WATER 



179 



stratum of the land and is used as such by 

 many land animals that are active in win- 

 ter; witness the excursion of arctic foxes 

 over sea ice far from shore. Ice is more or 

 less transparent to both heat and light; it 

 is not readily permeable to gases. As a re- 

 sult, ice-covered lakes may be lighted suf- 

 ficiently to make some photosynthesis pos- 

 sible, and their waters are warmed by sun- 

 light that penetrates through unmelted ice. 

 Such lakes become stagnant if long frozen 

 over, with an accompanying decrease in 

 dissolved oxygen and an increase in carbon 

 dioxide. 



Ice expands and contracts with tempera- 

 ture changes and often forms pressure 

 ridges along weak points; these may pile 

 high in Arctic seas, especially near land 

 where tidal pressures vary. Expansion of 

 warming ice may exert strong outward 

 pressure against banks of frozen rivers or 

 shores of lakes. The entire frozen area of 

 a river bank may thus be broken free from 

 the unfrozen subsoil and piled in ridges 

 parallel to the channel. Similar action may 

 bring sand bars above water, even though 

 they are usually submerged and are sur- 

 rounded by deeper water. Ice has little ten- 

 sile strength and readily cracks from 

 contraction with falling temperature. 



The presence of salt lowers the freezing 

 point of sea water so that it remains liquid 

 to —1.91° C. When sea water does freeze, 

 the ice crystals themselves are probably salt 

 free; they form a matrix that encloses 

 enough somewhat concentrated sea water so 

 that the ice crystals, together with the en- 

 closed sea brine, if melted together, will 

 approximate the same composition and con- 

 centration of ions found in the sea water 

 before it was frozen. If freezing occurs 

 slowly, there may be a marked reduction in 

 the salinity of the melted sea ice. In 

 one instance sea water with a salinity of 

 ■30 °/„„ frozen at —16° C. yielded brackish 

 water with a salinity of 5.6 °/oo As the 

 temperature rises, the sea ice surrounding 

 the entrapped brine melts, and small pores 

 appear through which the brine in the up- 

 per ice can trickle down to lower levels. Old 

 sea ice, exposed above the water level, may 

 come to yield practically fresh, potable 

 water unless it is contaminated by salt 

 spray. When large amounts of such old ice 

 thaw with some rapidity, a layer of water 

 with lowered salinity spreads over the sur- 

 face of the surrounding sea. 



Being frozen in ice is not necessarily 

 fatal for many animals (see p. 99). Whole 

 resistant communities, such as the bryocoles 

 of the tundra and of bogs, are frozen each 

 winter. Some hardy forms among groups 

 like the protozoans, tardigrades, and rotifers 

 exist, although they are frozen during most 

 of the year or perhaps for several years at a 

 time (Murphy, 1928). Shallow lakes in 

 high latitudes freeze to the bottom each 

 winter and still support a fairly rich animal 

 life. So-called anchor ice may form on the 

 bottom during the night in cold weather or 

 even in daytime when cloudy winter skies 

 keep back the heat of the sun. Anchor ice 

 is especially likely to form over dark rocks 

 that rapidly radiate their heat to the water 

 above. Typically, the ice melts with the re- 

 turn of direct radiation (Church, 1942). 

 Anchor ice may be destructive to sensitive, 

 bottom-dwelling animals. 



The grinding of small bits of ice or of 

 large masses of it breaks dowTi Uving things 

 exposed to its action and may be quite 

 destructive even with short exposure. Such 

 wave-driven ice also exerts strong eroding 

 force along shore lines. It is one of the rea- 

 sons why winter-killing is more extensive in 

 intertidal or adtidal communities along the 

 sea shore than it is in somewhat deeper 

 water (Alice, 1919). 



SNOW 



Snow is an integral factor in the physical 

 environment. It is important particularly in 

 the higher latitudes and on the higher parts 

 of mountains where the snow cover may 

 last through the year. Its significance is in- 

 creased by the large amount of land surface 

 in northern Eurasia and North America. The 

 limits of snow to the south and in lower 

 latitudes vary with the general climate and 

 with local conditions. Snow extends farther 

 south in continental climates than in those 

 characterized by marine influences, al- 

 though this general rule is modified by 

 humidity, wind direction, topography, other 

 local regional considerations, and by ocean 

 currents. The snow line is pushed north- 

 ward by the Gulf Stream on the European 

 coast and by the Kuroshio in the western 

 and northern Pacific Ocean. It extends far- 

 ther south along the shores of Canada and 

 New England under the influence of the 

 Laborador Current, and the cold Oyashio 

 Current has a similiar effect in Japan and 

 in adjacent parts of Asia. The snow line 



