Plankton

Immediately following it comes the reproduction of the nektic and benthic animals. Hosts of Fishes, Crustacea, Mollusca, Echino derms and worms spawn their eggs into the water, or deposit them on the sea bottom. The eggs hatch out and the waters of the sea teem with larvae. The latter must feed and they utilize the vege table plankton (Diatoms, algal spores, etc.). The production of animal larvae lasts into the early summer (or even later with some species), but while it goes on there is a diminution in the abundance of the Diatoms. Those indispensable food-stuffs which accumulated during the winter months are largely used up while the vernal maximum of vegetable plankton lasts. The extraor dinary abundance of Diatoms therefore falls off during the months of June to August, but about then the Peridinians begin to increase. The summer months are characterized then by a relative scarcity of Diatoms, a great increase in the Peridinians, an abundance of larvae belonging to many groups of inverte brates and a great abundance in the Copepods.

In the autumn or early winter the Diatoms sometimes increase again—there is an autumnal maximum. But this is a very brief phase and with the shortening of the day and the drop in sea temperature towards the annual minimum all kinds of planktonic life in the sea decrease. In this very summary account of the seasonal change stress has been laid on certain physical agencies —light, temperature and inorganic food substances—that are re garded as of importance, but we must never forget that intrinsic causes are also operative. Heredity has implanted, in the organ izations of all marine organisms, a cyclical tendency so that the annual outbursts of reproductive activity are stimulated, or set off, qo to speak, by the seasonal changes.

The Abundance of the Plankton.

There is no part of the seas and oceans where planktonic organisms do not occur. Even in the greatest depths of the ocean we suspect that certain micro crustacea exist. Under the ice, in polar regions, rich collections may be taken and there are abundant algal organisms in the pools that form in hollows on the surfaces of ice-floes in the summer. Snow and ice may actually be tinted with Diatoms and other organisms. The richest plankton in any part of the world-ocean is that found just on the borders of the polar seas, in the regions of melting ice. In general the temperate and cold seas are rich in plankton, while the tropical and warm waters are relatively poor.

What absolute quantities of plankton-organisms exist in the sea? There is so much variation with regions and seasons that it is difficult to give definite figures without going into great detail.

A long series of experimental hauls made in the Irish sea gave a general average of one or two organisms in every cubic centi metre of water, while at the periods of greatest abundance each cubic centimetre contained about a dozen organisms. But this in cluded only the larger planktonic individuals, and it is generally the case that the micro-f orms are far more numerous. In some experi ments made at Plymouth it was shown that there must have been at least several hundreds of individual organisms (macro- and micro-species) in every cubic centimetre of sea water.

These estimates refer to the numbers of organisms in the sea water at a particular moment, and we ought also to consider the rate at which they reproduce and grow. There is not the knowl edge at present which would enable us to make such estimates, but some approximate idea of the total mass of living material that is produced in a sea area can be obtained by indirect methods.

We must note, first, that all animal life in the sea depends on vegetable life : the animals can only subsist by eating other ani mals, or plants, but the plants can use as their source of food stuff the carbonic acid that is dissolved in the water. Utilizing the energy of sunlight they can synthesize this carbonic acid into cellulose and other vegetable substance. Now it is mainly the Diatoms, Peridinians (and other plankton organisms that have the vegetable mode of nutrition) that do this, for the total mass of all the rooted algae must be very small in comparison with the mass of the vegetable plankton. It is possible, by routine chemical methods, to find out how much carbonic acid is removed from solution in a sea area, during the spring and summer seasons, and then converted into carbohydrate by the activity of the vege table plankton. The result is very surprising: at least 300,000 kilog. of dry organic carbohydrate must be produced during a single season per square kilometre of sea surface. Or, expressed in more familiar terms, a spring and summer of organic produc tion in the sea gives origin to about ten tons of moist vegetable substance per acre of sea. This is the marine harvest due to the activity of the vegetable plankton : at present man reaps only a very small fraction of it in the shape of the fishes and other edible animals that he catches.