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32 GUNNAR LOHAMMARmake up only a small fraction of the total area ofthe country. The majority of the Swedish lakes aresituated in the uplands where the oligotrophictype prevails. Transitional cases are of coursecommon and any boundary between categories ismore or less arbitrary. Many oligotrophic watershave been auxotrophified through human influence.As an example Lake Luossajarvi situated withinthe subalpine birchwood region at an altitude of500 m thanks to the vicinity of the mining townof Kiruna shows intense water bloom in summer(SKUJA 1964).Fig. 2 a. Example of an eutrophic aquatic specie$ thatreaches far north. From Hulten's Atlas.and Central Jamtland, the lakes are more or lesscalcareous and some of them also rich in otherrespects.The eutrophic regions previously mentionedOMNIPRESENT SPECIES.-Some aquatic plantsoccur all over Sweden-except for the alpine regions-or within the greater part of the country, bothin oligotrophic and eutrophic environments. Thethree most important helophytes of the Swedishlakes are good examples: Phragmites, Scirpuslacustris and Equisetum fluviatile. Similar widedistribution areas have for example Eleocharispalustris, E. acicularis, Nuphar luteum Polygonumamphibium, Potamogeton natans, P. gramineus, P.perfoliatus and Utricularia vulgaris. Most of themdecrease in frequency in the interior of NorthSweden towards their distributional limits, but theyare common in the coastal areas · near the Gulfof Bothnia as well as in the southern half of thecountry.Because of the very great differences with regardto climate and chemical conditions a rich differentiationin ecotypes is likely within such widespreadspecies. Only few have been investigated in thisrespect, e.g. Phragmites communis (BJORK 1963),Eleocharis palustris (STRANDHEDE 1961) and Polygonumamphibium (TuRESSON 1961), all found toconsist of many ecotypes.Fig. 2 b. A southern eutrophic aquatic and shore plant.Note the easterly trend in Fennoscandia. From Hulten'sAtlas.Acta Phytogeogr. Suec. 50VEGETATION OF EUTROPHIC WATERS .-Eutrophicwaters ·are characterized not only by the luuriantdevelopment of common species but also by theoccurrence of species that actually need an environmentrich in nutrients. Several species of the lattercategory have their northern limits in southern orEast-Central Sweden, others have wider distributions,reaching areas north of the Bothnian Bay oreven northern Lappland (cf. Figs. 2a, 2b, 6).