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86 JOH.AN SODERSTROM40 9t,6.1953 20.6 30.6 10.7Fig. l. Fluctuations in water-level and position of theFucus vesiculosus belt on a sheltered shore. Borgila Fjord,BohusHin, first half of summer.vertical extension that coincides with this gap.The microclimate in this part of the shore must bevery special, both in winter and summer. Verrucariamaura forms a belt only on the north and north-eastsides of the rocks (cf. Du RIETZ in NANNFELDT &Du RIETZ 1952).In summer the surface water temperature usuallyreaches values around l8°C. Lower temperaturesare met with in the outer parts of the archipelago,and higher in shallow parts of bays and inlets.A SHELTERED SITE.-To exemplify a shelteredinlet in BohusHin I have chosen Borgila Fjord, achannel that has much in common with a Scottishsea-loch. It has narrow entrances and is less than2 km long, exposure to waves being everywhere oflittle importance.The many seasonal changes make it difficult todescribe the littoral zonation in Borgila Fjord in afew words. The schematic profile (Fig. 3) presentsconditions at the end of July on an open shorefacing west.At the end of February Dumontia incrassata andMonostroma Grevillei, as well as other species thatlike these two disappear or are scarce in summer,leave their mark on the shores just beneath -thewater surface. If the winter has been icy Fucusvesiculosus and Fucus serratus are often wornaway, and where Fucus serratus usually grows insteada cover is formed by Laurencia pinnatifida,in the autumn seen as an epiphyte on Fucus serratus.In slightly exposed localities the Calothrixbelt has a lower part with Ulothrix flacca (situatedjust above the Fucus vesiculosus-Balanus balanoidesbelt and even entering its uppermost part as anundergrowth), and a little higher up Bangia fuscopurpureamixed with Ulothrix forms a distinct belt.A close examination of the bigger algae revealssporelings of Pylaiella and Cladophora. Species ofthese genera are later on to become the mostconspicuous ones among the epiphytic algae.In April the composition of the vegetation ismuch the same but Monostroma, Dumontia andLaurencia are beginning to show signs of destruction.Pylaiella litoralis is now well developed and isthe dominant epiphyte on Fucus vesiculosus. In theFucus serratus belt small specimens of Chondruscrispus begin to a pp ear and Scytosiphon lomentariais growing rapidly. Further down Chorda filumcan be found as about five cm long specimens growingin the shadow of Halidrys siliquosa covered byTrailliella intricata.In the beginning of May a belt with Spongomorphapallida and Acrosiphonia centralis develops withinthe Balanus belt--especially on shores facing north-and Scytosiphon lomentaria is a characteristicelement in the lower part of the Fucus vesiculosusbelt in sheltered localities. The gaps in the Fucusvegetation begin to be filled with sporelings somefew cm high. Dictyosiphon foeniculaceus is a commonepiphyte on Fucus serratus.When the summer warmth arrives in June speciesof Cladophora attain their full growth. For a monththe Fucus individuals carry delicate green tufts ofCladophora oblitterata and near the surface, onshores with some wave action, Cladophora glauces-60cm-LL WiVertical extensionif Fucus vesiculosusand .Balanus balanoides1s 10days15Fig. 2. Maximum duration of time of emergence at differentlevels on a completely sheltered shore in Borgila Fjordand the position of the Fucus vesiculosus - Balanus balanoidesbelt. June 2 to Aug. 15, 1953.Acta Phytogeogr. Suec. 50