Seaweed assemblage changes in the eastern Cantabrian Sea and their potential relationship to climate change

Rising sea-surface temperatures (SSTs) over the last three decades in the south-eastern part of the Bay of Biscay could be affecting phytobenthic assemblage distributions. This study assesses recent changes in species abundance and diversity along the western Basque coast by studying 18 locations in summer 1991 and in 2008. There were substantial changes in the structure of subtidal vegetation between the two surveys: 1) an increase in coralline algae; 2) changes in the distribution patterns of the canopy species; 3) disappearance of kelps; 4) increases in warm-water species, mainly morphologically simple forms; 5) introduction and expansion of non-indigenous species, and 6) increases in species richness and diversity. These results are consistent with the rise detected in SST, particularly in summer when SST was found to have risen by as much as 1 °C from 1980 to 2008. Furthermore, two extreme warming events occurred in the summers of 2003 and 2006. However, some biological shifts cannot be explained by warmer waters alone. Only limited data are available on changes in nutrient concentrations and water transparency, but these factors are also thought to be involved in the biological changes detected.