Kelp Communities Research Articles

Multifactorial effects of warming, low irradiance, and low salinity on Arctic kelps

Abstract. The Arctic is projected to warm by 2 to 5 °C by the end of the century. Warming causes melting of glaciers, shrinking of the areas covered by sea ice, and increased terrestrial runoff from snowfields and permafrost thawing. Warming, decreasing coastal underwater irradiance, and lower salinity are potentially threatening polar marine organisms, including kelps, that are key species of hard-bottom shallow communities. The present study investigates the physiological responses of four kelp species (Alaria esculenta, Laminaria digitata, Saccharina latissima, and Hedophyllum nigripes) to these environmental changes through a perturbation experiment in ex situ mesocosms. Kelps were exposed for 6 weeks to four experimental treatments: an unmanipulated control; a warming condition under the CO2 emission scenario SSP5-8.5; and two multifactorial conditions combining warming, low salinity, and low irradiance reproducing the future coastal Arctic exposed to terrestrial runoff under two CO2 emission scenarios (SSP2-4.5 and SSP5-8.5). The physiological effects on A. esculenta, L. digitata, and S. latissima were investigated, and gene expression patterns of S. latissima and H. nigripes were analyzed. Across all species and experimental treatments, growth rates were similar, underlying the acclimation potential of these species to future Arctic conditions. Specimens of A. esculenta increased their chlorophyll a content when exposed to low irradiance conditions, suggesting that they may be resilient to an increase in glacier and river runoff with the potential to become more dominant at greater depths. S. latissima showed a lower carbon : nitrogen (C : N) ratio under the SSP5-8.5 multifactorial conditions' treatment, suggesting tolerance to coastal erosion and permafrost thawing. In contrast, L. digitata showed no response to the conditions tested on any of the investigated physiological parameters. The down-regulation of genes coding for heat-shock proteins in H. nigripes and S. latissima underscores their ability to acclimate to heat stress, which portrays temperature as a key influencing factor. Based on these results, it is expected that kelp communities will undergo changes in species composition that will vary at local scale as a function of the changes in environmental drivers.

Summer primary production of Arctic kelp communities is more affected by duration than magnitude of simulated marine heatwaves.

Fjord systems in the Norwegian Arctic are experiencing an increasing frequency and magnitude of marine heatwaves. These episodic heat stress events can have varying degrees of acute impacts on primary production and nutrient uptake of mixed kelp communities, as well as modifying the biogeochemical cycling in nearshore systems where vast areas of kelp create structural habitat. To assess the impact of future marine heatwaves on kelp communities, we conducted a 23 day mesocosm experiment exposing mixed kelp communities to warming and heatwave scenarios projected for the year 2100. Three treatments were considered: a constant warming (+1.8°C from the control), a medium magnitude and long duration heatwave event (+2.8°C from the control for 13 days), and two short-term, more intense, heatwaves(5 day long scenarios with temperature peaks at +3.9°C from the control). The results show that both marine heatwave treatments reduced net community production, whereas the constant warm temperature treatment displayed no difference from the control. The long marine heatwave scenario resulted in reduced accumulated net community production, indicating that prolonged exposure had a greater severity than two high magnitude, short-term heatwave events. We estimated an 11°C temperature threshold at which negative effects to primary production appeared present. We highlight that marine heatwaves can induce sublethal effects on kelp communities by depressing net community production. These results are placed in the context of potential physiological resilience of kelp communities and implications of reduced net community production to future Arctic fjord environmental conditions.

Kelp forest community structure and demography in Kongsfjorden (Svalbard) across 25 years of Arctic warming.

The Arctic archipelago of Svalbard is a hotspot of global warming and many fjords experience a continuous increase in seawater temperature and glacial melt while sea-ice cover declines. In 1996/1998, 2012-2014, and 2021 macroalgal biomass and species diversity were quantified at the study site Hansneset, Kongsfjorden (W-Spitsbergen) in order to identify potential changes over time. In 2021, we repeated the earlier studies by stratified random sampling (1 × 1 m2, n = 3) along a sublittoral depth transect (0, 2.5, 5, 10, and 15 m) and investigated the lower depth limits of dominant brown algae between 3 and 19 m. The maximum fresh weight (FW) of all seaweeds was 11.5 kg m-2 at 2.5 m and to 99.9% constituted of kelp. Although biomass distribution along the depth transect in 2021 was not significantly different compared to 2012/2013, the digitate kelp community (Laminaria digitata/Hedophyllum nigripes) had transformed into an Alaria esculenta-dominated kelp forest. Consequently, a pronounced shift in kelp forest structure occurred over time as we demonstrate that biomass allocation to thallus parts is kelp species-specific. Over the past decade, kelp demography changed and in 2021 a balanced age structure of kelps (juveniles plus many older kelp individuals) was only apparent at 2.5 m. In addition, the abundances and lower depth limits of all dominant brown algae declined noticeably over the last 25 years while the red algal flora abundance remained unchanged at depth. We propose that the major factor driving the observed changes in the macroalgal community are alterations in underwater light climate, as insitu data showed increasing turbidity and decreasing irradiance since 2012 and 2017, respectively. As a consequence, the interplay between kelp forest retreat to lower depth levels caused by coastal darkening and potential macroalgal biomass gain with increasing temperatures will possibly intensify in the future with unforeseen consequences for melting Arctic coasts and fjord ecosystem services.

Productivity of mixed kelp communities in an Arctic fjord exhibit tolerance to a future climate

Arctic fjords are considered to be one of the ecosystems changing most rapidly in response to climate change. In the Svalbard archipelago, fjords are experiencing a shift in environmental conditions due to the Atlantification of Arctic waters and the retreat of sea-terminating glaciers. These environmental changes are predicted to facilitate expansion of large, brown macroalgae, into new ice-free regions. The potential resilience of macroalgal benthic communities in these fjord systems will depend on their response to combined pressures from freshening due to glacial melt, exposure to warmer waters, and increased turbidity from meltwater runoff which reduces light penetration. Current predictions, however, have a limited ability to elucidate the future impacts of multiple-drivers on macroalgal communities with respect to ecosystem function and biogeochemical cycling in Arctic fjords. To assess the impact of these combined future environmental changes on benthic productivity and resilience, we conducted a two-month mesocosm experiment exposing mixed kelp communities to three future conditions comprising increased temperature (+ 3.3 and + 5.3°C), seawater freshening by ∼ 3.0 and ∼ 5.0 units (i.e., salinity of 30 and 28, respectively), and decreased photosynthetically active radiation (PAR, - 25 and - 40 %). Exposure to these combined treatments resulted in non-significant differences in short-term productivity, and a tolerance of the photosynthetic capacity across the treatment conditions. We present the first robust estimates of mixed kelp community production in Kongsfjorden and place a median compensation irradiance of ∼12.5 mmol photons m−2 h−1 as the threshold for positive net community productivity. These results are discussed in the context of ecosystem productivity and biological tolerance of kelp communities in future Arctic fjord systems.

Bathymetric origin shapes the physiological responses of Pterygophora californica (Laminariales, Phaeophyceae) to deep marine heatwaves.

Kelp communities are experiencing exacerbated heat-related impacts from more intense, frequent, and deeper marine heatwaves (MHWs), imperiling the long-term survival of kelp forests in the climate change scenario. The occurrence of deep thermal anomalies is of critical importance, as elevated temperatures can impact kelp populations across their entire bathymetric range. This study evaluates the impact of MHWs on mature sporophytes of Pterygophora californica (walking kelp) from the bathymetric extremes (8-10 vs. 25-27 m) of a population situated in Baja California (Mexico). The location is near the southernmost point of the species's broad distribution (from Alaska to Mexico). The study investigated the ecophysiological responses (e.g., photobiology, nitrate uptake, oxidative stress) and growth of adult sporophytes through a two-phase experiment: warming simulating a MHW and a post-MHW phase without warming. Generally, the effects of warming differed depending on the bathymetric origin of the sporophytes. The MHW facilitated essential metabolic functions of deep-water sporophytes, including photosynthesis, and promoted their growth. In contrast, shallow-water sporophytes displayed metabolic stress, reduced growth, and oxidative damage. Upon the cessation of warming, certain responses, such as a decline in nitrate uptake and net productivity, became evident in shallow-water sporophytes, implying a delay in heat-stress response. This indicates that variation in temperatures can result in more prominent effects than warming alone. The greater heat tolerance of sporophytes in deeper waters shows convincing evidence that deep portions of P. californica populations have the potential to serve as refuges from the harmful impacts of MHWs on shallow reefs.

Loss, resilience and recovery of kelp forests in a region of rapid ocean warming.

Changes in kelp abundances on regional scales have been highly variable over the past half-century owing to strong effects of local and regional drivers. Here, we assess patterns and dominant environmental variables causing spatial and interspecific variability in kelp persistence and resilience to change in Nova Scotia over the past 40 years. We conducted a survey of macrophyte abundance at 251 sites spanning the Atlantic coast of Nova Scotia from 2019 to 2022. We use this dataset to describe spatial variability in kelp species abundances, compare species occurrences to surveys conducted in 1982 and assess changes in kelp abundance over the past 22 years. We then relate spatial and temporal patterns in abundance and resilience to environmental metrics. Our results show losses of sea urchins and the cold-tolerant kelp species Alaria esculenta, Saccorhiza dermatodea and Agarum clathratum in Nova Scotia since 1982 in favour of the more warm-tolerant kelps Saccharina latissima and Laminaria digitata. Kelp abundances have increased slightly since 2000, and Saccharina latissima and L. digitata are widely abundant in the region today. The highest kelp cover occurs on wave-exposed shores and at sites where temperatures have remained below thresholds for growth (21 °C) and mortality (23 °C). Moreover, kelp has recovered from turf dominance following losses at some sites during a warm period from 2010 to 2012. Our results indicate that dramatic changes in kelp community composition and a loss of sea urchin herbivory as a dominant driver of change in the system have occurred in Nova Scotia over the past 40 years. However, a broad-scale shift to turf-dominance has not occurred, as predicted, and our results suggest that resilience and persistence are still a feature of kelp forests in the region despite rapid warming over the past several decades.

Nearshore subtidal community response during and after sediment disturbance associated with dam removal

Dam removal is used increasingly to restore aquatic ecosystems and remove unnecessary or high-risk infrastructure. As the number of removals increases, there is a growing understanding about the hydrologic, geomorphic, and ecological responses to these removals. Most dam removal studies, however, focus on river and watershed responses to dam removal. The removal of two dams on the Elwha River provided a unique opportunity to characterize the response of nearshore (coastal) ecosystems. We conducted SCUBA surveys between 2011 and 2022 to quantify trajectories of change in a nearshore ecosystem during and after dam removal. We focused on the degree to which the abundances of kelp, benthic invertebrates, and fish changed in response to patterns of sediment fluxes during and after dam removal. Our findings point to two pathways of response depending on the disturbance mechanism and species type. Sites with persistent sediment deposition were characterized by wholesale community changes that did not recover to a before dam removal condition. Instead, the sites were colonized by new species that were largely absent prior to dam removal. Sites that experienced high turbidity but lacked persistent seafloor deposition were primarily characterized by a reduction in the abundance of kelp and other algae during dam removal and a rapid recovery after sediment flux to the nearshore declined. Dam removal influences on invertebrates and fish at these sites were more variable, benefiting some species and disadvantaging others. In addition to dam removal, sea star wasting syndrome and a marine heatwave exerted distinct controls on subtidal communities during the same period. The loss of the predatory sea star Pycnopodia helianthoides was associated with gains in some of its prey species, and kelp community changes reflected regional trends in ocean temperature and kelp abundance. The results presented here have important implications for understanding the response of marine ecosystems to future dam removals and similar sediment perturbation events.

Lethal effects of the harmful dinoflagellate Karenia selliformis (Gymnodiniales, Dinophyceae) on two juvenile kelp sporophytes Saccharina japonica and S. sculpera (Laminariales, Phaeophyceae)

A huge red tide caused mainly by Karenia selliformis in the eastern Hokkaido Pacific Ocean during autumn of 2021 was associated with mass mortalities of sea urchins, salmon, and other marine organisms. Bleaching of seaweeds, including kelps, was reported in the areas with red tide. Thus, K. selliformis is suspected to be harmful to kelp; however, the harmful effects are not yet known. This study aimed to verify the harmfulness of K. selliformis to cultured juvenile sporophytes of two commercially valuable kelp species, Saccharina japonica and Saccharina sculpera. Juvenile kelp sporophytes were exposed to seawater with four different densities of red tide-causing K. selliformis (0, 200, 1000, 2000 cells mL−1), and changes in thallus color over time were examined. Some thalli of both kelps were visibly bleached within 12 h of K. selliformis exposure; no thallus of the negative control was bleached. The ratio of bleached thalli differed significantly depending on the density of K. selliformis and kelp species; S. japonica was more sensitive than S. sculpera. During the follow-up period, most bleached thalli, especially those exposed to high-density K. selliformis seawater, finally withered. The median lethal density (LD50) of K. selliformis within 144 h after 48 h exposure was estimated as 218 cells mL−1 and 2285 cells mL−1 for juvenile thalli of S. japonica and S. sculpera, respectively. These results suggested the lethal effects of K. selliformis to kelp leaves, suggesting the potential risk posed to cultured and natural kelp communities by K. selliformis red tides.

Diverse stability responses of a sub-Antarctic invertebrate understorey community to experimental giant kelp removal

Ecological stability is crucial for understanding anthropogenic biodiversity loss and its consequences, especially when disturbances affect species that influence numerous others. The giant kelp Macrocystis pyrifera, an iconic foundation species, supports diverse communities on temperate and subpolar coasts. While giant kelp communities in temperate regions have been extensively studied, field-based manipulative research in subpolar latitudes is still limited. We investigated the resistance, resilience, and recovery of a community subjected to experimental giant kelp removal in a sub-Antarctic ecosystem. We simulated pulsed kelp loss caused by destructive storms and monitored the macrobenthic invertebrate understorey for 12 mo. The experimental disturbance elicited varying stability responses of the understorey. Understorey community biomass was strongly resistant to the disturbance but declined gradually (i.e. low resilience), showing incomplete recovery compared to undisturbed communities. Community density (total number of individuals per sample) increased notably following the removal (i.e. weak resistance) but ultimately returned to undisturbed levels (i.e. high resilience and complete recovery). Species composition exhibited low resistance and resilience, and no recovery from the disturbance. Secondary coextinctions of several understorey species accounted for the observed low compositional stability. In contrast to giant kelp communities in temperate latitudes, the slower recovery rates of kelp and the loss of key positive interactions likely contributed to the observed stability responses in this subpolar ecosystem’s invertebrate understorey. By examining the response of a sub-Antarctic understorey community to giant kelp removal, our study enhances our understanding of how foundation species sustain local biodiversity.

Macroalgae of the high-Arctic Severnaya Zemlya Archipelago

Abstract Most of the archipelagos of the Russian Arctic are still extremely poorly studied. New data have been obtained using SCUBA diving and on-shore sampling during the“Open Ocean: Arctic Archipelagos – 2019. Severnaya Zemlya” expedition, which provided a significant update to scarce information on the seaweeds of the Severnaya Zemlya Archipelago. The list of macroalgae of the archipelago includes 48 species to date, belonging to Chlorophyta (17 species), Rhodophyta (18), and Phaeophyceae (13). Twenty-eight species are recorded for the first time in the study area; three species are new for the entire Russian Arctic. The taxonomic status of some observed species is discussed. For the first time, a wide distribution of kelps is described in the sub-tidal zone in the high-Arctic areas of the Siberian shelf, which is severely affected by ice disturbance and glacial processes. The highest population densities of the kelp communities have been observed near the Krasnoflotskie Islands (the Kara Sea) and in the Akhmatov and Mikoyan bays (the Laptev Sea). Further studies of the flora and vegetation of both the Severnaya Zemlya Archipelago shelf and other Arctic archipelagos are required.

Comment on “Seaweed ecosystems may not mitigate CO2 emissions” by Gallagher et al. (2022)

Abstract The role of animal (and plant) respiration in assessing the true carbon sequestration potential of a system is vital to acknowledge, and addressed in Gallagher et al. (2022). However, within this article, there is confusion around the respiration of kelp once exported to open waters from kelp ecosystems but respired before sequestration. From their consideration of a closed kelp ecosystem (but with import of phytoplankton and export of kelp), respiration of phytoplankton transported into the system is correctly considered in their net respiration figures (but not the fixation of carbon dioxide by the phytoplankton outside the system, again correct for a closed system). However, the respiration of kelp exported from the closed system is also considered as part of the kelp community respiration. A closed system must remove this respiration of exported kelp from calculations. Alternatively, an open system must consider also the carbon fixation by phytoplankton. The outcome of redefining open and closed systems is that the systems examined in Gallagher et al. (2022) will be net sinks of carbon, although, as yet, the magnitude of this sink is poorly quantified.

El Niño and marine heatwaves: Ecological impacts on Oregon rocky intertidal kelp communities at local to regional scales

AbstractEl Niños and marine heatwaves (MHWs) are predicted to increase in frequency under greenhouse warming. The impact of climate oscillations like El Niño‐Southern Oscillation on coastal environments in the short term likely mimics those of climate change in the long term; therefore, El Niños may serve as a short‐term proxy for possible long‐term ecological responses to an increasingly variable climate. Understanding and prediction of ecosystem responses requires elucidating the mechanisms underlying different organizational scales (organism, space, and time). We analyzed spatiotemporal variation in the effect of the 2015–2016 El Niño and the overlapping 2014–2016 East Pacific MHW on three intertidal kelps (Hedophyllum sessile, Egregia menziesii, and Postelsia palmaeformis) at seven sites across 300 km of the Oregon coast and over three years post El Niño. We measured percent cover, density, maximum length, growth, and carbon : nitrogen (C:N) ratios monthly in spring/summer at each site from 2016 through 2018. Results revealed a complex interplay between spatial, temporal, and biological factors that modified the effects of these thermal anomalies on Oregon intertidal kelp populations. Our findings generally agree with prior literature showing detrimental effects of El Niño on kelp. However, El Niño and possibly MHW effects can be mitigated or amplified by environmental processes and kelp life history strategies. In our study, coastal upwelling provided regional relief for the kelp individuals with respect to their growth needs and mitigated the adverse effects of warming. On the other hand, we also found that coastal upwelling amplified, or compounded, detrimental effects of El Niño by increasing phytoplankton‐induced shading and mollusk grazing on juvenile and adult kelps, thereby reducing their density. Given the greater uncertainty associated with warming events and climate change in the California Current Upwelling System and its biological implications, our findings reiterate the importance of acquiring better understanding of how context‐specific underlying conditions modify ecosystem processes. More specifically, understanding how demographic traits and life history stages of kelp change with biological interactions and environmental forcing over temporal and spatial scales is crucial to anticipating future climate change ramifications.

Changes in kelp forest communities off Washington, USA, during and after the 2014-2016 marine heatwave and sea star wasting syndrome

Canopy-forming kelps are foundation species in many coastal ecosystems, but kelp-forest communities are subject to abrupt state changes caused by environmental drivers and trophic dynamics. We examined changes in kelp communities at 5 sites along the Olympic Coast of Washington State, USA, during and following the recent perturbations of anomalous warm-water events and sea star wasting syndrome (SSWS). Anomalously warm water in 2013 and 2014 corresponded with a loss of approximately 50% of Macrocystis pyrifera and Nereocystis luetkeana canopy. However, the canopy quickly recovered, and stipe density increased after 2015. Purple sea urchins Strongylocentrotus purpuratus increased in density 164-fold, largely at one site, but this increase was first observed in 2017 and peaked in 2019, after the warm period. Sea stars did not show recovery from SSWS, with several species including Pycnopodia helianthoides continuing to decline. The majority of variation in assemblage structure occurred at the site level for kelps, macroinvertebrates, and fishes, while year explained most of the variability for juvenile rockfishes Sebastes spp. We did not see strong top-down effects of urchins on kelp, suggesting that top-down impacts were not dominant regionally during this period. In contrast, we found evidence for a bottom-up influence of kelp habitat on juvenile rockfishes, as rockfish recruits occurred with higher probability where kelp stipe density was higher. Our analyses highlight the importance of spatial variation in structuring changes in kelp forest communities associated with disturbance and suggest that it is essential to ensure the protection of a diversity of kelp forests.

Ocean warming and multiple source populations increase the threat of an invasive bryozoan to kelp beds in the northwest Atlantic Ocean

Climate change is expected to create more favourable climatic conditions for many invasive species, increasing their abundance and range. One such invasive species is Membranipora membranacea, an epiphytic bryozoan causing defoliation of kelp beds in the northwest Atlantic Ocean (NWA). The impact of M. membranacea is directly linked to its abundance, which is anticipated to increase due to climate change. Additionally, further range expansion may threaten Arctic kelp beds in the future. We constructed a species distribution model (SDM) to predict the abundance of M. membranacea in the NWA under present and future climate scenarios. We also assessed the effect of a possible additional invasion of M. membranacea from populations in Norway. The projected future abundance distribution of M. membranacea in the NWA differed substantially depending on the future climate scenario employed, but the bryozoan was predicted to occur in the Arctic at low abundances regardless of the scenario. However, we also found that populations of M. membranacea in Norway achieve much higher abundances at lower temperatures than NWA populations and could pose a dire threat to kelp beds in the NWA and southern Canadian Arctic if introduced in these regions. Although the SDMs performed well under internal validation, estimating the impact of M. membranacea is complicated by the context-dependent response of kelp communities to coverage by the bryozoan. Nonetheless, this study provides valuable predictions of the response of an ecologically significant invasive species to climate change with findings of broader relevance to the study of other invasive organisms.

Multi-decadal decline in cover of giant kelp Macrocystis pyrifera at the southern limit of its Australian range

Knowledge of long-term and multi-scale trends in ecological systems is a vital component in understanding their dynamics. We used Landsat satellite imagery to develop the first long-term (1986-2015) data set describing the cover of dense surface canopies of giant kelp Macrocystis pyrifera around the entire coastline of Tasmania, Australia, and assessed the extent to which potential environmental drivers explain the dynamics of surface canopies at multiple spatial and temporal scales. Broad-scale temporal patterns in canopy cover are correlated with El Niño-Southern Oscillation events, while regional patterns are related to sea surface temperature and nutrient regimes are associated with the East Australian Current. Regression models developed to predict the presence or absence of giant kelp canopy emphasise the importance of sea surface temperature in these systems. Long-term decline in canopy cover is clearly evident in most regions, and in light of increasing thermal stress associated with a changing ocean climate, this raises concern for the future of this species as a major habitat-forming kelp in Australia and some other regions worldwide. Given that Tasmania represents the stronghold of the range of this species in Australia, but is a geographic trap in that there is no suitable habitat for M. pyrifera to the south, our findings support the Federal listing of giant kelp communities in Australia as an endangered marine community type.

Superior photosynthetic performance of the invasive kelp Undaria pinnatifida may contribute to continued range expansion in a wave-exposed kelp forest community

In New Zealand, the kelp Undaria pinnatifida has colonized nearly all major coastal ports and has spread to colonize diverse habitats over a range of hydrodynamic environments. We report one of the earliest surveys of the distribution of this species in Macrocystis pyrifera dominated kelp forest habitats in New Zealand alongside experiments comparing the photosynthetic physiology of U. pinnatifida to nine native co-occurring macroalgal species. Undaria pinnatifida was identified at four locations that had previously been reported to be uninvaded and was found throughout the depth range of the kelp forest. Highest densities were observed at shallow depths, reflecting the commonly accepted mode of invasion via sheltered rock pools. Photosynthesis versus Irradiance (P vs. E) curves were constructed for 10 dominant brown macroalgal species. Undaria pinnatifida exhibited greater photosynthetic ability than native macroalgae with maximal rates of photosynthesis at least 1.6-times higher than the habitat forming native kelp species Macrocystis pyrifera and Ecklonia radiata and more than 2.5 times higher than eight other native species. The photosynthetic efficiency of U. pinnatifida under low light was at least as high as macroalgae common on deeper sections of surveyed reefs (e.g. M. pyrifera and E. radiata). Collectively, this information shows that U. pinnatifida has superior photosynthetic performance across all ecologically relevant light intensities. Our data show that the photosynthetic physiology of U. pinnatifida is not likely to limit invasion into deeper sections of reef. This trend could continue, with the potential to fundamentally alter the structure of these wave-exposed kelp communities.

Regional differences in kelp forest interaction chains are influenced by both diffuse and localized stressors

AbstractOn temperate rocky reefs, overexploitation of high‐trophic‐level omnivores can result in the decimation of kelp forest habitats by releasing sea urchin population networks from top‐down control. However, the local dynamics of the resulting trophic cascades are context‐dependent. Here, we investigate the community‐wide patterns associated with both diffuse stressors and localized protection of high‐trophic‐level omnivores in kelp forest ecosystems by comparing communities among marine reserves and fished areas in two contrasting regions in terms of fishing intensity and land‐based stressors, Fiordland and the Marlborough Sounds, New Zealand. We find higher densities of the potential sea urchin predators, red rock lobsters (Jasus edwardsii) and banded wrasse (Notolabrus fucicola), in the Fiordland region, and larger effect sizes of fishing on the exploited fish communities in the Marlborough Sounds. Patterns in sea urchin density were consistent with the idea that high‐trophic‐level species, such as large fish and rock lobsters, regulate sea urchin population density, with lower densities of Evechinus chloroticus observed inside marine reserves, in both regions. Nevertheless, densities of E. chloroticus were generally high (>3 m2) in the Marlborough Sounds, likely above a grazing threshold in both fished and reserve sites. The proportion of habitat where sea urchins were absent was 29% in Marlborough Sounds and 90% in Fiordland. Consequently, we observe 49% barren habitat in Fiordland vs. 70%, and a larger effect of fishing on kelp community structure, in the Marlborough Sounds, where fishing effect sizes and land‐based stressors were more severe. We propose that a combination of diffuse stressors including regional overexploitation of important sea urchin predators, sedimentation, and warming of coastal waters likely contributed to regional differences in the responses of trophic interaction chains to localized reductions in fishing within marine reserves. The present study highlights how physiologically stressed and modified kelp forest ecosystems are more susceptible to detrimental phase shifts at a regional spatial scale.

Environmental heterogeneity mediates scale-dependent declines in kelp diversity on intertidal rocky shores.

Biodiversity loss is driven by interacting factors operating at different spatial scales. Yet, there remains uncertainty as to how fine-scale environmental conditions mediate biological responses to broad-scale stressors. We surveyed intertidal rocky shore kelp beds situated across a local gradient of wave action and evaluated changes in kelp diversity and abundance after more than two decades of broad scale stressors, most notably the 2013–2016 heat wave. Across all sites, species were less abundant on average in 2017 and 2018 than during 1993–1995 but changes in kelp diversity were dependent on wave exposure, with wave exposed habitats remaining stable and wave sheltered habitats experiencing near complete losses of kelp diversity. In this way, wave exposed sites have acted as refugia, maintaining regional kelp diversity despite widespread local declines. Fucoids, seagrasses and two stress-tolerant kelp species (Saccharina sessilis, Egregia menziesii) did not decline as observed in other kelps, and the invasive species Sargassum muticum increased significantly at wave sheltered sites. Long-term monitoring data from a centrally-located moderate site suggest that kelp communities were negatively impacted by the recent heatwave which may have driven observed losses throughout the region. Wave-sheltered shores, which saw the largest declines, are a very common habitat type in the Northeast Pacific and may be especially sensitive to losses in kelp diversity and abundance, with potential consequences for coastal productivity. Our findings highlight the importance of fine-scale environmental heterogeneity in mediating biological responses and demonstrate how incorporating differences between habitat patches can be essential to capturing scale-dependent biodiversity loss across the landscape.

Diversity and structure of epibenthic communities of the red algae zone in the White Sea

At high latitudes, the marine transitional zone between the highly productive kelp communities and the devoid of vegetation deep-waters is occupied mainly by number of red algae species, both encrusting and cespitose, which associate with rich and diverse fauna. However, zoologists and botanists never scrutinized these communities in equal manner up to present. We investigated the transition red algae zone in the White Sea in summer 2016 and 2017 using scuba equipment. The photic depth is limited to 18–23 m at different locations. Sessile species prevail among 315 epibenthic organisms in species number ( ~ 80%) and biomass ( ~ 90%). Communities of the red algae zone are rich in species (from 54 to 177). The most distinct differences were observed between the communities of Kandalaksha and Onega Bays. Vertical heterogeneity of the red algae zone was revealed; at most locations, Rhodophyta predominate in the upper subzone and Cirripedia predominate in the lower parts of the red algae zone. At the investigated depths, shallow-water and deep-water sets of species were defined. Interspecific spatial interactions between sessile organisms are revealed: 72.2 ± 3.5% of them were recorded as epibionts, and 29.1 ± 2.5% were registered as basibionts. These interactions provide evidence of the increase of species diversity: 32.7 ± 6.8% of sessile taxa were recorded exclusively as epibionts. Algae Phycodrys rubens, Odonthalia dentata, and Coccotylus truncatus, ascidian Styela rustica, barnacles Balanus crenatus, Verruca stroemia, and polychaete Pista maculata are foundation species, each supporting multi-level suites (up to four levels) of dependent species.

Organic-matter flow in kelp forest

Ecology and Climate![Figure][1] Laminaria ochroleuca is moving north as the climate warms. PHOTO: AGE FOTOSTOCK/ALAMY STOCK PHOTO As the global climate warms, there are shifts in the geographical distribution of organisms, which can be accompanied by changes in ecosystem functioning. Pessarrodona et al. have been investigating the ecosystem consequences of the climate-driven arrival of warm-temperate kelp forest communities to the northwestern coastlines of Europe. Cycling of organic matter in the ecosystem—through kelp growth, herbivory, and decomposition—was faster in the new communities relative to cycling in native cold-temperate kelp communities. Notably, decomposition of plant detritus occurred 6.5 times faster. The continued northward expansion of warm-temperate kelp can be expected to lead to shifts in the flow of organic matter through these ecosystems and to further changes in associated communities of consumer organisms. J. Ecol. 10.1111/1365-2745.13053 (2018). [1]: pending:yes