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Notes on the vegetation of lakes in the woodland of Lule Lappmark 229Fig. I. The broad lower geolittoralof Lake Satisjaure (eastern part),almost sterile except for a crustof algae or lichens on the largerboulders. Aug. 16, 1961. Photo .L. Granmark.so called because of the occurrence of the pearlbearingfreshwater mussel, Margaritana margaritifera).Each river arm originates through the confluenceof several feeder streams in the mountains andpasses through a series of large lakes. Below theScandes, both rivers flow through the type of gneissand granite landscape, described in "Lapplandeast of the mountains". As stated there, the localclimate is of a continental type. The duration ofwinter is usually seven months, and summer lastsonly two and a half months with a mean temperaturein July at about + 14.5°0. As the woodlandto some extent is situated in rain-shadow theannual precipitation is low. The mean value forKvikkjokk, at the western end of the Lilla Lulelake chain, is 580 mm, and for J okkmokk, at theeastern end, 490 mm. The water flow in the riversduring summer is not so low as one might perhapsexpect, because the maximum precipitation is inlate summer and a large part of the catchment areais in the high mountains, where snow-fields andglaciers thaw in summer and large amounts ofprecipitation fall at all seasons.Most of the great lakes in this district are situatedin old rift valleys and in some cases the lake basinswere further gouged out by glaciers during the Iceages. Moraines transported by these glaciers wereoften deposited in the eastern part of the basins,damming the usually long and narrow lakes. Suchlakes have great depths to the west but are shallo""'in their eastern parts. The best examples in theLule river system are the lakes Saggat and Satisjaure.With the exception of small ponds and lakesin the mire areas most of the lakes between theriver valleys are also of glacial origin.HuMIC LAKES.-The lakes are either. clearoligotrophicand highly transparent, or of the dystrophic,humic type, but transitions between the· two types are also frequent. Dystrophic lakes, frequentespecially in the vast peatland areas, suchas Sjaunja and Muddus, are generally shallow.From the surrounding woodland and peat theyreceive large quantities of dissolved humic materialgiving the water a colour from yellow to differentshades of . brown. Moreover the water of thesedystrophic lakes generally has a very low contentof dissolved electrolytes and a distinctly butusually not very strongly acid reaction. The transparencyin these waters is as a rule about two metres,and seldom more than 3.5 m. Unfortunately neitherthe water chemistry nor the microflora in theselakes has been studied. The only published informationavailable concerns the higher water vegetation(QuENNERSTEDT 1960, T. ARNBORG 1963). In someActa Phytogeogr. Suec. 50