Decreased depth penetration of Fucus vesicolosus (L.) since the 1940's indicates eutrophication of the Baltic Sea

Abstract

Although nutrient inputs to the Baltic Sea have increased drastically since the end of the last century, there is still little hard biological evidence of a general eutrophication of the Baltic Sea outside locally polluted areas. A revisit after 40 yr to some well-documented diving stations in the outer archipelago of the h a n d Sea gave us an opportunity to register any changes in benthic vegetation that could be linked to eutrophication, By mapping the vertical distribut~on in coverage of bladder wrack Fucus vesiculosus (L.) at 11 stations we observed that the lower limit of this alga had moved upwards at 10 stations from maximally 11.5 m in 1943/44 to 8.5 m in 1984. Also the depth of maximum development had withdrawn from 5 to 6 m in 1943/44 to 3 to 4 m in 1984, while coverage a t these depths was about the same: 58 % and 51 % respectively. The deepest specimens today at 8.5 m had the same dwarfed appearance as those found at 11.5 m in the 1940's; at that time growth at 8 .5 m was luxuriant During both studies the decrease in F. vesiculosus coverage with depth towards the lower limit could be approximately fitted to an exponentially decreasing light attenuation curve. Since the structure of Baltic hardbottom communities is almost totally governed by abiotic factors, the changes in depth penetration are probably caused by decreased transparency of the water column due to eutrophication. Results indicate that the decreased water transparency arises from a 40 to 50 % increase in summer values of chlorophyll a and nutrients in the offshore surface water of the Baltic Sea since the 1940's.