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Detritus is widely considered to be dead organic matter. However, recent studies have shown that kelp detritus can be functionally equivalent to the attached plant. This is significant, because detritus makes up ~ 42% of all plant biomass. In the Northeast Atlantic, studies on detrital photobiology have been restricted to the genus Laminaria. Here, we present data from a 46 d in situ experiment investigating the effect of decomposition on detrital photosynthetic pigment concentrations and stoichiometry in four Northeast Atlantic kelp species from three genera. We corroborate the lack of a decomposition effect on pigments of cold temperate Laminaria species as identified by previous studies but show that the photophysiology of other kelp genera responds differently. While Saccharina latissima displays an idiosyncratic parabolic response, the annual and thermally plastic kelp Saccorhiza polyschides seems to become less pigmented with increasing detrital age. If S. polyschides becomes increasingly dominant in future parts of a warmer Northeast Atlantic, this may translate to lower detrital photosynthetic potential at the forest scale. However, given our somewhat ambiguous data on this species, this conjecture requires further investigation into detrital photosynthesis. [ABSTRACT FROM AUTHOR]

Restoration of foundation species in historical habitat may be difficult if adult facilitation is obligatory for survival of early life stages.On intertidal Mediterranean coasts, large‐scale loss of the dominant forest‐forming macroalga Ericaria amentacea have prompted restoration efforts using recruits. Yet, early life stages may be more susceptible to the abiotic stress that characterizes their habitat.We tested strategies to enhance resilience of lab‐cultured juveniles of E. amentacea to environmental stress in historical habitat lacking conspecifics. Juveniles were exposed in culture to fluctuations of the dominant physical stressors, irradiance and temperature, and then outplanted in upper and lower zones of their native intertidal range.Without adult canopy, juvenile outplant survival was limited to the lower tidal range, with nearly complete mortality in the upper zone. Survival was also strongly determined by spatial clumping of recruits within the outplant substrate. Longer‐term growth in the lower zone was enhanced by fluctuating mild stress in culture, with variable irradiance and concurrent heat pulses increasing post‐outplant cover by 40%–60% after 4 months. Clumping also promoted growth across experimental treatments.Synthesis and applications. Reliance on self‐facilitation feedbacks is a common barrier to foundation species restoration in high‐stress habitats. Our results suggest that without adult habitat amelioration, environmental stress limits recruit survival in intertidal algal forest. Yet, exposure to transient, low environmental stress in culture and outplanting of clumped individuals may confer resilience and allow successful establishment of early life stages in zones of reduced abiotic stress, providing a source for gradual colonization of more high‐stress areas. [ABSTRACT FROM AUTHOR]

Photosynthetic features of different canopy-forming macroalgae of the order Fucales (Phaeophyceae) living in shallow and sheltered environments show a high homogeneity when compared with other morphologically similar species living across a depth gradient. Photosynthesis at saturation (situated around 5 mg O2 gAFDM 1 h1) and photosynthetic efficiency [around 0.4 mg O2·m2 s (μmol photon·gAFDM·h)-1] are relatively low, while dark respiration (around 1 mg O2 gAFDM 1 h1) and light at compensation (around 24 μmol photon·m-2·s-1) are relatively high, as it corresponds to the characteristics of “sun” plants. C:N and C:P ratios suggest a strong nutrient limitation for growth and photosynthesis, in agreement with the low dissolved nutrient levels usually found in shallow Mediterranean waters. Homogeneity in photosynthetic features points to a good local adaptation of the different species to the prevailing light conditions but opens the question of which are the factors allowing the coexistence of different species of Fucales in sheltered and shallow Mediterranean environments. [ABSTRACT FROM AUTHOR]

Abstract To understand the thermal plasticity of a coastal foundation species across its latitudinal distribution, we assess physiological responses to high temperature stress in the kelp Laminaria digitata in combination with population genetic characteristics and relate heat resilience to genetic features and phylogeography. We hypothesize that populations from Arctic and cold‐temperate locations are less heat resilient than populations from warm distributional edges. Using meristems of natural L. digitata populations from six locations ranging between Kongsfjorden, Spitsbergen (79°N), and Quiberon, France (47°N), we performed a common‐garden heat stress experiment applying 15°C to 23°C over eight days. We assessed growth, photosynthetic quantum yield, carbon and nitrogen storage, and xanthophyll pigment contents as response traits. Population connectivity and genetic diversity were analyzed with microsatellite markers. Results from the heat stress experiment suggest that the upper temperature limit of L. digitata is nearly identical across its distribution range, but subtle differences in growth and stress responses were revealed for three populations from the species’ ecological range margins. Two populations at the species’ warm distribution limit showed higher temperature tolerance compared to other populations in growth at 19°C and recovery from 21°C (Quiberon, France), and photosynthetic quantum yield and xanthophyll pigment responses at 23°C (Helgoland, Germany). In L. digitata from the northernmost population (Spitsbergen, Norway), quantum yield indicated the highest heat sensitivity. Microsatellite genotyping revealed all sampled populations to be genetically distinct, with a strong hierarchical structure between southern and northern clades. Genetic diversity was lowest in the isolated population of the North Sea island of Helgoland and highest in Roscoff in the English Channel. All together, these results support the hypothesis of moderate local differentiation across L. digitata's European distribution, whereas effects are likely too weak to ameliorate the species’ capacity to withstand ocean warming and marine heatwaves at the southern range edge.

Marine forests and kelps as their foundation species are threatened by ocean warming especially at the warm distributional edges. Previously identified genetic divergence and ecotypic differentiation within kelp species may allow to produce more resilient lineages by intraspecific outbreeding among populations. In a mechanistic investigation of heat stress, heterosis (hybrid vigour), and underlying gene expression patterns, we assessed the thermal performance of inbred (selfings) and outbred (reciprocal crosses) sporophytes of the N-Atlantic kelp Laminaria digitata among clonal isolates from two divergent populations; one from the temperate North Sea (Helgoland) and one from the Arctic (Spitsbergen). First, we investigated the upper thermal tolerance of microscopic sporophytes in a 14-day experiment applying sublethal to lethal 20–23°C. The upper survival temperature of microscopic sporophytes was lower for the inbred Arctic selfing (21°C) than for the temperate selfing and the reciprocal crosses (22°C). Only in the temperate selfing, 4.5% of sporophytes survived 23°C. We then subjected 4–7 cm long sporophytes to a control temperature (10°C), moderate (19°C) and sublethal to lethal heat stress (20.5°C) for 18 days to assess gene expression in addition to physiological parameters. Growth and optimum quantum yield decreased similarly in the reciprocal crosses and the temperate selfing at 19 and 20.5°C, while inbred Arctic sporophytes died within seven days at both 19 and 20.5°C. In response to 20.5°C, 252 genes were constitutively regulated across all surviving lineages, which we use to describe metabolic regulation patterns in response to heat stress in kelp. At sublethal 20.5°C, ca. 150 genes were differentially expressed by either crossed lineage in comparison to the temperate selfing, indicating that they maintained a growth response similar to the temperate selfing with differential metabolic regulation during sublethal heat stress. Subtle differences in physiology and the differential expression of nine genes between the reciprocal crosses at 20.5°C indicate that female and male gametophytes may contribute differently to offspring traits. We consider potential inbreeding depression in the Spitsbergen selfing and quantify the better performance of both crosses using heterosis-related parameters. We discuss the potential and risks of outbreeding to produce more resilient crops for mariculture and marine forest restoration.

Compared to its terrestrial counterpart, work at sea to collect scientific data calls for an efficient workflow and particular skills from personnel, particularly when the study involves several observation variables. Therefore, a procedure that incorporates both the underwater and on-board phases in a coordinated manner is required. Although there exist published and validated sampling protocols for benthic habitats, in the existing literature it is more difficult to find workflows or recommendations for use prior to data collection. Using a kelp forest ecological assessment study as a model, we present a detailed workflow for sampling, both on-board and under hyperbaric conditions, so that it can be used as an example for subsequent evaluations, as a standardized method for systematic study in blue carbon ecosystems. cosystems.

Canopy-forming brown algae support highly productive ecosystems whose decline has been attributed to the interplay of several anthropogenic disturbances. Climate change could have disruptive effects on the biology of these species, but the role of temperature in the development of early life stages is poorly understood. The aim of this study was to assess the response of Ericaria giacconei, a winter-reproducing Southern–Mediterranean endemic species, to thermal stress by testing five temperatures (12, 15, 18, 24, and 28°C) on adults and early stages. Chlorophyll a fluorescence of adult plants was measured at 0, 24, 72, and 120 h on nine fronds in each of the three aquaria per treatment. To assess egg release, zygote settlement, and embryo growth rate, approximately 1,200 receptacles were cultured on six Petri dishes per temperature treatment, and 10 random subsections of 2 ×2 mm were examined in three Petri dishes at 0, 20, 44, and 92 h after fertilization. Adult plants showed a plastic physiological response, and thermal stress had no significant effect on PSII efficiency. Embryos fully developed only at 12 and 15°C. Mortality increased at 18 and 24°C, and no zygotes survived at 28°C. In a scenario of further increasing temperatures, the effects of warming could affect the recruitment of E. giacconei and increase its vulnerability to further stresses. These effects on the survival of early stages, which are the bottleneck for the long-term survival of the species, should be taken into account in conservation and restoration measures to maintain canopy-forming macroalgal populations and associated biodiversity and ecosystem services.

Kelp species function as foundation organisms along almost one third of the world's coastlines, where they underpin diverse and productive forest habitats. The three-dimensional structure of the forest modifies local environmental conditions, which in some regions allows rich understorey macroalgal and macrofaunal assemblages to develop. These understorey assemblages, however, represent somewhat overlooked components of kelp forest ecosystems, with fundamental information on their structure and the processes that shape them often lacking. Here, we used an underwater suction sampler to characterize understorey macroalgal and macrofaunal assemblages within Laminaria hyperborea forests across the mid northeast Atlantic (United Kingdom). Specifically, we investigated spatial and temporal variation in the structure, abundance, diversity, and evenness of assemblages at twelve sites nested in four regions of the UK distributed along a 9° latitudinal gradient that encompassed a ∼ 2.5 °C gradient in mean sea surface temperature. Collectively, the samples comprised of 2.4 kg of understorey macroalgae from five functional groups, and 65,323 macrofaunal individuals from 179 taxa, demonstrating that understorey assemblages represent important repositories of biodiversity within coastal marine systems. Univariate assemblage metrics exhibited significant site-level variation, while the multivariate structure of assemblages exhibited both site and regional-level variation, and all metrics exhibited significant temporal variation. The observed variation was attributed in part to complex interactions between sea surface temperature, wave exposure and habitat structure, while the high site-level variation suggests that unquantified local-scale processes are also important influencers of understorey assemblages. Given that kelp forests are becoming increasingly threatened by multiple stressors, our results provide important evidence to support enhanced management of kelp forests, as well as baseline information that can serve as a benchmark to monitor, restore, and ultimately conserve these forests and their associated ecosystem services. • Kelp understorey macroalgal and macrofaunal assemblages are abundant and diverse but exhibit spatiotemporal variation. • Sea surface temperature, wave exposure, and habitat structure are important drivers of macrofaunal assemblage structure. • Results provide a baseline upon which to detect change in understorey macroalgal and macrofaunal assemblages. [ABSTRACT FROM AUTHOR]

Kelp species function as important foundation organisms in coastal marine ecosystems where they provide biogenic habitat and ameliorate environmental conditions, often facilitating the development of diverse understorey assemblages. The structure of kelp forests is influenced by a variety of environmental factors, changes in which can result in profound shifts in ecological structure and functioning. Intense storm-induced wave action in particular, can severely impact kelp forest ecosystems. Given that storms are anticipated to increase in frequency and intensity in response to anthropogenic climate change, it is critical to understand their potential impacts on kelp forest ecosystems. During the 2021/22 northeast Atlantic storm season, the United Kingdom (UK) was subject to several intense storms, of which the first and most severe was Storm Arwen. Due to the unusual northerly wind direction, the greatest impacts of Storm Arwen were felt along the northeast coast of the UK where wind gusts exceeded 90 km/h, and inshore significant wave heights of 7.2 m and wave periods of 9.3 s were recorded. Here, we investigated temporal and spatial variation in the structure of L. hyperborea forests and associated understorey assemblages along the northeast coast of the UK over the 2021/22 storm season. We found significant changes in the cover, density, length, biomass, and age structure of L. hyperborea populations and the composition of understorey assemblages following the storm season, particularly at our most north facing site. We suggest continuous monitoring of these systems to further our understanding of temporal variation and potential recovery trajectories, alongside enhanced management to promote resilience to future perturbations. • Spatio-temporal variation in the structure, abundance, and composition of L. hyperborea forests and their associated understorey macroalgae and macrofauna assemblages along the northeast coast of the UK was assessed between two timepoints spanning the 2021/22 northeast Atlantic storm season. • Storm Arwen, which was characterised by an unusual northerly wind direction, was one of the most impactful storms along the northeast coast with winds gusts exceeding 90 km/h and significant wave heights of over 7m. • We found significant variation in the cover, density, length, biomass, and age structure of L. hyperborea populations and the composition of understorey assemblages between pre and post storm timepoints. • Differences between timepoints were generally most pronounced at the most north facing site. • The results suggest that the storm season likely had an impact on the kelp forest ecosystem, but seasonal variation may also have contributed to observed differences between timepoints. [ABSTRACT FROM AUTHOR]

Dispersal is a central process that affects population growth, gene flow, and ultimately species persistence. Here we investigate the extent to which gene flow occurs between fragmented populations of the deep-water brown algae Ericaria zosteroides (Turner) Greville (Sargassaceae, Fucales). These investigations were performed at different spatial scales from the bay of Marseille (western Provence) to Corsica. As dispersal of zygotes is shown to be limited over distances beyond a few meters, we used a multidisciplinary approach, based on Lagrangian modeling and population genomics to test the hypothesis that drifting of fertile parts of thallus (eggs on fertile branches), mediated by ocean currents, enable occasional gene flow between populations. Therefore we assessed the respective contribution of oceanographic connectivity, geographical isolation, and seawater temperatures to the genetic structure of this species. The genetic structure was assessed using 10,755 neutral SNPs and 12 outlier SNPs genotyped by dd-RAD sequencing in 261 individuals of E. zosteroides. We find that oceanographic connectivity is the best predictor of genetic structure, while differentiation in outlier SNPs can be explained by the depth of populations, as emphasized by the minimum seawater temperature predictor. However, further investigations will be necessary for clarifying how depth drives adaptive genetic differentiation in E. zosteroides. Our analyses revealed that local hydrodynamic conditions are correlated with the very high divergence of one population in the Bay of Marseille. Overall, the levels of gene flow mediated by drifting were certainly not sufficient to counteract differentiation by local genetic drift, but enough to allow colonization several kilometers away. This study stresses the need to consider secondary dispersal mechanisms of presumed low dispersal marine species to improve inference of population connectivity.

Sargassum species form large beds that play an important role in coastal ecosystems. The beds are abundant and Sargassum is often used as food and in Asian medicine in Vietnam. As a base for future use and conservation, we surveyed 14 species of Sargassum at eight locations during April-May of 2018 and 2019. Sargassum species from Vietnam were divisible into northern and southern groups; the northern group comprised Sargassum graminifolium, Sargassum herklotsii, and Sargassum cotoense, which occur in the North and North Central zones. The southern group comprised Sargassum bicorne, Sargassum feldmannii, Sargassum ilicifolium, Sargassum swartzii, and two endemic species, Sargassum phamhoangii and Sargassum quinhonense, which occur in the Central to South West zones. Three species, Sargassum aquifolium, Sargassum oligocystum, and Sargassum polycystum, were widespread along the long coast of Vietnam. This is the first report on distribution patterns of Sargassum species along the long coast of Vietnam. Our study provides baseline data for evaluating ecological disturbance and risks of Sargassum-associated ecosystems. [ABSTRACT FROM AUTHOR]

To understand the thermal plasticity of a coastal foundation species across its latitudinal distribution, we assess physiological responses to high temperature stress in the kelp Laminaria digitata in combination with population genetic characteristics and relate heat resilience to genetic features and phylogeography. We hypothesize that populations from Arctic and cold‐temperate locations are less heat resilient than populations from warm distributional edges. Using meristems of natural L. digitata populations from six locations ranging between Kongsfjorden, Spitsbergen (79°N), and Quiberon, France (47°N), we performed a common‐garden heat stress experiment applying 15°C to 23°C over eight days. We assessed growth, photosynthetic quantum yield, carbon and nitrogen storage, and xanthophyll pigment contents as response traits. Population connectivity and genetic diversity were analyzed with microsatellite markers. Results from the heat stress experiment suggest that the upper temperature limit of L. digitata is nearly identical across its distribution range, but subtle differences in growth and stress responses were revealed for three populations from the species' ecological range margins. Two populations at the species' warm distribution limit showed higher temperature tolerance compared to other populations in growth at 19°C and recovery from 21°C (Quiberon, France), and photosynthetic quantum yield and xanthophyll pigment responses at 23°C (Helgoland, Germany). In L. digitata from the northernmost population (Spitsbergen, Norway), quantum yield indicated the highest heat sensitivity. Microsatellite genotyping revealed all sampled populations to be genetically distinct, with a strong hierarchical structure between southern and northern clades. Genetic diversity was lowest in the isolated population of the North Sea island of Helgoland and highest in Roscoff in the English Channel. All together, these results support the hypothesis of moderate local differentiation across L. digitata's European distribution, whereas effects are likely too weak to ameliorate the species' capacity to withstand ocean warming and marine heatwaves at the southern range edge. [ABSTRACT FROM AUTHOR]

Summary Transplanting experiments were carried out to determine whether the small type sporophytes with short stipe of Ecklonia cava Kjellman (Laminariales, Phaeophyta) growing in a locality with warm temperatures, change into larger type with a long stipe when transplanted to a locality with cooler temperatures. Juvenile E. cava sporophytes, having a stipe shorter than 5 cm long were collected from Tei in Tosa Bay (southern Japan) (seawater temperature 15-29°C) and transplanted to Nabeta Bay (central Japan) (seawater temperature 13-25°C), where larger type E. cava sporophytes characterized by long stipe (ca 1 m) grow. They were attached to artificial reefs at the sea bottom (9 m depth) in Nabeta Bay to monitor their growth. For comparison, juvenile E. cava sporophytes of almost similar size growing in Nabeta Bay were also transplanted in the same way to the same experimental site. Observations of growth of sporophytes from Tei and Nabeta were carried out monthly for 2 years from November 1995 to October 1997. The transplanted Tei and Nabeta sporophytes showed an increase in stipe length and diameter from winter to spring, whereas almost no increase was observed during summer and autumn. At the end of the study period, the stipe of Nabeta sporophytes reached 25.6 cm in length and 17.0 mm in diameter, whereas that of Tei sporophytes reached 11.1 cm in length and 11.2 mm in diameter. The primary blade length was 16.0 cm in Nabeta sporophytes, whereas it was 5.5 cm in Tei sporophytes. Thus, Tei sporophytes still remained smaller than Nabeta sporophytes even under the same environmental conditions. [ABSTRACT FROM AUTHOR]

Summary Sporophytes of Ecklonia cava Kjellman (Laminariales, Phaeophyta) were collected seasonally from within 3-4 replicate, 1-m2 quadrates, haphazardly placed in dense assemblages at 6-9 m depth in Tei, Tosa Bay (southern Japan; water temperature 15-29°C) from 1995 to 1996, and in Nabeta Bay, Shimoda (central Japan; water temperature 13-25°C) from 1996 to 1997. Growth rings were checked for all samples and mean values of each biometric parameter of sporophytes 1-year-old and over were compared. The plant length (stipe length + primary blade length) was always shorter in Tei sporophytes (24-52 cm) than Nabeta sporophytes (70-100 cm), the difference being mainly owing to the shorter stipe length in Tei sporophytes (7-14 cm) than in Nabeta sporophytes (54-83 cm). However, the primary blade length was sometimes longer in Tei sporophytes (12-38 cm) than in Nabeta sporophytes (14-21 cm). Stipe diameter, longest bladelet length and primary blade width were mostly less in Tei sporo­phytes (8.6-12.4 mm, 5.5-7.0 cm and 23.4-38.0 cm, respectively) than Nabeta sporophytes (16.4-20.2 mm, 9.2-12.0 cm and 43.0-52.6 cm, respectively). Nevertheless, the number of bladelets of sporophytes from Tei (15-28) and Nabeta (18-29) were within the same range. At Tei, 32-43% of the sporophytes had wrinkled blades in summer and autumn, whereas wrinkled primary blades or bladelets were not observed in Nabeta sporophytes throughout the experimental period. These morphometric differences of E. cava between the two localities are suggested to be dependent on environmental factors, especially seawater temperature. [ABSTRACT FROM AUTHOR]

SUMMARY Characteristics of photosynthesis and respiration of bladelets were compared between Ecklonia cava Kjellman sporophytes growing in a warmer temperate locality (Tei, Kochi Pref., southern Japan) and in a cooler temperate locality (Nabeta, Shizuoka Pref., central Japan). Photosynthesis and respiration were measured with a differential gas-volumeter (Productmeter). In photosynthesis–light curves at 20°C, the rate of net photosynthesis was almost the same at light intensities lower than 25 μmol m–2 s–1 and the light-saturation occurred at 200–400 μmol m–2s–1 in plants of both localities. The light-saturated net photosynthetic rates were higher in winter and spring than in summer and autumn in both plants. The optimum temperature for net photosynthesis at 400 μmol m–2 s–1 was 27°C throughout the year in the Tei plant and 25–27°C in the Nabeta plant. The decrease of net photosynthetic rates in the supraoptimal temperature range up to 29°C was sharper in winter and spring than in summer and autumn in both plants, being smaller in the Tei plant than in the Nabeta plant in all seasons. The dark respiration rate always increased with water temperature rise in both plants. No clear differences were found in the dark respiration rate between Tei and Nabeta plants except that when measured against dry weight, the Tei plant showed a slightly lower rate as compared with the Nabeta plant. [ABSTRACT FROM AUTHOR]