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96 PER ERIK LINDGRENThe latter two were only observed upstream of theOrmo dam which was constructed in order toprevent brackish water from penetrating furtherupstream at high waters in the sea.TABLE 3. The estuarian zonation of dominants in theN ordre Firth.Outer part:Stora KalvenMiddle part:Bass enUlothrix-Urospora communityFucusvesiculosusFucusserratusLaminariasaccharinaPhormidium·· Litus lineEnteromorpha ahlnerianaFucusvesiculosusFucusserratusInner part:DrottningskaretcoriumUlothrix zonataOladophora glomerata.Z'.tlytilusedulisVERTICAL ZONATION.-The zonation of dominantsis evident from Table 3. In the outer estuary it issimilar to that in the southern inner transitionaldistrict, except for the absence of a Fucus edentatushorizon. The downward displacement of the F.vesiculosus and F. serratus belts in the middleestuary is interesting, and gives room for a horizonof Oladophora glomerata above them. This alga offresh and brackish waters is only found sparsely atStora Kalven further out. In the southern area itappears scattered as far as the outer transitionaldistrict. As stated above, it has a deep greencolour in the estuaries.ConclusionsAs this paper is condensed from a large unpublishedmaterial, the conclusions must be presentedwithout much of the evidence on which theyare based. Attempts have been made to comparethe algal response to the salinity gradient with thereach of various species around the coasts to thesouth and further, into the Baltic and Bothnianbasins. As to the pollution gradient, comparisoncould be made with the conditions found outsideHelsingfors by HA.YREN (1921) and in the innermostpart of the Oslofjord by GRENAGER (1957). However,obvious differences in the indicative status ofmany algae exist in these areas.The presence of Oladophora rupestris at lowsalinity, the absence of Spirulina subsalsa and therarity of Porphyra and Ulva, etc., are signs of alow degree of pollution in the northern area, whichmay be regarded as oligosaprobic.The greater degree of pollution in the southernestuary is evident from several indications. (1) Theposition far out, at higher salinity than expectedfrom distribution data, of the inner limits of Cladophorarupestris, Fucus spiralis (nearly exclusively inthe off-shore district) and Trailliella intricata (commonin the off-shore district). (2) The occurrence ofa Fucus edentatus horizon in the two transitionaldistricts. (3) The composition of the Enteromorphahorizon, with little E. ahlneriana (the chief dominantin the northern estuary). (4) The mass occurrenceof Mytilus edulis in the two inner districts.( 5) The occurrence of Spirulina subsalsa in thesame districts. (6) The mass occurrence of (detached)Ulva lactuca f. latissima in the same districts, wherethe floatsam lines on the shores are made up chieflyof this species and Enteromorpha. (7) The gray"pubescence" on Oladophora glomerata.Further upstream, in the Gota River, accordingto G. STALBERG (unpubl.) occur such organisms asZoothamnion commune (an infusorium), Beggiatoaalba and Cladothrix dichotoma (bacteria, the latteraccording to KoLKWITZ 1926 a juvenile form ofSphaerotilus natans). They all indicate polysaprobicconditions (HA.YREN 1921) ..Acta Phytogeog1·. Suec. 50