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Coastal algae oft Goteborg 93., ,. .... ..,· 'i):0 •()Fig. I. Sketch-map of the archipelagooff the mouths of N ordreand Gota Rivers. City centre andindustrial areas in black, otherdensely built-over areas hatched.Figures refer to salinity stations(Fig. 2), letters to samplingstations in the rivers and estuaries(Table 1).0sJOkmlina officinalis). Even though the majority seemfirst of all to need a high salinity, some may havetheir inner limits within the off-shore district becauseof other reasons, e.g. the lower transparencyor other unfavourable features, including influenceof pollution, in the waters farther inshore. However,the off-shore district is to be regarded as nearlyunpolluted (katharobic) although Porphyra umbilicalisf. laciniata occurs in its inner part.Some species, e.g. Corallina, Polysiphonia Brodiaei,Chaetomorpha aerea, Spongomorpha pallida,inhabit strongly exposed rocky shores not foundfarther inshore; the latter alga is in this districtalmost certainly independent of the salinity gradient,for it enters the Baltic waters (W JERN1952).THE OUTER TRANSITIONAL DISTRICT.-This areaincludes the wide open firths of the central archipelago(Dana Firth, most of Hake Firth), and thewaters between the Gota estuary and the AskimFirth to the south. It was earlier characterized asthe Fucus edentatus-Calothrix district (LINDGRENop. c.). Species finding their inshore limit in thisdistrict include, e.g., Laminaria digitata, Halidryssiliquosa, Rhodomela confervoides ( =subfusca), Cladophorarupestris. Again, at least the latter two arehardly limited by salinity. According to WrTTROCK(1878), Cladophora rupestris occurs on the Bothniancoast (at about 4 %o salinity), and it is encounteredin the Nordre estuary (locality Bassen, see Table 1).It is probably intolerant of too strong pollution;cf. GRENAGER (1957) who statesthatitis absentfromActa Phytogeog.r. Suec. 50