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30 GUNNAR LOHAMMARby compact underwater growth. In sheltered bayswith the latter kind of vegetation, the surface temperaturemay reach about + 30°C.As previously mentioned there is a great differencein the thickness and duration of lake ice inSweden. On large lakes in the extreme south theice during some winters is hardly reliable, whereason lakes at the highest altitudes in the Lapponianmountains the ice cover does not disappear everysummer. Small lakes near the south and south-westcoast are on an average ice covered a bout the middleof December and ice free from the latter part ofMarch. Within the interior forested area in northernmostSweden the freezing of small lakes begins inearly October and the ice breaks up about the firstof June. Large and deep lakes become ice coveredlater and even in Lappland they may remain openuntil December (cf. Fig. 1).ChemistrySoLUTES.-The contents of dissolved substancesin lake waters vary within wide limits. As a consequenceof the predominance of Archaean rock andcoarse silicious drift deposits, the majority ofSwedish lakes have low contents of dissolvedminerals. The lowest values are generally measuredin northern Sweden. Electrical conductivity, givenas u20 · 106, is there very frequently below 25 and insome areas of hard rock at high elevation even about10. In South and East-Central Sweden values ofabout 50 are usual for lakes in silicious areas withoutmuch cultural influence. Only exceptionally arevalues as low as 25 or lower met with. The differencemay be related to the smaller evaporation in northernSweden and the slower chemical weatheringdue to the lower soil temperatures.In the large parts of Sweden that have risen fromthe sea fairly recently the continuous leaching ofthe former sea bottoms has an influence on thecontent of solutes in the water; for instance regardingchloride concentration which is greater inyoung land at low altitude. The composition of theprecipitation (E. ERIKSSON 1960) is also differentand close to the West Coast the chloride content ofthe lakes is evidently increased through salt derivedfrom the sea (LYSEN 1960). This effect is demonstrableeven further inland, in the oligotrophicActa Phytogeogr. Suec. 50lakes of the South Swedish upland (MALMER 1961),with higher values for Na and Cl in the westernparts as compared with the central and eastern,and much higher when the lakes of SmaJand, etc.,are compared with more northern Swedish lakes.Most of the latter also show lower figures forsulphate.The sedimentary areas, in particular those withclay or calcareous material, show much highervalues for electrolytes. In the province of Upplandespecially the northern and eastern parts arecovered by drift and clay deposits owing theirhigh content of lime to disintegrated materialderived from submarine limestone further north.Despite the prevailing acid bedrock, the lakes hereare rich in electrolytes with conductivity values ofabout 200 or 300 and occasionally as high as 500.A few lakes in Cambro-Silurian areas show similarhigh values, e.g. at Rattvik in the province ofDalarna. Even as far north as the Abisko valleysome small pools with calcareous bottom depositshave an equally high conductivity, e.g. a Charatarn there. In very shallow lowland lakes thereare great temporal variations due to alternationof periods of high precipitation and of strongevaporation, and other circumstances. In a shallownearly cut-off bay of Lake Malaren, the Hjalstaviken,situated about 50 km north-west of Stockholmand famous for its bird life, values between395 and 730 have been observed. The average ofseven measurements during three years was 518.Values for residue on evaporation in mg/1 roughlycoincide with the values for conductivity given asu20 · 106. However humic waters poor in electrolytesand waters containing collodial clay bothgive higher figures for residues on evaporation,and the waters with very high content of electrolytesgive lower figures for residue on evaporationthan for conductivity.PHOSPHORUS AND NITROGEN.-P and N are keyelements for the productivity in the lakes, beinggenerally in minimum. Analytical values for totalP and total N have a limited significance becausethey do not tell the rate of liberation from thoseorganic substances in which they are included.Humic brown water may give higher values for these