Epibioses of Gullmarsfjorden: An underwater stereophotographical transect analysis in comparison with the investigations Of Gislén in 1926–29

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Abstract
During the years 1926–29 Torsten Gislén pioneered quantitative investigations on hard-bottom epibioses in Gullmarsfjorden on the Swedish west coast using hard-hat diving equipment. At stations in the fjord previously studied by Gislén, series of transects on vertical rock walls were stereophotographed in 1986–87 to a depth of 30 m. The quantitative distribution of epifauna and epiflora was estimated by using point-sampling technique. Although comparison ofepibioses are difficult due to a large degree of heterogeneity and patchiness, both among and within sites, the results show that the vertical distribution of macro-algae has dirnished, especially toward the inner part of the fjord. The available space below the macro-algal belt is now occupied mainly by Metridium senile and sheets of blue-green algae. The reduction started before 1966 and may be explained by a gradual decrease in the surface water transparency although this is difficult to show conclusively. A non-reversed pollution effect caused by previous sewage dumping from a sulphite pulp mill before 1966, in combination with interspecific competition, may be of importance. Only a few changes in the distribution of epifauna below the algal belt have taken place which cannot be explained by patchy distribution and interannual variations. During 1926–29 Ascidia obliqua was abundant in the central and inner part of the fjord below the 15 m level, but is now relatively rare. At 5-18 m depth the anthozoan Metridium senile has increased in abundance while the perennial ascidian, Ascidia mentula, has decreased in abundance at all stations.