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The vegetation of Swedish lakes 29Fig. l. Left: average dates forice formation and break-up; right:average duration of ice cover; sG•for lakes of moderate size and2.depth. From Angstrom 1958.(/o• 2. 4" 2.-,o•4,"greater than it is today. Many at first water-filledshallow depressions have been transferred intomires or are gradually becoming filled in with mudand peat. In consequence, lake shores consistingof peat are very common, in particular aroundsmall lakes or tarns with a quagmire fringe. Theincrement of the quagmires is often counteractedby erosion as seen along many of the small lakesin the woodlands of North Sweden.Lake climateAs discussed elsewhere ("Features of land andclimate") the temperature in July is about thesame in all the lowland of Sweden. From the Gulfof Bothnia summer temperatures decrease towardsthe west with the increasing altitude. In smalllakes the surface temperature is not very differentfrom that of the air even though fluctuations aresmaller. Large and deep lakes are heated moreslowly, and the surface water does not reach suchhigh temperatures as in small lakes. In the Abiskovalley on the south side of Lake Tornetrask innorthernmost Lappland small lakes may show temperaturesof about +20°C at the end of a few daysof nice summer weather. In the large and deepLake Tornetrask itself the surface water abovedeep-water areas will rarely reach + l0°C. Thelarge lakes in Lappland are always too cool forswimming but shallow lakeside lagoons may bequite agreeable for this purpose in the later partof summer.It is obvious that nearby sites can offer aquaticplants very different climatic conditions, and in thesame bay considerable micro-climatic differenceshave been recorded. If two similar areas with calmwater are compared, an area with tall dense reedsof Phragmites is cooler than one with direct insolation,especially when water circulation is hamperedActa Phytogeog.r. Suec. 50