Long-term changes in the sublittoral zonation of brown algae in the southern Bothnian Sea

Abstract

The algal vegetation of five diving profiles situated at exposed sites of the outer Öregrund archipelago, southern Bothnian Sea, Sweden, was investigated by SCUBA diving in 1996. The vegetation of the same profiles had been described earlier in 1943–4 and 1984. In the present study, special attention was paid to the occurrence of the structurally important belt-forming brown algae Fucus vesiculosus and Sphacelaria arctica in comparison with the situation 52–53 and 12 years earlier. Prominent differences between the 1940s and 1996 were found. The weighted average depth of the F. vesiculosus belt in 1996 was about 1·7 m shallower than in the 1940s, and the lower distribution limit of this species was about 2·5 m shallower. At its lower end the Fucus belt was replaced by epilithic red algae, mainly Furcellaria lumbricalis, Rhodomela confervoides and Polysiphonia fucoides. These results were similar to those of the 1984 study and show neither an improvement nor a deterioration of the belt during the past 12 years. In four of the five diving profiles, a conspicuous belt of S. arctica occurred in the lower sublittoral, similar to that observed in the 1940s, but in one of the five profiles the S. arctica belt was heavily impoverished. These results contrast with the 1984 study which reported that S. arctica had practically disappeared from all profiles. The epiphytic cover of annual filamentous algae, dominated by Pilayella littoralis and Ceramium gobii, was higher in 1996 than in 1943–4. The results of this comparative study are discussed in relation to the general eutrophication of the Baltic Sea.