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), 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 B12 (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: 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