Long-term surveys reveal abrupt canopy loss with immediate changes in diversity and functional traits

Cumulative human pressures are threatening canopy-forming macroalgae worldwide, putting at risk the key ecosystem functions they provide. Concurrent with declines in Gelidium corneum meadows, the present study investigates year-on-year changes in associated assemblages over a 21-year period (1996-2017) in the southeastern Bay of Biscay. The results show a non-linear loss of G. corneum that causes an abrupt shift towards new assemblages dominated by caespitose and encrusting forms. The rapid decline of morphologically complex algae and perennial forms is accompanied by a decrease in both epiphytic and epibenthic suspensivores and the loss of numerous associated species of fauna and flora. Along with the decline in canopy, a marked decrease in algae and invertebrate abundance is registered. After the canopy loss, a notable rise in algal density and a net increase in flora and fauna richness and evenness are detected. Moreover, a significant increase in the temporal variability of assemblages is identified, denoting greater stress after the shift. The implications of the changes observed and the consequences of the shift to a new state for the functioning of the ecosystem are discussed.