Long-term changes of macroalgal vegetation in the Skagerrak area

The algal vegetation at three rocky-shore localities on the Swedish Skagerr ak coast with different environmental conditions was studied in 1960-1961 b y SCUBA diving. The same localities were revisited in the summer of 1997, u sing the same methods for recording the vegetation. Detailed descriptions o f the vegetation profiles are presented to allow the recording of future ch anges in these profiles, and extensive ecological herbaria are kept of both the 1960-1961 and the 1997 investigations. The abundance and depth distrib ution of 78 macroalgal species were recorded in 1997 and community composit ion was compared with that of the early 1960's. Increases of perennial red algae with delicate foliaceous thalli (Delesseria sanguinea, Phycodrys rube ns) were conspicuous at the two localities most exposed to wave action, whe reas perennial red algae with tougher foliaceous thalli (Phyllophora trunca ta and Phyllophora pseudoceranoides) prevailed at the more sheltered locali ty with most sedimentation. It is hypothesised that increased abundances of delicate species with a large growth potential are caused by eutrophicatio n, but that this effect may be counteracted when eutrophication results in a high load of sedimentation. Tougher species are designed better to withst and 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 lac k of competition from D. sanguinea and P. rubens. The abundance of filament ous algae had increased since the early 1960's at all three localities, whi ch may be attributed to eutrophication as well, but no changes were observe d in the large perennial brown algae. Decaying loose-lying algae occurred a t all localities, but algal mat formation was only abundant at the most she ltered locality. The observed changes in the attached algal vegetation of t he exposed sites may be an indication of a general large-scale eutrophicati on of the Skagerrak.