fulltext - DiVA

202 NILS QUENNERSTEDTsists of irregular, gelatinous, dense clusters, about0.5 cm across, which resemble green marmaladespread on the rocks. This Coleochaete species hasbeen observed in such profusion among the rapidsof Lilla Lule river that most of the boulders arecoloured green around the water-line. It is foundalso in the Torne and Kalix rivers (cf. PEKKARI}and the river Ljungan (Fig. 3), although not yetobserved in other rivers.As previously stated, the gradual reduction inwater-level, which takes place in late summer andautumn, results in a change in colour, affecingSpirogyra lapponica and Zygnema melanosporum.Scattered grey patches, situated at the same heightabove water as the uppermost fertile Zygnemales,and increasing in density and size as they approachthe water-line, indicate the presence of anotherquantitatively important component in the autumnvegetation of the main rivers, i.e. the diatom Didymospheniageminata. The colonies of this species,which, in late summer, begin to flourish at thewater-line, are usually very soon left high and dryas the water sinks, appearing on the rock surfaceas grey flecks about 2 mm in size. At lower levels,with longer submersion time, the colonies growlarger before drying out. In the zone below autumnlow-water, where growth continues undisturbed,grey and brown tufts, 2-3 cm in diameter, are to bef:!een. They can grow so densely that they coverthe greater part of the boulder surface directlyfacing the current. Within two or three weekstime, a homogenous blanket of Didymospheniahas been formed that generally survives throughoutthe winter, in those rapids which are not iced-over.It has, as a matter of fact, been observed evenunder the ice. Most of the living Didymospheniacells seem to gradually disappear from the resistivemucous coating, which they have themselvesproduced, whereupo:p. the brown colouring diminishesand a dirty-grey tone becomes predominant.Even when the Didymosphenia tufts are nohigher than t cm, they are already inhabited bymicro-algae, especiaHy Achnanthes minutissima v.cryptocephala and also Anomoeoneis exilis, Ceratoneisarcus v. linearis, Diatoma elongatum, Synedraacus, etc.The Didymosphenia vegetation even thrives inrivers where the natural flow has been controlled,and the macroscopic Didymosphenia tufts are seenin all the main northern rivers and in many of theirlarge tributaries from the Torne river in the northto Ljusnan in the south. (Pite river, which hasnot yet been investigated, most certainly supportsthe same vegetation.) Smaller streams, situatedentirely within the North Swedish conifer zone,do not appear to contain this species, which is,however, found in relatively small rivers at higherlevels, e.g. in the Abisko region (SKUJA 1964, pp.418-419, 440), in company with Phormidiumvalderianum (a Cyanophyceae resembling Didymospheniain manner of growth) and also togetherwith M ougeotiopsis calospora, Spirogyra fluviatilisand Zygnema melanosporum. It would appear thatDidymosphenia cannot exist in rivers where thewater-level sinks below a certain minimum.With the succeeding year's spring flood, theDidymosphenia blanket is torn away from itssubstratum and swept away with the current. Verylarge quantities of this vegetation, resemblinglarge, dirty tufts of cotton-wool, are carried awaydownstream, and cause considerable annoyance tofishermen by fouling their equipment (V ALLIN1951).A hitherto unmentioned alga group, the Florideae(Rhodophyceae), whose taxonomy and ecology hasbeen studied by IsRAELSON (1942), is not of anyquantitative importanc in the large North Swedishrivers, with the exception of the lower rapids ofthe Torne river, although it is often present in thesmaller waterways in the forest districts. Lemaneafluviatilis and L. condensata are common in theKukkolankoski rapids in the Torne river (cf.PEKKARI), where Audouinella (Chantransia) Hermanniand Sirodotia suecica are also often encountered.Audouinella Hermanni was observed(September, 1961) in large quantities in the Muonioriver. Both Audouinella Hermanni and Sirodotiasuecica, together with Lemanea condensata alsooccur in the Kamlunge rapids in the Kalix river.Batrachospermum (e.g. B. moniliforme) is found insmall quantities in large rivers but only occasionallyand far apart.In the north Swedish alpine region, where limestoneand dolomite occur, SKUJA (1964, pp. 359-Acta Phytogeogr. Suec. 50