About a hundred years ago, scientists in their search for further understanding of life in the sea first used fine nets and found sometimes many and sometimes few small floating animals and plants drifting in the water at the mercy of the currents and winds that drive them. As finer nets were used or alternatively as more refined methods of collection were devised, the plants were found to outnumber the animals vastly. Almost all the microscopic and unicellular material was proved to be of plant origin and we today cannot be surprised to hear that they were limited to the uppermost layers, where alone light is adequate for plant growth; although the small animals caught with such nets might be found at almost any depth.
These minute plants and animals are collectively termed the plankton, because of their drifting life. This is not a systematic unit, but a convenient grouping of organisms according to their mode of life. Representatives of nearly every main type of animal from fish to Protozoa are found in zooplankton, some being planktonic all their lives, and others like the young of crabs and mussels and the baby fish, being planktonic only their earliest life. After this they settle on to the bottom or grow into fish that are strong enough to stem the currents. All derive their nourishment, often directly, from the microscopic plants or phytoplankton.
Compared with animal plankton, the plants are relatively poor specialized and comprise only a few groups of algae, of which diatoms and flagellates are the best known. They have many relatives on land and in the fresh water but their numbers are incalculably greater in the sea. Even the numbers to be caught in a small net may run into many millions, while despite their individual minute size at times they may be sufficiently abundant to color the water. Further unlike those animals and plants which live for a year or more, these unicellular plants can reproduce so fast that the number caught at any one time may give only a slight idea of the total top that a particular volume of water may be providing.
During their flowering, almost as large quantities may be produced and eaten daily by the animals as can be caught in the net at any moment.
The grass of the sea. Upon it as far we know almost all other forms depend, including the commercial fishes, altogether these form only a small fraction of the animals in the sea. The abundance of the plants varies from season to season. They depend not only on the light and dissolve carbon dioxide but also on the varying quantities of nutrient salts. In the deeper waters these nutrient salts are more or less uniformly distributed, but in the surface layers of temperature latitudes they may be quickly reduced by plant growth; and if they are not speedily replenished, growth slows or even ceases until water movements bring some of the deeper supplies towards the surface once more. This overturn can occur seasonally at the times of autumn and winter storms, but in some regions there is an intermittent or even constant upwelling of deep and usually fertile water, which maintains there a denser phytoplankton that elsewhere. In one way or the other the shallow seas and oceanic banks are region, which are fairly regularly bathed by nutrient-rich water and it is no coincidence that it is on such banks that our great fisheries have been founded. They are the indirect result of these rich supplies of the nutrients, and of the amazing power of the plants to concentrate and combine them into proteins and oils and resembling those of land plants as well as minute quantities of complex compound vitamins, antibiotics, and so on, which are also typical of plants.
For a truly remarkable thing about the plants is their power of concentrating the nutrients they need. Relative to seawater they concentrate carbon, phosphorus and nitrogen. Then in turn the animal plankton and the bottom animals sift the basic plant food from the water and concentrate these animals further.
Thus the route from plant to fish is not necessarily direct, although for the herring and its relatives it is nearly so. The plant plankton and the greater part of the herring’s food consist just of such small plankton animals. For all its size, the whale feeds directly on animal plankton, as does the shark. So rich is its food in the Antarctic that in two years the blue whale can grow to a length of 60 feet and a weight of 50 tons of plankton alone and reach 200 or 300 tons when fully grown.