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A vista on the marine vegetation 23STEDT (1928) found a true although much reducedlittoral with its fucaceous belts. At the stormypromontory of Kullen, the exposed rocks have afringe of "F. vesiculosus compressus f. racemosa",a narrow plant without vesicles; in more shelteredplaces F. spiralis still grows at the water's edge,and also Ascophyllum in a belt a few centimetresbroad just above the permanently submerged F.serratus. The normal, West Coast type of F. vesiculosusis also present where some shelter is offeredand extends some distance down from the surface(LEVRING 1935, p. 29).Further to the south, in the Oresund, F. spiralis,"F. vesiculosus compressus f. racemosa" and Ascophyllum(as well as Porphyra and N emalion) passout of the flora, and the littoral Fucaceae formationdisappears. It is replaced by a shifting vegetationof ephemeral algae such as Pylaiella, diaphanoidCeramium, Scytosiphon, Cladophora cf. glomerata,and Enteromorpha, the latter especiaily in pollutedwater (SJOSTEDT 1928).In the older Swedish phycological literature (KJELL­MAN 1878, SVEDELIUS 1901, KYLIN 1907) the consequenceof these conditions was regarded to be a downwarddisplacement of the lower boundary of the littoral.Thus the infralittoral (sublittoral) was said tobegin at the shift from fucaceous to laminariaceousvegetation, at 3-4 m on the Skagerrak. In the Balticproper SVEDELIUS placed the boundary of his lowerlittoral where the dense vegetation of Fucus vesiculosusdisappeared. However, this delimitation was difficultto carry out. Although of interest as a perception ofthe "downward process", this classification of thebelts was abandoned since SERNANDER's treatise waspublished (1917).The belt of temporary inundation in the Baltic,about t m high, dries up at low waters in earlyspring and then becomes completely void of algae.No permanent growth can be established because ofthese periods of dryness, and perenial algae arethus absent from the water's edge in the Baltic.Rapidly growing algae of the type obligatelybound to the surface of the water colonize at intervals,in short periods in spring spp. of Ulothrix andUrospora penicilliformis, occasionally Bangia. Althoughthese algae disappear whenever the waterrecedes and leaves them to dry up, some maysurvive in moist crevices facing north. In JuneUlothrix zonata and from July to early autumnlarger algae such as Dictyosiphon chordaria andspecies in common with the infralittoral ( Ceramiumtenuicorne, Cladophora glomerata, Dictyosiphon foeniculaceus)colonize. Their colonization may registerthe temporary water level when diaspores of thespecies concerned were available (observations inthe Aland Sea).In winter U rospora penicilli form is and Bangiamay colonize several metres high up on the rock,just under the ledge-like "ice-foot", i.e. in theCalothrix- Verrucaria maura belt. This is the lowergeo-littoral of Du RIETZ (1950d), whereas the previouslydescribed "belt of summer-annual filiformalgae" (Du RrETZ 1925a, 1930c, 1932b) is called thehydro-littoral.This vegetation extends far north in the Balticseas. On the southern Bothnian Sea large sectionsof the coast are very poor, and there it is reducedor even absent but is established in polluted areas.It also grows on the edges of rocky basins and bouldercoves of the "white-bird skerries" inhabited bycolonies of gulls (W lERN 1952, Fig. 74).The downward processAs mentioned, a downward displacement of algalspecies is observable along the Swedish coast, allthe way from the Skagerrak to the Gulf of Bothnia.The successive downward dislocation and final disappearanceof many truly marine species is obviouslyin some way, directly or indirectly, relatedto the reduction in salinity, but the first steps also,as previously dealt with, to the reduction of thetide. Other conditions such as the inwardly increasingseverity of the winter may exert an influence,but the total effect runs parallel to a general fadingof the typically marine environment.Possibly one might even extrapolate a continuationinto the freshwater, following a series of furtherreduced electrolyte content, e.g. from the eutrophiclowland lakes to certain parts of the mountains wherethe rocks are poor and the conductivity in lakes andstreams as low as about 10 · 10-6 or even lower. Thusextended, the process involves a reduction of the sharpcontrast between land and water that prevails on thesea to a state where there is comparatively little differencebetween terrestrial conditions (at least duringrain) and the lacustrine habitat. Many species-inActa Phytogeog1·. S.uec. 50