fulltext - DiVA

16 MATS WJERNmetres below the surface. Beside the mentionedspecies one encounters Laminaria saccharina butnever L. digitata.Starting from the pure organic mud as one extreme,the "warped" vegetation forms a series oftransitional stages leading to vegetation in a"cradled" position, the extreme type of the latterbeing the crystal clean barnacle facies on the mostexposed rocks, with small red algae growing closeto the surface.Broad archipelagos occupy large parts of Sweden'scoasts, and a great work remains to be doneuntil we know the varying vegetation of thesewide waters. As one sails inshore, the ceasing of theswell and the decrease of the transparency of thewater as well as the increased deposition of sedimentshave a great influence on the distributionand composition of underwater vegetation.In archipelagos of the Baltic the most profoundchange in the vegetation may set in already whenone is passing the outer fringe of an archipelago.The sheltered sides of even the outermost skerries,at a moderate depth, lose the freshness typical ofthe shores that are absolutely unprotected. Thiscontrast is not so striking in that part of Bohuslanwhich receives the rough long waves of the NorthSea over the open Skagerrak.Outside of the isolated off-shore group of isletsand skerries called Vaderoarna, the sea may duringheavy westerly gales break over banks at a depthof about 18 m (probably Laminaria hyperboreabottoms). Further south, the coast is protected bythe spit of Skagen which acts as a breakwater. Inthe Baltic, the exposed shallows break the wavesonly at a much lesser depth, but here a new factormust be taken into account: the pack ice which hasbeen observed to run aground on rocks at a depthof many metres (cf. ice map, Fig. 3).It is probable that tidal currents contribute tokeep the vegetation clean of sediments in the archipelagosof Bohuslan. In the Baltic, however, destituteof a real tide, the currents are due to outflowsor influenced by changes in water level related tothe shifts in wind direction and air pressure. U nfortunatelywe know too little about the currentswithin the archipelagos, especially the bottomcurrents, and are too dependent on deduction fromActa Phytogeog1·. Suec. 50the general physiography of the system of inletsand "fjards" (inshore basins).Along certain Baltic coasts a submarine archipelago,off-shore of the skerries, has been observedto act as a screen in front of the visible skerries,as for instance, off the coast of SmiUand and in theouter Stockholm Archipelago. Although the effectof this screen may escape notice near the watersurface around these outer skerries, which carry avegetation seemingly typical of strong exposure,the shelter is perceptible in the deeper infralittoral.Where we would have expected a beautiful andstrong-growing vegetation of Rhodornela subfusca,Polysiphonia nigrescens, Sphacelaria arctica withAglaothamnion roseum, and outside Sma1and Fucusserratus, instead Furcellaria fastigiata is found togrow with entangled, loose individuals of PhyllophoraBrodiaei, Ph. membranifolia, Rhodomela subfusca,together with a great amount of the Balticsmall-sized Mytilus edulis. This indicates a somewhatsheltered aspect of the infralittoral algalgrowth of the Baltic, and constitutes a "warped"vegetation, to be compared with the above-mentioned"warped" vegetation of Bohuslan as foundin the Gullmar Fjord, as a matter of fact even withsome dominant species in common.This "warped" vegetation of the Baltic is notrestricted to growth on the rocks. In its more shelteredaspects, it covers even certain bottoms ofclay or of clay with fine sand in the "fjards",either as loose-lying or as loose-and-entangled inthe byssus of clusters of Mytilus. A similar waveringtransition between an attached and a loose-lyingway of living is also characteristic of the warpedvegetation of Fucus vesiculosus in the inner Balticarchipelagos (we have to realize that Fucus vesiculosusis an infralittoral plant in the Baltic, growingon the rocks from about 0.5 down to 10 m andmore). Even on the rock one may find a mixture ofattached and loose plants, and on the clay or theorganic mud occurs a dense cover of loose-lyingcomparatively big warped plants. In these mats ofloose-lying Fucus may sometimes be found shootsof loose-and-entangled Zostera marina. They growerect, looking as if they were attached, but theirroots are black, affected by the decomposition ofthe lower stratum of the thick Fucus cover.