Decades of biomass loss in the shallow rocky subtidal vegetation of the south-eastern Bay of Biscay

Abstract

This study seeks to assess changes over time in the structure of subtidal macroalgal assemblages across depth in the south-eastern Bay of Biscay. The results reveal a large-scale decline in total macroalgal biomass between 1982 and 2014. However, the temporal pattern of shift differs from one depth to another: total biomass decreased at depths from 3 to 10 m, but increased at depths of 2 and 11 m. The strong decrease in biomass detected in the 3–10-m depth range is a consequence of a sharp net decline in large macroalgae biomass which was not offset by increased biomass of small species, mainly corresponding to turf-forming algae. The dominant canopy-forming Gelidium corneum in 1982 had practically disappeared by the end of the study period and its biomass loss was far from being offset by the small increase detected in the fucoid Gongolaria baccata. By contrast, at depths of 2 and 11 m, the most notable result is an increase in large species, mainly Halopithys incurva and Codium decorticatum at 2 m and G. baccata at 11 m; however, at both depth levels, a new canopy was far from being developed. These findings evidence that biomass and habitat provision, two pivotal roles of canopy-forming species in ecosystem functioning, have been altered. Further research into potential changes in primary productivity and biodiversity linked to the shift detected in assemblage structure needs to be conducted in order to get information for conservation and management decisions associated with the loss of habitat-forming macroalgae.