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ASPE CTS OF THE SOUTHVertical Zonation of Littoral Algae in BohusHinBy JOHAN SODERSTROMFrom a marine botanist's point of view the provinceof BohusHin is remarkable because of its diminishedand irregular tides, the shoreward and southwarddecreasing salinity and the great difference betweensummer and winter temperature in the surfacewater. There is a variation from fairly strong exposureat the outermost skerries to complete shelterin the inner parts of coves and inlets. A number ofnarrow sounds complete the picture of an archipelagothat provides a wide variety of ecologicalproblems.The semidiurnal fluctuations of the water levelhave in BohusHin an amplitude of one foot approximately.Alterations due to weather conditionsare of a greater magnitude; during periods of strongwesterly winds and low air pressure the watersurface may rise more than one and a half metrea hove the low levels prevailing during periods ofeasterly winds and high air pressure. These irregularchanges make it difficult to study the relationsbetween zonation and water levels. However, observationsin the sheltered inlets indicate that thecommon littoral algae have comparatively littleadvantage from the irregular changes, a few shortperiods of high water not being sufficient to allowa colonization of truly marine plants in the upperparts of the littoral. On these completely shelteredshores, the periods when the algae are emerged increaserapidly upward to several days or even weeksfrom a level about 30 cm above the summer low waterline. Nea this level one finds the uppermostspecimens of Fucus vesiculosus and also roughly theupper limit of Balanus balanoides as occurringplentifully.A complication due to seasonal variations mustbe observed. During late winter and spring thereare usually long periods of very low water (cf.GILLNER 1960, Fig. 9, and his contribution to thisbook) in combination with weather conditions thateffectively keep the rocks dry above the watersurface. The colonization of spedes that are fertileduring this time is therefore limited to levelsbeneath what would be expected from observationsin summer. Some winters the low water is coincidentwith formation of thick ice, and when such a winteris followed by a sunny spring the result will be acondition of the littoral vegetation very differentfrom what would be found if a mild winter hadbeen followed by a rainy spring.The surface salinity which is around 30 %o in theopen waters outside the archipelago of Bohuslandecreases to around 20 %o on the southern parts ofthe Swedish West Coast. A similar, but inward decreaseis met with in many inlets. There are alsoconsiderable seasonal changes and in the inner partof the firths the salinity may change so rapidly thatthere is a difference from one day to the next ofmore than 10 %o . Littoral algae in the innermostparts of some firths may have to endure periodswhen the salinity is only one tenth of what is normalalong the coast.In cold winters broad waters may be coveredwith ice, in other winters ice is only formed inshallow bays. Even when no ice is formed the rocksmay be covered by snow for a considerable timeduring January and February. Owing to the relativewarmth of the water a snow-free zone is formedjust above the water surface, having a verticalextension of 2-3 dm . In late winter and early springthis gap between snow and water is inhabited by anephemeral growth of Bangia fuscopurpurea. TheVerrucaria maura- Calothrix scopulorum belt has aActa Phytogeog.r. Suec. 50