will, within a reasonable range of error, give a fair representation of the plankton in the area for ten miles around it; thus it has been felt that a series of such stations twenty miles apart will give an adequate quantitative survey. Some plankton organisms are much more patchy in their distribution than others; for some kinds such a method may give quite an adequate picture, but for others it may be hopelessly misleading. Very early in my career as a marine naturalist I had an experience which I will recall because it so well illustrates this very point; it was an episode which had a marked effect on much of my later work. In 1921, soon after leaving the University, I was appointed as Assistant Naturalist on the staff of the Fisheries Laboratory of the Ministry of Agriculture and Fisheries at Lowestoft and was delighted to be allowed to study the plankton in relation to herring. In March of the following year, through the illness of a senior, I found myself, at the last moment of sailing, as naturalist in charge of a cruise on that grand old research trawler the George Bligh.
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