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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.

The idea that species diversity is causally related with ecosystem performance has stimulated a growing number of manipulative experiments in terrestrial and marine systems. These experiments may confound effects of species number and community composition. The effects of diversity can be due to the number of species per se (or other functional/taxonomic categorisation), to the probability of including species with particularly important traits (sampling effect), or to an interaction between both attributes. We test these hypotheses in a field experiment conducted in Helgoland island (German Bight, North Sea), in which we manipulated the number of functional groups and assemblage composition. Artificial substrata (PVC) made up of 16 (2.5 χ 2.5 cm) settlement panels were exposed at a range of depths and sites to natural colonisation for 6 months. Colonisers were categorised into 4 functional groups and panels rearranged to create assemblages of defined functional diversity. Diversity treatments (2 levels) consist of (i) 4 monocultures (one per functional group) and (ii) 1 mixed culture including all groups. Diversity treatments were fully crossed with two density treatments (100 and 50 % species cover). Clearance rate, biomass production, and community composition are measured from January until June 2007 to assess temporal stability of the dependent variables. Our result will help to identify the mechanisms associated with ecosystem responses to changes in biodiversity.

Herbivory is particularly intense in tropical benthic communities, suggesting preference of constitutive, rather than inducible, anti-herbivory defense. The objective of the study was to examine whether anti-herbivore defenses in the red alga Hypnea pannosa J. Agardh and the brown algae Sargassum asperifolium Hering and G. Martens ex J. Agardh and Cystoseira myrica (S.G. Gmelin) C. Agardh could be induced and subsequently reduced in response to grazing by the amphipod Cymadusa filosa Savigny. During a 14-day treatment phase, algae were exposed to amphipod grazing or were left ungrazed (control). Subsequently, one subset of algae was used in feeding assays, whereas another was cultivated for additional 14 days without consumers (recovery phase). At the end of each phase, bioassays were conducted to detect defensive traits in terms of differences in consumption rates of grazed and control pieces of live algae and agar-based food containing nonpolar algal extracts. Consumption of grazed live S. asperifolium and H. pannosa specimens was lower than of control algae. Furthermore, nonpolar extracts of grazed S. asperifolium and C. myrica were less preferred than those from control algae. Defensive responses were exclusively detected after the treatment phase, although strong preference of ungrazed H. pannosa and C. myrica over grazed conspecifics continued throughout the recovery phase. These findings suggest that phenotypic plasticity in anti-herbivory defense of marine macroalgae 1) might be more common than previously shown, 2) could be switched on and off within 2 weeks, and 3) can be found in nonpolar algal extracts.

Species richness is the most widely used measure of biodiversity. It is considered crucial for testing numerous ecological theories. While local species richness is easily determined by sampling, the quantification of regional richness relies on more or less complete species inventories, expert estimation, or mathematical extrapolation from a number of replicated local samplings. However the accuracy of such extrapolations is rarely known. In this study, we compare the common estimators MM (Michaelis-Menten), Chao1, Chao2, ACE (Abundance-based Coverage Estimator), and the first and second order Jackknifes against the asymptote of the species accumulation curve, which we use as an estimate of true regional richness. Subsequently, we quantified the role of sample size, i.e., number of replicates, for precision, accuracy, and bias of the estimation. These replicates were sub-sets of three large data sets of benthic assemblages from the NE Atlantic: (i) soft-bottom sediment communities in the Western Baltic (n = 70); (ii) hard-bottom communities from emergent rock on the Island of Helgoland, North Sea (n = 52), and (iii) hardbottom assemblages grown on artificial substrata in Madeira Island, Portugal (n = 56). For all community types, Jack2 showed a better performance in terms of bias and accuracy while MM exhibited the highest precision. However, in virtually all cases and across all sampling efforts, the estimators underestimated the regional species richness, regardless of habitat type, or selected estimator. Generally, the amount of underestimation decreased with sampling effort. A logarithmic function was applied to quantify the bias caused by low replication using the best estimator, Jack2. The bias was more obvious in the soft-bottom environment, followed by the natural hard-bottom and the artificial hard-bottom habitats, respectively. If a weaker estimator in terms of performance is chosen for this quantification, more replicates are required to obtain a reliable estimation of regional richness.

The Arctic is warming rapidly. Atmospheric temperatures in the Barents Sea region are increasing 5 – 7 times faster than the global average. As a result, Arctic sea ice extent is declining by nearly 12% per decade during the summer months. The study site in Kongsfjorden, Svalbard, has been mostly ice free since 2006. These changes in the physical environment likely affect the ecology of coastal Arctic ecosystems. Increased underwater irradiance seems to improve the growth of kelp forests on rock shores. This thesis is a continuation of a time series to estimate historic developments in abundance, biomass, and taxonomic composition of seaweed-associated fauna, comparing data from 2021 with data from 2012/13 and 1996/98. At each time point, samples were collected along the same gradient from 2.5 m to 15 m water depth. There is limited evidence for a difference in taxonomic composition between 2021 and 2012/13, while taxonomic composition in 1996/98 was significantly different from both other time points. Moreover, both faunal abundance and biomass increased about two-fold between 2012/13 and 2021. These results might suggest that the seaweed-associated fauna at Hansneset reached a new stable state after initial shifts in the taxonomic composition between 1996/98 and 2012/13. Differences in taxonomic composition, based on biomass data, were largely a result of an increase in cirripedian and decrease in bryozoan biomass. These taxonomic groups possibly show different responses to some biotic and abiotic factors, such as macroalgae cover or sedimentation.