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Highlights • Records of hard-bottom communities show regional differences in community dynamics. • Regionally, signs of regime shift were detected. • Shift can be explained by the decline of the foundation species Mytilus sp. • Modelling process revealed three environmental variables explaining the decline. • Regional differences in larval dispersal could explain contrary Mytilus recoveries. Abstract Ecological processes modulate ecosystem functioning and services. Foundation species are those exerting intense control on such processes as both their existence and loss have profound implications on the structure of ecological communities. For the distinction between random fluctuations and directional regime shifts in community composition, long-term records are of strategic need. In this study we present the monitoring of benthic hard-bottom communities over 11 years along seven stations in the SW Baltic Sea. Regional differences were found between the communities of Kiel and Lübeck bights, with the former area displaying signs of regime shift. The decline and near disappearance of the foundational species Mytilus edulis from settlement panels deployed in Kiel Bight correlated with three environmental variables: sea surface temperature, water current speed and chlorophyll a concentration. Thus, low spring temperatures, in some cases reinforced by local maxima of chlorophyll a, correlated with reduced recruitment of Mytilus. Moreover, regional differences of larval dispersal and population connectivity could explain the rapid recovery after disturbance of the mussel populations in Lübeck Bight in contrast to Kiel Bight. Our findings underscore the relevance of long-term monitoring programmes to detect the interactive impacts of global climatic and regional environmental drivers.

Mechanistic knowledge on drivers of benthic community traits is key to predict the effects of global change, particularly for the Arctic, where climate change is paramount while its causalities on biota have been scarcely studied. Manipulative field experiments were conducted in two intertidal sedimentary sites (Thiisbukta and Longyearbyen) on Svalbard (Norway) from May to August 2017. At both sites, a complete randomized block design was used to assess the effects of detached seaweed presence on species composition, and biomass of soft-bottom communities. Species composition was significantly distinct between both sites, with the communities of Thiisbukta being more divers, more populated, and more productive in terms of biomass. Here, the presence of detached seaweeds had no effect on any of the tested community responses. In Longyearbyen, however, the number of individuals and biomass were both significantly lower in plots covered by detached seaweeds than in un-manipulated controls. We conclude that detached seaweeds may negatively affect the structure and productivity of Arctic soft-bottom communities, but this response is strongly site-dependent as the more diverse community appeared resistant to the experimental manipulations.

Stress regimes defined as the synchronous or sequential action of abiotic and biotic stresses determine the performance and distribution of species. The natural patterns of stress to which species are more or less well adapted have recently started to shift and alter under the influence of global change. This was the motivation to review our knowledge on the stress ecology of a benthic key player, the macroalgal genus Fucus. We first provide a comprehensive review of the genus as an ecological model including what is currently known about the major lineages of Fucus species with respect to hybridization, ecotypic differentiation and speciation; as well as life history, population structure and geographic distribution. We then review our current understanding of both extrinsic (abiotic/biotic) and intrinsic (genetic) stress(es) on Fucus species and how they interact with each other. It is concluded that (i) interactive stress effects appear to be equally distributed over additive, antagonistic and synergistic categories at the level of single experiments, but are predominantly additive when averaged over all studies in a meta-analysis of 41 experiments; (ii) juvenile and adult responses to stress frequently differ and (iii) several species or particular populations of Fucus may be relatively unaffected by climate change as a consequence of pre-adapted ecotypes that collectively express wide physiological tolerences. Future research on Fucus should (i) include additional species, (ii) include marginal populations as models for responses to environmental stress; (iii) assess a wider range of stress combinations, including their temporal fluctuations; (iv) better differentiate between stress sensitivity of juvenile versus adult stages; (v) include a functional genomic component in order to better integrate Fucus’ ecological and evolutionary responses to stress regimes and (vi) utilize a multivariate modelling approach in order to develop and understand interaction networks.

Stress regimes defined as the synchronous or sequential action of abiotic and biotic stresses determine the performance and distribution of species. The natural patterns of stress to which species are more or less well adapted have recently started to shift and alter under the influence of global change. This was the motivation to review our knowledge on the stress ecology of a benthic key player, the macroalgal genus Fucus. We first provide a comprehensive review of the genus as an ecological model including what is currently known about the major lineages of Fucus species with respect to hybridization, ecotypic differentiation and speciation; as well as life history, population structure and geographic distribution. We then review our current understanding of both extrinsic (abiotic/biotic) and intrinsic (genetic) stress(es) on Fucus species and how they interact with each other.It is concluded that (i) interactive stress effects appear to be equally distributed over additive, antagonistic and synergistic categories at the level of single experiments, but are predominantly additive when averaged over all studies in a meta-analysis of 41 experiments; (ii) juvenile and adult responses to stress frequently differ and (iii) several species or particular populations of Fucus may be relatively unaffected by climate change as a consequence of pre-adapted ecotypes that collectively express wide physiological tolerences.Future research on Fucus should (i) include additional species, (ii) include marginal populations as models for responses to environmental stress; (iii) assess a wider range of stress combinations, including their temporal fluctuations; (iv) better differentiate between stress sensitivity of juvenile versus adult stages; (v) include a functional genomic component in order to better integrate Fucus' ecological and evolutionary responses to stress regimes and (vi) utilize a multivariate modelling approach in order to develop and understand interaction networks.