Long-term changes in the macroalgal vegetation of the inner Gullmar Fjord, Swedish Skagerrak coast

We examined long-term changes in the macroalgal vegetation at Stora Borno Island in the inner Gullmar Fjord on the Swedish Skagerrak coast. This was made possible by access to a 1941 diving investigation. The same sites were reinvestigated in 1998. Community composition and depth distributions of species were compared and changes were analyzed with focus on functional groups (size, thallus shape, and life-history traits). We discovered a significant decrease in the depth extension of macroalgal species and a dramatic decline of species richness in the lower littoral (below 1.6 m of depth) compared with 57 years earlier. Ordination analysis revealed that there was a significant difference in the community composition between the two study periods. In general, small (<10 cm), thin, filamentous, and aseasonal ephemerals increased in relative abundance, whereas larger (>10 cm), coarsely branched, and perennial algae decreased. Calibrations of individual species to local sediment cover, using canonical correspondence analysis, indicated that part of the change in species composition was related to sediment load. Furthermore, large-scale climate differences (NAO Winter Index) between the study periods indicated a higher impact of Baltic Sea and Kattegat water in the nutrient dynamics of the fjord in the 1998 study. We concluded that the observed long-term changes in the macroalgal community at Stora Borno Island were consistent with an increased nutrient availability.