LONG-TERM CHANGES IN THE SUBLITTORAL ZONATION OF BROWN-ALGAE IN THE SOUTHERN BOTHNIAN SEA

The algal vegetation of five diving profiles situated at exposed sites of the outer Oregrund archipelago, southern Bothnian Sea, Sweden, was investigated by SCUBA diving in 1996. The vegetation of the same prof iles had been described earlier in 1943-4 and 1984. In the present stu dy, special attention was paid to the occurrence of the structurally i mportant belt-forming brown algae Fucus vesiculosus and Sphacelaria ar ctica in comparison with the situation 52-53 and 12 years earlier. Pro minent differences between the 1940s and 1996 were found. The weighted average depth of the F. vesiculosus belt in 1996 was about 1.7 m shal lower than in the 1940s, and the lower distribution limit of this spec ies was about 2.5 m shallower. At its lower end the Fucus belt was rep laced by epilithic red algae, mainly Furcellaria lumbricalis, Rhodomel a confervoides and Polysiphonia fucoides. These results were similar t o those of the 1984 study and show neither an improvement nor a deteri oration of the belt during the past 12 years. In four of the five divi ng profiles, a conspicuous belt of S. arctica occurred in the lower su blittoral, similar to that observed in the 1940s, but in one of the fi ve profiles the S. arctica belt was heavily impoverished. These result s contrast with the 1984 study which reported that S. arctica had prac tically disappeared from all profiles. The epiphytic cover of annual f ilamentous algae, dominated by Pilayella littoralis and Ceramium gobii , was higher in 1996 than in 1943-4. The results of this comparative s tudy are discussed in relation to the general eutrophication of the Ba ltic Sea.