Maas, Die craspedoten Medusen der Plankton Expedition, by permission of Lipsius and Tischer.
Plankton Expedition, ii.
Ilydrophytes.These include plants of the plankton, or micro- W
Several decades ago all marine organisms became grouped together in three great categories: (1) the Benthos, or bottom-living, rooted or sedentary forms; (2) the Nekton, or actively swimming animals; and (3) the Plankton, or drifting (usually) microscopic organisms, which have little power of locomotion (see 21.720).
The plankton is divided into (a) the Zoo-plankton (such as the minute crustacea and the eggs and larva of fishes and many other marine animals); and (b) the Phyto-plankton, that is, the minute algae, diatoms, peridinians, some flagellate protozoa, spores of alga, etc. The investigation of the plankton from a new point of view, begun by Hansen in 1889, was continued by Lohmann at Kiel, by Cleve in Sweden, by Gran and Ostenfeldt in Norway and Denmark, and by Herdman, Allen and others in England.
In general the plankton - and especially the phyto-plankton of the polar and temperate seas - is much more abundant than is that of the sub-tropical and tropical zones.
All forms of plankton are more abundant in the shallow coastal waters of relatively low salinity.
Finally, the plankton (and again the vegetable forms in particular) are practically restricted to the upper hundred fathoms or so of the sea.
- These are the materials which are utilized by the vegetable plankton in the synthesis of living material: they are water, carbonic acid, nitrates and nitrites of calcium, magnesium and other earthy and alkaline metals, phosphates, silica, traces of salts containing iron, sulphur, potassium and a few other elements.
Therefore the animal organisms, as a rule, reproduce in the spring or early summer just after the vernal phyto-plankton maximum.
Following that again is a less well-marked maximum of phyto-plankton in the autumn, occurring just after the period of highest sea temperature.
The plankton, both animal and vegetable, attains its minimal values and many of the larger forms of animal life pass into a kind of condition of hibernation.
The Plankton Expedition brought back, and H.
Plankton Expedition, ii., 1897) has described, a few larval brachiopods of undetermined genera, two of which at least were pelagic, or at any rate taken far from the coast.
Distribution is treated of under ZOOLOGICAL DISTRIBUTION, PLANKTON, PALAEONTOLOGY and PLANTS: Distribution.
Depths greater than 2500 fathoms. Viewed as a whole this deposit may be taken as a partial precipitation of the plankton living in the upper waters of the open sea.
A small proportion of organic matter including the fat globules of the plankton is mixed with the calcium carbonate, the amount according to Giimbel's analysis being about 1 part in 1000.
Schott gives the following as the result of measurements of transparency by means of a white disk at 23 stations in the open ocean, where quantitative observations of the plankton under i square metre of surface were made at the same time.
There is a distinct relationship between colour and transparency in the ocean; the most transparent water which is the most free from plankton is always the purest blue, while an increasing turbidity is usually associated with an increasing tint of green.
Jacobsen on some occasions found water in the surface layers of the Baltic supersaturated with oxygen, which he ascribed to the action of the chlorophyll in vegetable plankton; in other cases when examining the nearly stagnant water from deep basins he found a deficiency of oxygen due no doubt to the withdrawal of oxygen from solution, by the respiration of the animals and by the oxidation of the deposits on the bottom.
A temperature of 40.1 ° F., the carbonic acid amounts to 51 J5 cc. per litre, and the oxygen only to 2.19 cc. Vegetable plankton in sunlight can reverse this process, assimilating the carbon of the carbonic acid and restoring the oxygen to solution, as was proved by Martin Knudsen and Ostenfeld in the case of diatoms. Little is known as yet of the distribution of carbonic acid in the oceans, but the amount present seems to increase with the salinity as shown by the four observations quoted: Water from Gulf of Finland of 3.2 per mille salinity =17.2 cc. C02 Western Baltic of 14.2 North Atlantic of .0, , 49'0 Eastern Mediter ranean of 39.o, , =53'0, , Unfortunately the very numerous determinations of carbonic acid made by J.
The differences of salinity support this method, and, especially in the northern European seas, often prove a sharper criterion of the boundaries than temperature itself; this is especially the case at the entrance to the Baltic. Evidence drawn from drift-wood, wrecks or special drift bottles is less distinct but still interesting and often useful; this method of investigation includes the use of icebergs as indicators of the trend of currents and also of plankton, the minute swimming or drifting organisms so abundant at the surface of the sea.
The connexion between variations of current strength and the conditions of existence and distribution of plankton are no less important, especially as they act directly or indirectly on the life-conditions of food fishes.
Makaroff, The Yermak in the Ice (in Russian) (St Petersburg, 1901); The Norwegian North Atlantic Expedition (on the " Voringen "), 1876-1878 (Christiania, 1880-1900); Expeditions scientifiques du " Travailleur " et du " Talisman," 1880-1883 (Paris, 1891 et seq.); Die Ergebnisse der Plankton-Expedition, 1889 (Kiel, 1892 et seq.); Resultats des campagnes scientifiques accomplies sur son yacht par Albert I e ' Prince Souverain de Monaco (Monaco, from 1889); The Danish " Ingolf " Expedition, 1806 (Copenhagen, 1900); Prof. Luksch, Expeditionen S.M.
Medusae thus form an important constituent of the plankton or floating fauna of the ocean, and compete with fish and other animals for the food-supply furnished by minuter forms of life.
The phenomenon of seasonal dimorphism is of especial moment for the plankton dwellers.
A, Notholca longispina, lorica only; b, Anuraea aculeata, like the former, a floating pelagic type (plankton proper); c, Synchaeta stylata; corona with accessory antennae and sensory styles; auricles for swimming - an actively swimming pelagic type (nekton); d, Pterodina patina, with bdelloid corona and retractile foot with terminal ciliated cup; e, Distyla gissensis partly extended; f, Rattulus tigris.
Widely different are the habits of the plankton forms, which float or swim near the surface, and are often provided with long cuticular extensions for this purpose (fig.
Asplanchnaceae, plankton, dwellers in small pools, are, however, ovoid, and Trochosphaera is spherical and must owe its floating powers to the low density of the liquid in its enormously dilated bodycavity.
Together with Peridiniaceae they constitute the bulk of marine plankton, and thus play an important part in the support of marine animal life.
To the aggregate of such forms, both animal and vegetable, the term plankton has been applied, and the investigation of the vegetable plankton, both freshwater and marine, has been pursued in recent times with energy and success.
The German Plankton Expedition of 1889 added greatly to our knowledge of the floating vegetable life of the North Atlantic Ocean, while many laboratories established on the shores of inland seas and lakes have rendered a similar service in the case of our freshwater phyto-plankton.
The organisms constituting this plankton are mostly unicellular, of ten aggregated together in colonies, and the remarkable structure which they exhibit has added a new chapter to the story of adaptation to environment.
The families Diatomaceae, Peridiniaceae and Protococcaceae are best represented in the pelagic plankton, while in addition the Volvocaceae are an important element in freshwater plankton.
Schutt, Die Peridinien der Plankton-Expedition (Kiel and Leipzig, 1895); R.
(Amphipoda), Sars (1890-1895), demanding attention, and the quite recent Amphipoda of the Hirondelle, Chevreux (1900), and Hyperiidea of the Plankton-Expedition, Vosseler (1901).
The zoological collections of the "Pola" expeditions show that certain well-defined districts are extremely rich in plankton, while others are correspondingly poor; and it appears that the latter occur in districts surrounded by currents of relatively low temperature, while the richer parts are where the movements of water are blocked by irregularities in the coast-line.
Vanhi ffen, " Die Akalephen der Plankton-Expedition," Ergebn.
Many of the larger species of Crustacea are used as food by man, the most valuable being the lobster, which is caught in large quantities on both sides of the North Altantic. Perhaps the most important of all Crustacea, however, with respect to the part which they play in the economy of nature, are the minute pelagic Copepoda, of which incalculable myriads form an important constituent of the " plankton " in all the seas of the globe.
It is on the plankton that a great part of the higher animal life of the sea ultimately depends for food.