fulltext - DiVA

20 MATS W.LERNthe West Coast is caused by the northward "Balticcurrent" which has a variable vertical extension,in the southern Kattegat perhaps 20-30 m, ineastern Skagerrak 10-20 m.Baltic traits may be traced in the West Coastvegetation, and as expected they are most markedin its southern part; see KYLIN (1907) who studiedthe whole West Coast. The Baltic current must beregarded as a source of brackish-water componentsin the otherwise prevailingly purely marine vegetationof the West Coast, e.g., Cladophora glomerataand probably a diaphanoid Ceramium not yetcleared up taxonomically.During the year water of different type andorigin, with a higher salinity than that of the surfacecurrent, travels southward below the latter, affecting,as previously mentioned, the algal vegetationmainly at greater depth.The contrast is most marked when the current isstrongest. This happens in spring and summer.From the works of BRATTSTROM (1941) and GISLEN(1929) we quote the following salinity values (in %o):OresundThreshold, averageNorthern part, average(at bottom 32-34)Fladen, KattegatJune-Sept.Oct.-Febr.Grisbadarna, Skagerrak(off Koster, N. Bohuslan)May-JulyOct.-Febr.(all the year > 32 at 40 m)Surface10.613-1416-2021-2617-2326-3120 m30ea 33ea 2931-32These figures may serve as a background tothe fact that the most marine formation of theSwedish vegetation, the Laminaria hyperborea"forest" with its characteristic undergrowth includingseveral red algae of the Atlantic, does notreach the surface of the water but extends southinto the Kattegat in deep water. This type ofmarine vegetation apparently also has a preferencefor a position where it is moved about by the swell,as observed by diving on the British coast (JOANNAM. KAIN 1960, 1962).The bottom water of higher salinity is broughtfar to the south. A comparatively rich marine floramay therefore be met with, chiefly in the deeperwaters, as far south along the Swedish West Coastas Oresund, and certain species even close to thethreshold in its southern part.South of Denmark, bottom water, diluted toabout 17 %o salinity, overflows the Darss thresholdand enters the Baltic proper, being its main sourceof salinity, whereas the outflowing surface waterhas here about 8! %o (JENSEN 1940). In the Oresundthe surface water flow may occasionally shift directionand when south-going may introduce somesaltier water into the Baltic. The bottom waterwith its very high salinity reaches and overflowsthe Oresund threshold only on rare occasions.Enough of the saltier bottom water is brought intocirculation to increase the salinity of the northwardsurface current rapidly, especially in the Oresundand southern Kattegat.The Baltic current thus consists only to a smallerpart of water of Baltic origin, but is carrying mixedwater towards the north.As previously stated the thresholds in the Oresundbetween Copenhagen and Malmo are at 8 to 7 m. Inthe strong current, the vegetation on this flint bottom("Flintrannan") is peculiarly dark-coloured, veryclean, poor in species and almost devoid of epiphytes.Beside Lithoderma, Lithothamnion Lenormandii andH ildenbrandia prototypus on the flints and Dumontiaincrassata on the small-sized Mytilus (occurring here ingreat quantity), this vegetation seems largely to consistof "loose-and-entangled" plants fixed by Mytilus.. Ceramium rubrum and 0. cf. tenuicorne grow ascushion-formed coatings, but for the rest there arefew species in scattered individuals: small Laminariasaccharina, Rhodomela subfusca, M embranoptera alata,Chaetomorpha linum and Furcellaria fastigiata. (Investigationwith "Sunbeam", July 18, 1964, station162/64. )South of the threshold in the southern Oresund,Baltic water prevails throughout the zone of algalgrowth. This causes the deep-water vegetation tobe quite markedly changed on passing that threshold,and more so than is the case with the vegetationof the upper horizon. (The disappearance of thelast trace of an intertidal belt takes place furthernorth in the Oresund.)The counter-clockwise circulation (R. WITTING1912a and b) of the surface water in the Baltic seasleads to a slightly higher surface salinity on theirActa Phytogeogr. S'ttec. 50