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18 MATS WlERNmon in shallow water, the others in comparativelydeep water.We may focus our attention on those factorswhich a fjord in Bohuslan and the Baltic (includingOresund) have in common. In this connection wemay emphasize the ice cover (see the map, Fig. 3)and its consequences, such as shelter, dep:>sition ofsediments, reduced light and temporary reductionof the oxygen content, as well as lower watertemperatures, and particularly more markedchanges in temperature.Firm iceCemented packice and floesPack"iceIce floesOpen water100 km...__...__ ...The Baltic-a cul-de-sacWe might well compare the Baltic to a giganticthreshold fjord, with the shallower of its thresholdsin the southern part of the Oresund, betweenCopenhagen and Malmo, at a depth of 8-7 m. Themain entrance to the Baltic is however throughFig. 3. Maximum extension of sea ice in an average winter.In hard winters, the pack ice becomes largely frozen togetherand extends seawards over the floe II).arked areasand even more. From Atlas of Finland, with additions byDr. C. J. Ostman, in Angstrom 1958.Acta PhytogeogT. Suec. 50the Great Belt, which has a deep central furrow,whereas the threshold is here at the DarsserSchwelle (ea. 18 m) between Denmark and Germany.The typical Baltic conditions thus beginonly east of the Darss and south of the Oresundthresholds.The outward influence of Baltic waters reachesfar beyond its thresholds, affecting Kattegat andeven Skagerrak, mainly with regard to the surfacewaters. As to the hydrology of the transitional seas,see JENSEN (1940), STEEMANN NIELSEN (1940),BRATTSTROM (1941).The South Norwegian and entire Swedish coastforms a continuous gradient from oceanic to fjordlikeand finally rock-pool and near-freshwater conditions,a gradient regarding several factors, apartfrom the two most obvious ones, the salinity andthe tide.Even on the part of the West Coast borderingthe Skagerrak, the interior waters, as we have