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. 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, 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 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, 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