fulltext - DiVA

The major rivers of Northern Sweden 201Fig. 2. Lilla Lule river near Jokkmokk, Lappland. Akkatj rapids at autumn low water. The boulders partly covered byColeochaete divergens v. catharactarum around the water-line up to a height of 2-3 dm. Spirogym lapponica and Zygnemamelanospor·um frequently occur at the water-line. The dark mosses are probably Schistidium Agassizii. Oct. 9, 1957.Photo N. Quennerstedt.period, when the seething waters render detailedobservation impossible. Investigations can only beundertaken when the spate has greatly diminished,and then the dried-out surfaces may be more orless covered by a grey coating, chiefly composedof diatom frustules, and, to a lesser degree, ofdried Zygnemales and other algae.Towards late summer, in August and sometimesin July, long green veils of Zygnemales and Oedogonialesappear at the then-prevailing water line. Aluxuriance of Bulbochaete appears as a mucilaginouslayer on the vertical, current-washed sides ofboulders, and in addition a profusion of smallalgae, mostly diatoms but also desmids, appear. Asa rule the algae grow in greater quantity on thecurrent-washed surfaces than on the surfaces facingcalmer water.The Zygnemales are usually sterile and consequentlydifficult to identify. IsRAELSON (1949) hashowever succeeded in distinguishing taxonomicunits amongst sterile species of M ougeotia, Zygnemaand Spirogyra by investigating a wide range ofspecimens. He has also summarised the regionaldistribution of certain attached Zygnemales inSweden. Two fertile species are, however, foundregularly in certain rapids of northern rivers,especially during late summer, when the waterlevelsinks towards minimum value. Around andsomewhat above the descending water-line are seenfertile Spirogyra lapponica or Zygnema melanosporum,chocolate-brown or blue-black, respectively,instead of the usual bright green filamentsof the vegetative phase. Both of these species,described by LAGERHEIM (l884a, b), were seentogether in the Lilla Lule river and they have alsobeen recorded from other rivers, chiefly due toJSRAELSON (op. c.) and PEKKARI (this volume).Another filamentous alga, M ougeotiopsis calospora,indigenous to a great part of Scandinavia, has beenobserved on the same North-Swedish habitats asSpirogyra lapponica and Zygnema melanosporum.At the time when the Zygnemales attain fertility,a green layer of Coleochaete, described by LAGER­HElM (l884a, b) as C. divergens v. catharactarum,appears on the current-exposed surfaces of thesehabitats in Lilla Lule river (Fig. 2). The layer con-Acta Phytogeog.?·. Suec. 50