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94 PER ERIK LINDGRENTABLE I. Variation (min.-max.) in salinity and signs of pollution in the two river branches and estuaries.Biochemical Total coliformSalinity 02 demand bacteria (37°C)Station Locality %o mg/1 per lOO mlThe fork at K ungalvA. Bredungen river 0.04- 0.06 0.7- 1.8 440- 4,800N ordre RiverB. Ormo river 0.06- 0.19 0.9- 4.5 720- 2,800c. Korseberga inner estuary 0.28-12.6 0.5- 3.3 400- 1,900D. Bassen outer estuary 2.65-16.4 0.7- 5.9 40- 1,000Gota RiverE. Alelyckan river 0.04- 0.06 0.4- 2.1 420- 3,600F. Kampegatan city 0.09- 1.14 0.9-55 10,000-1,200,000G. Rya nabbe inner estuary 3.40-15.0 0.3-14.3 12,000- 830,000H. Nya Alvsborg outer estuary 6.7 -14.6 1.1-15.8 26,000- 850 000The data are taken from investigations made by " Gota Alvs Vattenvardsforbund "on June 11,and Sept. 3, Hl58, June 10, and Sept. 9, 1959, July 20, and Aug. 31, 1960 and June 13, 1963.the interior of the Oslofjord for that reason. Thereare considerable salinity variations and evidentsigns of a weak pollution all over this district,which may be regarded as oligosaprobic.THE INNER TRANSITIONAL DISTRICT.-This districtmainly comprises the Rivo Firth, i.e. thetransition from estuary to open firths. It was calledthe Fucus edentatus- Ulothrix district in my previouspaper. Among the algae ceasing inwardswithin this district could be mentioned, e.g.,Bryopsis plumosa, Elachista fucicola, Polysiphoniaviolacea, Furcellaria fastigiata.This district shows strong variation in salinityand at the same time strong signs of pollution. Itmay be classed as mesosaprobic. Indications are forinstance the ample growth of Spirulina subsalsa(chiefly as a blue-green film on the shells of theabundant Mytilus edulis), great masses of detachedUlva lactuca £. latissima, etc.THE EsTUARY.-This highly polluted area knownas the Alvsborg Firth and referred to as the Spirulinasubsalsa -Phormidium district in my earlierpaper, shows extreme salinity variations and is unsuitablefor most marine algae. However, it isreached by Fucus serratus, F. vesiculosus, F. eden-tatus, Chondrus crispus, Phyllophora membranifolia,Pylaiella rupincola and Porphyra umbilical is f. laciniata.When Cladophora glomerata grows here, it isdark green but covered by diatoms (Melosira, etc.)and vorticellids in such quantity as to make itappear grey-brown.VERTICAL ZONATION.-The prevailing type ofzonation of dominants in the various districts isevident from the generalized and highly schematicaldiagram, Table 2.The lower part of the zonation was studied bydiving at a number of localities, and detailed profileswere obtained. There is considerable variationbetween profiles in the same district, due to localconditions, e.g. slope, depth and exposure. Spacedoes not allow the presentation of this material,nor of the many autecological details that appearfrom these studies.The waters off the mouth of the Nordre RiverThe extensive outer parts of these waters havenot yet been studied in sufficient detail to allow asubdivision into districts. Only the inner, estuarianparts will be dealt with here, from the NordreFirth to the river mouth. This area includes onlywaters of strongly reduced salinity. The river water.Acta Phytogeogr. Suec. 50