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Notes on aquatic vegetation 213Fig. 3. Boulders covered byLemanea, looking like goats hair.The Kukkolankoski rapids, Torneriver, at fairly low waters. July l 0,1963. Photo S. Pekkari.open throughout the winter, despite the smallerflow. A visit at the Kamlunge rapid in April 1964showed large parts of the river to be ice-free. Belowthe snow-covered ice sheet, the algal growth nowproved to be extremely poor, but within the open,well lighted parts a richly developed algal vegetationwas observed. However, the green of the latesummer and autumn was mostly replaced by agrayish discoloration, found to be caused by diatoms,above all Didymosphenia geminata, or inplaces, only its stalks. Green patches were formedby M icrospora willeana and there were plenty ofyoung individuals of Lemanea, Sirodotia suecicaand Audouinella Hermanii.The northernmost part of the Bothnian BayHardly any coast or estuary in the world showsa gradient from the genuine fresh to a very slightlybrackish water comparable to that of the northernmostpart of the Bothnian Bay. Within a ratherwide area a many-sided ecotone exists, with arichly developed transitional vegetation. It constitutesthe lowest step of the salinity gradient whichelsewhere is mostly abrupt, narrow in space andfluctuating in position. The Kalix, Torne and Kemirivers discharge large amounts of freshwater fromtheir combined catchment area (equal to about 25 %of the area of Sweden, although the Kemi andpartly the Torne river drains Finnish lands). Betweenthe mouths of the Kemi and Torne riversthe distance is only about 15 km, and the width ofthe archipelago that shelters the estuarian watersfrom the open sea is about 40 km, as measured fromthe MalOren lighthouse to the Torne river-mouth.The salinity at Maloren in August is normally 2.5to 3 %o, and the transparency 4 to 4.5 m (at sea,near the centre of the Bothnian Bay, about 8 m).At Salmis, 7 km west of the Torne river-mouth, thesalinity drops to 0.25 %o or lower, almost down tozero at intervals. In the river-mouth itself, thesalinity is of course mostly none and the transparencyabout 3 m.The water composition varies strongly in time,due to the direction and strength of the wind. Atnorthern winds, the lighter and warmer river waterdrifts off-shore, causing a drop in salinity, e.g. withabout 1 %o atMaloren, but also a rise of saltier bottomwater next to the coast, increasing the salinity atActa Phytogeog.r. Suec. 50