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.

中文翻译：

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长期调查显示冠层突然丧失，多样性和功能性状立即发生变化

累积的人类压力正在威胁着全世界形成冠层的大型藻类，使它们提供的关键生态系统功能处于危险之中。与 Gelidium 角质草甸减少的同时，本研究调查了比斯开湾东南部 21 年期间（1996-2017 年）相关组合的逐年变化。结果显示 G.corneum 的非线性损失会导致突然转向以丛生和结壳形式为主的新组合。形态复杂的藻类和多年生形式的迅速减少伴随着附生和底栖悬食动物的减少以及许多相关动植物物种的丧失。随着冠层的下降，藻类和无脊椎动物的丰度显着下降。树冠消失后，发现藻类密度显着增加，动植物丰富度和均匀度净增加。此外，确定了组合的时间变异性的显着增加，表明转变后压力更大。讨论了观察到的变化的影响以及转变为生态系统功能的新状态的后果。