It is shown to be probable that such effects actually occurred
about the time of the last maximum (A.D. 1433). There is evidence
that, towards the close of the mediaeval period, great storms and
tidal inundations occurred on the shores of the North Sea and Baltic,
and in the course of these floods, culminating in 1297, the Zuider Zee
was formed from a lake that existed in its neighbourhood, by the
breaking down of dykes. (Similar effects can be seen on a small
scale, even in our own times, as the result of exceptionally big tides.)
Severe winters were experienced and the Baltic was frequently frozen
over so that there was solid ice communication between Sweden and
Denmark across the Belts and Sound: this happened in the 1 3th,
I4th and 1 5th centuries but not in the l6th. There have been great
differences in the seas round Iceland and Greenland with regard to
the presence of ice: from the gth to the I2th centuries there is no
evidence (in contemporary accounts) of the presence of much ice in
the sea off Greenland, nor was much ice carried by the Labrador cur-
rent, but from the I3th century onwards we do have evidence that
there was very troublesome ice off Greenland. Hence from the loth
to the I2th centuries there was great intercourse with Iceland and
Greenland on the part of the English, Swedish and Danish, but at the
end of the I3th century some change occurred, resulting in the
southerly emigration of the Eskimos and the extinction of European
civilization in Greenland. At the present time the S. and E. coasts
are icebound, and the W. coast, though icebergs are present in the
adjoining sea, is clear. Many economic changes probably occurred
in consequence of the variations in tide-generating force, as, for
instance, the decline in the mediaeval Baltic herring fisheries con-
trolled by the Hanseatic League.
Hydrobiology. The study of marine life has in recent years become more general, and has become associated with very pre- cise investigations into the chemical composition of sea-water, changes in chemical equilibrium, the effect of variations in salin- ity 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 cate- gories: (i) 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 larvae of fishes and many other marine animals) ; and (6) the Phyto-plankton, that is, th.2 minute algae, diatoms, peridinians, some flagellate protozoa, spores -of algae, 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 re- sults 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.
Distribution of the Plankton. In general the plankton and es- pecially 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. Deeper than this, microscopic life is scanty; there is practically no reproduction and growth. These facts of distribution are due to certain conditions that govern the production of organic substance in the oceans.
Holozoic and Holyphytic Organisms. These terms relate to the modes of nutrition. Typical animals are holozoic, that is, they obtain their food by eating the tissues of other animals and plants: they take their food substances in the organized forms of proteids, fats and carbohydrates. Typical plants are hplophytic, that is, they obtain their food substances from purely mineral sources. Water and carbonic acid are synthesized, under the action of sunlight, to form sugar, starch or some other carbohydrate and this is then com- bined with simple nitrogenous salts to form proteid. Fats doubtless originate by the " cleavage " of the synthetically formed proteids, or from carbohydrates. Now dead animal substance and the excreta of animals decompose in the long run into carbonic acid, water and mineral salts, and so there is a continual destruction of animal sub- stance both on the land and in the sea. Animals cannot make use of these decomposition products, but the plants can. Therefore all life in the sea (as on land) depends on the power which the holophytic organisms possess of synthesizing mineral substances into organized tissues. This is mainly effected, in the sea, by the phyto-plankton.
Ultimate Food Substances in the Sea. These are the materials
which are utilized by the vegetable plankton in the synthesis of liv- ing material: they are water, carbonic acid, nitrates and nitrites of calcium, magnesium and other earthy and alkaline metals, phos- phates, silica, traces of salts containing iron, sulphur, potassium and a few other elements. Except the water, all are present in the sea in exceedingly small proportion. The source of the carbon of or- ganic tissues is carbonic acid ; that of the nitrogen in the proteids is the nitrates, nitrites and salts of ammonia dissolved in sea- water ; the material of the shells or other skeletons is the silica, phosphate and calcium of the salts of sea-water (and, in rare cases, the salts of stron- tium). All these substances exist as only a fraction of one part or, at most, a few parts, per million of water. Carbonic acid is the most abundant and it may be contained in sea-water in the proportion of about 50 milligrammes per litre (that is, 50 per million). All of this is not available, for carbonic acid is present as such m solution, as bicar- bonate (of magnesium mainly) and as normal carbonate. Only the " free" carbonic acid and that of the bicarbonate can be utilized in the process of photosynthesis by the diatoms and algae.
Mineral nitrogenous compounds (nitrates, nitrites and ammonia) are much more rare. The distribution is very interesting and it has been shown that the water of the Antarctic Ocean contains about 0-5 part per million of nitrogen in the above forms. The North Atlantic contains, on the average, about 0-15 part per million and the equatorial seas little more than about o- 1 part per million. The proportion varies with the temperature. There is more inorganic nitrogen in the sea near the land than in mid-ocean and there is more at the sea bottom than near the surface; finally, there is more in the later winter than at any other season. Silica (which is required for the skeletons of diatoms, radiolaria, peridinians, etc.) is present in about the same concentration, but it is now suspected that a source of this substance may be clay washed down from the land and present in the sea in the colloidal form. Phosphates, neces- sary for the formation of skeletons and also for the nucleo-proteid of cells, are about as scarce as nitrogen. In the case of all these sub- stances the quantities involved are so very small, and the difficulties of estimation are therefore so great, that the information we possess is by no means satisfactory. Clearly, however, the vast quantity of living substance in the ocean is built up from materials that are pres- ent in the sea-water as an exceedingly dilute solution, and the solu- tion is dilute just because organisms are incessantly utilizing it. It follows, too, that when there is a number of substances, all essential for the elaboration of living material, and when one of these is present in minimal proportion, that one substance rules the production, just as the effective strength of a chain depends on the weakest link. This is Liebig's " law of the minimum."
Seasonal Periodicities of Life in the Sea. In the temperate seas the two great features are: (i) the outburst of vegetable life in the spring; and (2) the vernal or summer phase of reproduction among animals. The low temperature of the winter allows (indirectly) an accumulation of the essential nitrogenous mineral salts, but as the minimal temperature is passed (in Feb. or March) and the days begin to lengthen the phyto-planktonic organisms begin to reproduce. Carbonic acid is taken from the water and synthesized (by the media- tion of light energy) into carbohydrate. The carbonic acid is taken from solution and then bicarbonate (usually that of magnesium) dis- sociates into carbonic acid and normal carbonate, and the process of photosynthesis ceases when there is no more bicarbonate in solution. The result of this is that the alkalinity of the sea-water increases and the hydrogen-ion concentration decreases. Perfectly pure distilled sea-water dissociates, to an infinitesimal degree, into hydrogen (H) and hydroxyl (HO) ions, so that one litre of such water contains
I X io 7 , or
10,000,000
part of a gram-molecule of either hydro-
gen or hydroxyl (a gramme-molecule of hydrogen is 2 grammes, or of hy- droxyl 1 7 grammes). Pure water, then, has a hydrogen-ion concentra- tion of io" 7 but sea-water gives (because of the mixture of the salts in solution) the concentration lo 8-2 and when photosynthesis by the larger algae, or diatoms, is very active this figure falls to about IO" 9 ' 1 . That is, the concentration of H-ions decreases and that of the HO-ions increases; the water becomes more alkaline because the carbonic acid of the bicarbonate has been abstracted by the phyto- plankton to the extent that normal carbonate is left. When that condition is attained photosynthesis slows down and ceases.
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 larvae.
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 KT 8 ' 1 in Dec. to about IO 8 ' 4 in April. This corresponds to an in- creased alkalinity represented by about 2 c.c. of N/ioo standard alkali, and that difference means that the carbon of about 8-8 mil- ligrammesof carbonic acid has been built up (by photosynthesis) into carbohydrate during the period during which the change in. alkalin-r
