Page:EB1922 - Volume 31.djvu/1223



Hydrobiology.—The study of marine life has in recent years become more general, and has become associated with very precise investigations into the chemical composition of sea-water, changes in chemical equilibrium, the effect of variations in salinity and temperature, the processes set up by marine bacteria, and so on. The investigation of the microscopic pelagic life of the sea has also developed to a great extent. 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 ). The plankton is divided into (a) the Zoö-plankton (such as the minute crustacea and the eggs and larvæ of fishes and many other marine animals); and (b) the Phyto-plankton, that is, the minute algae, diatoms, peridinians, some flagellate protozoa, spores of algæ, 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. Hansen’s early results were much criticized and the original methods very greatly modified and improved. It became clear that only very rough estimates of the numbers of planktonic organisms in a volume of sea-water as large as (say) 10 cubic metres could be made, but that these estimates could nevertheless be trusted to show very marked regional and seasonal differences.

{{Fine block|The spring outburst of plant life in the sea culminates about April, just about the time when the temperature of the water begins to rise rapidly. The increasing temperature raises the rate of animal metabolism, while the higher alkalinity is a stimulus to cell-division. Therefore the animal organisms, as a rule, reproduce in the spring or early summer just after the vernal phyto-plankton maximum. From then onwards the plant organisms diminish because they are eaten by the animal larvæ.

The numerical values are, it is to be noted, exceedingly small. Experiments made by Moore and Whitley at Port Erin in the Isle of Man show that the hydrogen-ion concentration falls from about 10$−7$ in Dec. to about 10$1⁄10,000,000$ in April. This corresponds to an increased alkalinity represented by about 2 c.c. of N/100 standard alkali, and that difference means that the carbon of about 8·8 milligrammes of carbonic acid has been built up (by photosynthesis) into carbohydrate during the period during which the change in {{hws|alkalin|alkalinity}}