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Foundation species play a disproportionate role in maintaining biodiversity and ecosystem functioning. Improved understanding of how environmental factors influence the distribution and population structure of foundation species therefore contributes to management and conservation of entire ecosystems. We surveyed subtidal kelp forests within four regions of the U.K., distributed over 9° of latitude and a mean sea temperature gradient of ~ 2.5°C. Our aims were: (1) to examine relationships between light availability and the structure and depth distribution of Laminaria hyperborea populations and (2) to determine whether depth‐related patterns were consistent across regions with different temperature regimes. We recorded marked depth‐related shifts in structure with decreasing light levels strongly correlated with declines in kelp density, cover, plant biomass, standing biomass, plant length, and age. We also recorded an effect of latitude; populations at our two colder, northernmost regions exhibited greater wet weight and length and higher standing biomass than populations in the warmer southern regions when under similar or even reduced light conditions, indicating an interactive effect of latitude, most likely related to temperature variability. We show that shifts in kelp population structure along depth gradients are strongly driven by light availability, although regional variability in the strength and nature of these relationships may be promoted by other factors such as temperature. Maximum depth penetration, standing biomass, plant density, and plant weight are useful indicators of light availability and, over time, could be monitored to detect changes in the quality of the overlying water column. [ABSTRACT FROM AUTHOR]

Enhanced persistence of stress sensitive species in harsh environments due to amelioration of physical stress by habitat‐forming species has been widely documented. By contrast, less attention has been given to positive species interactions that may enhance stress tolerance in habitat‐forming species, with positive cascading effects on biodiversity and ecosystem functioning.We used a combination of field surveys and experiments to assess the effects of the epiphyte Jania rubens, on its subtidal macroalgal host Halopithys incurva, and their cascading effects on the associated invertebrate community, along a depth gradient associated with increasing stress (i.e. light stress at shallower depths).Correlative studies showed that the biomass and photosynthetic efficiency of H. incurva increased with depth, along with a decrease of epiphyte biomass. The experimental manipulation of epiphytes (removal in shallow water and addition in deep water) revealed that their effects on H. incurva physiology varied from positive in shallow habitats to negative in deep habitats. These results suggest that reduction of light/UV stress by J. rubens allows H. incurva to persist in shallow‐water habitats that would be otherwise unsuitable. In addition, colonization by large clumps of J. rubens increased the abundance and diversity of associated invertebrates in shallow habitat, suggesting that positive feedbacks between H. incurva, acting as the primary habitat‐former, and J. rubens, acting as a secondary habitat‐former, generate a facilitation cascade.Synthesis. Our study suggests that the persistence of some habitat‐forming species and their ability to support biodiversity may rely upon physical stress reduction by other species (e.g. epiphytes in our case) that are generally neglected when planning conservation strategies. A deeper understanding of context dependency in positive species interactions is essential for predicting patterns of species distribution under increasingly stressful climate scenarios. [ABSTRACT FROM AUTHOR]

Disturbances are increasing in size and frequency with climate change, facilitating species that opportunistically exploit areas where habitat‐forming foundation species have been removed. Although it is well‐recognized that consumers, disease and weedy space‐holders can affect foundation species' resistance to and recovery from disturbance, how predators influence their resilience is less clear.In salt marshareas de‐vegetated by drought and intensive snail Littoraria irrorata grazing (hereafter, 'die‐offs'), we monitored bird use and experimentally manipulated bird and nekton access to the vegetated borders of die‐off mudflats across periods of both vegetation die‐off and regrowth to explore how these predators mediate the resilience of cordgrass Spartina alterniflora, the foundation species that structures US Atlantic coast salt marshes.Surveys revealed that birds, especially probers that agitate soils, forage year‐round for invertebrates in die‐off mudflats in our study area but not in adjacent vegetated areas.During periods of die‐off, cordgrass borders accessible to bird and nekton predators retreated >3‐times slower and snail densities were halved, relative to predator exclusion cages. In predator‐accessible plots, slower border retreat corresponded to greater snail infection by a bird host‐dependent trematode parasite. During recovery, cordgrass borders revegetated more quickly, and snail densities declined faster over time in unmanipulated controls relative to predator exclusions.Synthesis. These findings suggest that birds, through their transmission of parasites to snails, appear to act synergistically with snail‐consuming nekton to slow cordgrass loss after drought‐snail disturbances. Predator access also corresponds to faster cordgrass recovery as environmental conditions improve, although the mechanisms behind this need further investigation. Thus, predators that opportunistically forage within disturbances have the potential to suppress consumer impacts through multiple mechanisms, including consumption and disease transmission, thereby bolstering foundation species' resilience and modulating whole ecosystem responses to climate change. [ABSTRACT FROM AUTHOR]

Facilitation of foundation species is critical to the structure, function and persistence of ecosystems. Understanding the dependence of the strength of this facilitation on environmental conditions is important for informed ecosystem management and for predicting the impacts of global change. In coastal seagrass habitats, chemosymbiotic lucinid bivalves can facilitate seagrasses by decreasing potentially toxic levels of sulphide in sediment porewater. However, variation in the strength of lucinid–seagrass facilitation with environmental context has not been experimentally investigated.We tested the hypothesis that the presence of the tiger lucine Codakia orbicularis becomes more important to the growth and survival of the seagrass Thalassia testudinum under decreased light availability and increased sulphide stress. In a mesocosm experiment, we reduced average ambient‐light to T. testudinum by 64% and/or increased sediment porewater sulphide concentrations by ~200% and compared growth and tissue chemistry of T. testudinum with and without C. orbicularis.We found that T. testudinum was better able to maintain growth under shading and sulphide stress when C. orbicularis was present. C. orbicularis strongly decreased sediment porewater sulphide, an effect that minimized sulphur build‐up in seagrass tissue and was likely achieved through bioirrigation as well as chemoautotrophy. The relative effects of C. orbicularis on T. testudinum growth were strongest in the presence of environmental stressors.Synthesis. The strength of lucinid–seagrass facilitation increases under environmental conditions that hinder the ability of seagrass to detoxify sulphide. Our results provide evidence of a potential mechanism by which the spatiotemporal association between lucinids and seagrasses is maintained and support the incorporation of interspecific facilitation into conservation and restoration strategies for foundation species in the face of increasing anthropogenic impact and global change. [ABSTRACT FROM AUTHOR]

Kelp species are found along ~ 25% of the world's coastlines, where they provide myriad of ecological goods and services. However, compared with many other terrestrial and marine vegetated ecosystems, kelp forests have been critically understudied in many regions, leading to pressing knowledge gaps that hinder management and conservation efforts. We conducted a large-scale survey of understorey macroalgae within subtidal kelp forests dominated by Laminaria hyperborea at 12 sites, nested within 4 regions in the United Kingdom (UK). Regions spanned ~ 9° in latitude and encompassed a gradient in average sea surface temperature of ~ 2.5 °C. We employed a combination of traditional hand-harvesting of quadrat samples and a novel 3D photogrammetry technique to quantify crustose coralline algae. The structure of understorey macroalgal assemblages was highly variable but showed clear partitioning at the spatial scales of both regions and sites. At the regional-scale, we observed a general increase in richness, diversity and biomass from north to south, most likely due to biogeographical context, ocean climate and the structure and composition of overlying kelp canopies. Site level variation was most likely driven by concurrent variability in wave exposure and kelp canopy structure. Our study shows that understorey macroalgal assemblages represent a rich and abundant component of kelp forests in the northeast Atlantic, with high biodiversity value that warrant conservation measures. As these kelp forest ecosystems are structured by multiple physical and biological processes, current and predicted environmental change will likely alter the diversity and composition of understorey macroalgal assemblages. [ABSTRACT FROM AUTHOR]

Aim: Kelp forests provide habitat and food that supports a high diversity of flora and fauna. While numerous studies have described macroinvertebrates associated with kelp blades, stipes and holdfasts, a key kelp forest microhabitat, epilithic understory algae, remains poorly studied. Here, we used a macroecological approach and artificial seaweed units (ASUs) to explore the effects of ocean climate, wave exposure and habitat complexity on understory algal associated macroinvertebrate assemblages within Laminaria hyperborea forests in the United Kingdom. Location: 9° latitudinal gradient along the north and west coasts of the United Kingdom. Methods: Replicate ASUs comprising four different habitat complexities were deployed under mature L. hyperborea at 2 sites (along a wave exposure gradient, separated by km) within each of 4 locations (separated by 100s km) nested within two regions (warm and cold, spanning 9° of latitude). After 5 months in situ, the ASUs were collected and macroinvertebrates were identified to species level and enumerated. Results: Habitat complexity and wave exposure both influenced macroinvertebrate assemblage structure, but results also showed clear effects of ocean climate, with macroinvertebrate assemblages differing between warm and cool regions, primarily driven by higher diversity and evenness in the warmer region and greater abundance in the cooler region. Main conclusions: Predicted warming and a shift to less complex turf‐forming algal assemblages are likely to alter the structure of macroinvertebrate assemblages associated with understory algae, with potential implications for kelp forest food web dynamics. [ABSTRACT FROM AUTHOR]

We describe the change of operational instrument for the routine measurement of column-averaged dry-air mole fraction of several greenhouse gases (denoted 푋gas) at the Lauder Total Carbon Column Observing Network (TCCON) site and the steps taken to demonstrate comparability between the two observation systems following a systematic methodology Further, we intercompare retrieved 푋gas values during an intensive intercomparison period during October and November 2018, when both instruments were performing optimally, and on subsequent, less frequent occasions. The average difference between the two observing systems was found to be well below the expected level of uncertainty for TCCON retrievals for all compared species. In the case of 푋 퐶푂2 the average difference was 0.0264 ± 0.0465% (0.11 ± 0.19µmolmol-1). [ABSTRACT FROM AUTHOR]

Rates and drivers of primary productivity are well understood for many terrestrial ecosystems, but remain poorly resolved for many marine ecosystems, particularly those within in coastal benthic environments. We quantified net primary productivity (NPP) using two methods as well as carbon standing stock within kelp forests (Laminaria hyperborea) at multiple subtidal habitats in the United Kingdom (UK). Study sites spanned 9° in latitude and encompassed a gradient in average temperature of ~ 2.5 °C. In addition to temperature, we measured other factors (e.g. light intensity, water motion, nutrients, sea urchin density) that may influence productivity. Although estimates of NPP were highly variable between sites, ranging from 166 to 738 g C m-2 yr-1, our study-wide average of 340 g C m-2 yr-1 indicated that L. hyperborea forests are highly productive. We observed clear differences between NPP and carbon standing stock between our cold northernmost sites and our warm southernmost sites, with NPP and standing stock being around 1.5 and 2.5 times greater in the northern sites, respectively. Ocean temperature was identified as a likely driver of productivity, with reduced NPP and standing stock observed in warmer waters. Light availability was also strongly linked with carbon accumulation and storage, with increased light levels positively correlated with NPP and standing stock. Across its geographical range, total NPP from L. hyperborea is estimated at ~ 7.61 Tg C yr-1. This biomass production is likely to be important for local food webs, as a trophic subsidy to distant habitats and for inshore carbon cycling and (potentially) carbon sequestration. However, given the strong links with temperature, continued ocean warming in the northeast Atlantic may reduce primary productivity of this foundation species, as optimal temperatures for growth and performance are surpassed. [ABSTRACT FROM AUTHOR]

Global warming mediates and maintains the tropicalization of temperate marine ecosystems. Recent studies have demonstrated that this process causes shifts from algal forests to denuded non‐canopy states in temperate reefs. It has been suggested that these changes would incur significant consequences to ecosystem functioning.In this study, we tested how tropicalization affects habitat‐provisioning functions and carbon turnover of a shallow reef in the fast‐warming and highly invaded southeastern Mediterranean Sea, in‐situ. On a single shallow reef, we conducted measurements of these functions in three habitats: dwindling native brown algal (Cystoseira) forest, dominant turf (formed by overgrazing of tropical rabbitfish) and expanding tropical shrubs dominated by red calcifying algae (Galaxaura rugosa).Algal forest was an autotrophic net carbon sink and provided habitat for high species diversity and the largest community biomass. The denuded turf was heterotrophic and provided habitat for the lowest species diversity and community biomass. While diversity was as high in tropical shrubs as in the algal forest, it had lower biomass and functioned as a heterotrophic net carbon source.Synthesis. Our study exemplifies possible functional consequences of tropicalization‐driven regime shifts on shallow temperate rocky reefs, and how these can invert the net trophic state and carbon balance. [ABSTRACT FROM AUTHOR]

Propagule pressure is acknowledged as a key determinant of invasion success. Nonetheless, the role of morphological or physiological attributes of propagules (i.e. their quality) in regulating invader establishment has been little explored. In particular, no study has investigated how the presence of propagules differing in quality within an inoculum influences establishment across heterogeneous landscapes.We experimentally tested the hypothesis that the quality (+Fronds+Rhizoids; +Fronds−Rhizoids; −Fronds+Rhizoids) and the diversity (1, 2 and 3 fragment types) of vegetative fragments of the seaweed Caulerpa taxifolia determine their establishment success across seascapes consisting of bare sediments and patches of the seagrass Zostera muelleri exposed to different disturbance intensities (control, seagrass canopy clipping and total removal).After 6 weeks, seaweed biomass, stolon and frond length, frond and rhizoid number were generally greater in unvegetated habitats (bare sediments and total seagrass removal) than full or reduced seagrass canopies. The type and the diversity of types of fragments inoculated had significant effects on the final biomass and morphological features of C. taxifolia only in vegetated habitats. In control plots, inocula of fragments retaining both fronds and rhizoids achieved higher biomass, developed longer stolons and more fronds. In canopy clipping plots, mixed inocula of +Fronds+Rhizoids and −Fronds+Rhizoids fragments had the greatest biomass and stolon length.Synthesis. Assessing how propagules differing in quality perform in different habitats might be not sufficient to draw a comprehensive picture of invasion risk, as their establishment can be modulated by both negative and positive interactions among them. Propagule composition should be, therefore, considered as a further dimension of propagule pressure. Our results also suggest that the relevance of specific propagule traits for invader establishment decreases from intact to degraded habitats. Considering propagule size in terms of amount of competent propagules, rather than an absolute measure, would refine our ability of predicting invasion risk across habitats differing in biotic or abiotic conditions. [ABSTRACT FROM AUTHOR]

Summary: Kelp forests represent some of the most diverse and productive habitats on Earth, and provide a range of ecosystem goods and services on which human populations depend. As the distribution and ecophysiology of kelp species is strongly influenced by temperature, recent warming trends in many regions have been linked with concurrent changes in kelp populations, communities and ecosystems. Over the past decade, the number of reports of ocean warming impacts on kelp forests has risen sharply. Here, I synthesise recent studies to highlight general patterns and trends. While kelp responses to climate change vary greatly between ocean basins, regions and species, there is compelling evidence to show that ocean warming poses an unequivocal threat to the persistence and integrity of kelp forest ecosystems in coming decades. [ABSTRACT FROM AUTHOR]

Anthropogenic climate change is driving the redistribution of species at a global scale. For marine species, populations at trailing edges often live very close to their upper thermal limits and, as such, poleward range contractions are one of the most pervasive effects of ongoing and predicted warming. However, the mechanics of processes driving such contractions are poorly understood. Here, we examined the response of the habitat forming kelp, Laminaria digitata, to realistic terrestrial heatwave simulations akin to those experienced by intertidal populations persisting at the trailing range edge in the northeast Atlantic (SW England). We conducted experiments in both spring and autumn to determine temporal variability in the effects of heatwaves. In spring, heatwave scenarios caused minimal stress to L. digitata but in autumn all scenarios tested resulted in tissue being nonviable by the end of each assay. The effects of heatwave scenarios were only apparent after consecutive exposures, indicating erosion of resilience over time. Monthly field surveys corroborated experimental evidence as the prevalence of bleaching (an indication of physiological stress and tissue damage) in natural populations was greatest in autumn and early winter. Overall, our data showed that L. digitata populations in SW England persist close to their upper physiological limits for emersion stress in autumn. As the intensity of extreme warming events is likely to increase with anthropogenic climate change, thermal conditions experienced during periods of emersion will soon exceed physiological thresholds and will likely induce widespread mortality and consequent changes at the population level. [ABSTRACT FROM AUTHOR]