Long-term changes of macroalgal vegetation in the Skagerrak area

The algal vegetation at three rocky-shore localities on the Swedish Skagerrak coast with different environmental conditions was studied in 1960-1961 by SCUBA diving. The same localities were revisited in the summer of 1997, using the same methods for recording the vegetation. Detailed descriptions of the vegetation profiles are presented to allow the recording of future changes in these profiles, and extensive ecological herbaria are kept of both the 1960-1961 and the 1997 investigations. The abundance and depth distribution of 78 macroalgal species were recorded in 1997 and community composition was compared with that of the early 1960's. Increases of perennial red algae with delicate foliaceous thalli (Delesseria sanguinea, Phycodrys rubens) were conspicuous at the two localities most exposed to wave action, whereas perennial red algae with tougher foliaceous thalli (Phyllophora truncata and Phyllophora pseudoceranoides) prevailed at the more sheltered locality with most sedimentation. It is hypothesised that increased abundances of delicate species with a large growth potential are caused by eutrophication, but that this effect may be counteracted when eutrophication results in a high load of sedimentation. Tougher species are designed better to withstand a heavy load of sediment. Other perennial red algae with tougher thalli (Chondrus crispus, Furcellaria lumbricalis) also decreased at the exposed sites, but not at the site with most sedimentation, possibly because of lack of competition from D. sanguinea and P. rubens. The abundance of filamentous algae had increased since the early 1960's at all three localities, which may be attributed to eutrophication as well, but no changes were observed in the large perennial brown algae. Decaying loose-lying algae occurred at all localities, but algal mat formation was only abundant at the most sheltered locality. The observed changes in the attached algal vegetation of the exposed sites may be an indication of a general large-scale eutrophication of the Skagerrak.