Large-scale shift from sugar kelp (Saccharina latissima) to ephemeral algae along the south and west coast of Norway

Abstract

Sugar kelp forests (Saccharina latissima) normally dominate the sublittoral rocky sea bed in medium exposed to sheltered areas of the Norwegian coast. In 2002, a large-scale disappearance of sugar kelp was observed, and a survey of more than 600 sites along the coast of southern Norway (58–63°N, 6–11°E) during 2004–2009 recorded a large-scale shift from anticipated sugar kelp forests to communities dominated by filamentous, ephemeral macroalgae. The loss of sugar kelp was most severe in the Skagerrak region, and a comparison with earlier studies supported the observed changes in the community structure and regional differences between the Skagerrak region and the west coast. Filamentous red algae dominated the sublittoral communities at the Skagerrak coast, whereas coarsely branched and filamentous brown algae were most abundant at the west coast sites with low or no abundance of sugar kelp. The cause of this large-scale shift is not clear, but our observations suggest eutrophication (nutrient and particle pollution) and climate change (increase in temperature) as the two main drivers synergistically contributing, alongside other factors, to the demise of S. latissima. If the ephemeral algae community represents an alternative stable state, it will have implications for marine coastal zone production and management.