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.
1 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.
2 See Gardiner (1937).
CHAPTER 5 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). 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