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Changes in depth distribution and biomass of sublittoral seaweeds at the island of Helgoland (German Bight, North Sea): a comparison between 1970 and 2005

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Gehling, C. and Bartsch, I. (2007): Changes in depth distribution and biomass of sublittoral seaweeds at the island of Helgoland (German Bight, North Sea): a comparison between 1970 and 2005 , 42nd European Marine Biology Symposium, Kiel, GermanyAugust 2007. .
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Abstract:

Abstract Recent investigations of the intertidal macrophyto- and zoobenthos of the island of Helgoland (North Sea) revealed that species composition and spread has changed within the last decades. In order to evaluate the situation in the subtidal, a semi-quantitative study from the late 1960s was repeated with comparable methods in 2005. For this purpose characteristics of the Laminaria forest were investigated in 37 dives following a 1580 m long vertical transect in the North of the island Helgoland between 0 and 14 m below MLWS. Data collection included semi-quantitative assessment of coverage of dominating brown seaweeds (Fucus serratus, Laminaria digitata, L. hyperborea, Saccharina latissima, Sargassum muticum). These data were used to define vegetation zones. Within each zone all macroalgal species of three (to six) random 1 m-²- quadrates were estimated quantitatively. Additionally, in six depth steps (0.5, 2, 4, 6, 8 and 10 m below MLWS) a biomass survey, including morphometric measurements, was performed. Comparison of recent data with those from the late 1960s showed some characteristic changes. Species composition of dominating brown seaweeds showed a decline of S. latissima in the northern part of the island and an appearance of S. muticum. The vertical distribution of L. hyperborea broadened into greater depths and lower limits of various understorey seaweeds showed a downward shift of at least 2 m as well. Biomass data also show this development. There is a shift of the biomass maximum of L. hyperborea from 2 m in 1967 to 4 m in 2005. This overall downward extension of the Laminaria forest correlates with the increase in Secchi depths observed within the last decades.

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