HYDROGR. SECTION — APPENDIX D (58) 
this layer may sometimes reach the continental slope at varying depths below 
some 400 metres. 
Unperiodical variations of the hydrographical conditions have proved to be 
much greater than previously expected. Through many observations with regard 
to these variations new problems have arisen that have a very pneu bearing 
upon hydrographical investigations. 
Regular periodic variations have been found in some cases. Thus the coast- 
al currents extend far seawards in spring and summer, at the same time growing 
shallower; in autumn and winter the coast-water withdraws towards the coast, 
and its depth then increases. 
VII. Arctic Sea: 
The investigations have succeeded in giving an account, very detailed in 
certain particulars, of the yearly course of the variations in temperature and sali- 
nity in the different parts of the Arctic area, especially as regards the south- 
western part of the area under investigation, (viz. at the Murman coast and in 
different parts of the North Cape Current). The branches of the Gulf Stream, various 
cold currents, great differences between the various coastal areas, and other fac- 
tors, cause the physical-geographical conditions to present a very varied and 
complicated picture, which, upon the whole, rests unaltered from one year to an- 
other; nevertheless, — apart from the yearly fluctuations — not inconsiderable 
variations may be found in the details both with regard to temperatures and sali- 
nities. Large unperiodic variations take place here, and the limits of the branches 
of the Gulf Stream alter from season to season, these shiftings varying in extent 
from one year to another. The system of Gulf Stream branches and their conti- 
nuations as undercurrents is of a very great importance for the climate of the con- 
tinent and of the sea, affecting the occurrence of drift-ice, navigation, etc. 
B. Plankton Investigations 
The plankton Investigations have shown that areas of the sea which differ 
in hydrographical respects, are also different in regard to the composition of the 
plankton, and that this difference is qualitative as well as quantitative. The Baltic 
basin proper, for instance, possesses a decidedly less abundant plankton-fauna than 
the shallower western area, namely the Belts and the Cattegat. In the same 
manner, the Norwegian Sea and the North Atlantic possess a fauna less abundant 
than those of the shallow North Sea and of the coastal waters of Iceland. 
In the areas more closely examined the plankton organisms show a yearly 
periodicity that, in spite of variations from one year to another, yet always takes 
