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Understanding the effects of ecological disturbances in coastal habitats is crucial and timely as these are anticipated to increase in intensity and frequency in the future due to increasing human pressure. In this study we used directed kelp trawling as a scientific tool to quantify the impacts of broad-scale disturbance on community structure and function. We tested the ecosystem-wide effects of this disturbance in a BACI design using two 15 km2 areas. The disturbance had a substantial impact on the kelp forests in this study, removing 2986 tons of kelp and causing a 26% loss of total kelp canopy at trawled stations. This loss created a 67% reduction of epiphytes, an 89% reduction of invertebrates and altered the fish populations living within these habitats. The effect of habitat loss on fish was variable and depended on how the different species used the habitat structure. Our results show that large-scale experimental disturbances on habitat-forming species have ecological consequences that extend beyond the decline of the single species to affect multiple trophic levels of the broader ecosystem. Our findings have relevance for understanding how increasing anthropogenic disturbances, including kelp trawling and increased storm frequency caused by climate change, may alter ecosystem structure and function.

Recognition of the potential for vegetated coastal ecosystems to store and sequester carbon has led to their increasing inclusion into global carbon budgets and carbon offset schemes. However, kelp forests have been overlooked in evaluations of this ‘blue carbon’, which have been limited to tidal marshes, mangrove forests, and seagrass beds. We determined the continental-scale contribution to blue carbon from kelp forests in Australia using areal extent, biomass, and productivity measures from across the entire Great Southern Reef. We reveal that these kelp forests represent 10.3–22.7 Tg C and contribute 1.3–2.8 Tg C year−1 in sequestered production, amounting to more than 30% of total blue carbon stored and sequestered around the Australian continent, and ~ 3% of the total global blue carbon. We conclude that the omission of kelp forests from blue carbon assessments significantly underestimates the carbon storage and sequestration potential from vegetated coastal ecosystems globally.

Forests are key components of the global carbon cycle and dominate mitigation strategies for climate change and biodiversity loss. In contrast, the importance of and threats facing ocean forests—kelp forests—are comparatively underappreciated. Yet, increasing the global area of kelp forests would enhance biodiversity and drawdown CO2, mitigating climate change.

Kelp forests are in decline globally and large-scale intervention could be required to halt the loss of these valuable ecosystems. To date kelp forest restoration has had limited success and been expensive and unable to address the increasing scale of ecosystem deterioration. Here we developed and tested a new approach: “green gravel”. Small rocks were seeded with kelp and reared in the laboratory until 2–3 cm, before out-planting to the field. The out-planted kelp had high survival and growth over 9 months, even when dropped from the surface. This technique is cheap, simple, and does not require scuba diving or highly trained field workers. It can be up-scaled to treat large areas or even used to introduce genes from more resilient kelp populations onto vulnerable reefs. Green gravel thus overcomes some of the current major limitations of kelp restoration and provides a promising new defense against kelp forest decline.

Climate change is transforming marine ecosystems through the expansion and contraction of species’ ranges. Sea ice loss and warming temperatures are expected to expand habitat availability for macroalgae along long stretches of Arctic coastlines. To better understand the current distribution of kelp forests in the Eastern Canadian Arctic, kelps were sampled along the coasts for species identifications and percent cover. The sampling effort was supplemented with occurrence records from global biodiversity databases, searches in the literature, and museum records. Environmental information and occurrence records were used to develop ensemble models for predicting habitat suitability and a Random Forest model to predict kelp cover for the dominant kelp species in the region – Agarum clathratum, Alaria esculenta, and Laminariaceae species (Laminaria solidungula and Saccharina latissima). Ice thickness, sea temperature and salinity explained the highest percentage of kelp distribution. Both modeling approaches showed that the current extent of arctic kelps is potentially much greater than the available records suggest. These modeling approaches were projected into the future using predicted environmental data for 2050 and 2100 based on the most extreme emission scenario (RCP 8.5). The models agreed that predicted distribution of kelp in the Eastern Canadian Arctic is likely to expand to more northern locations under future emissions scenarios, with the exception of the endemic arctic kelp L. solidungula, which is more likely to lose a significant proportion of suitable habitat. However, there were differences among species regarding predicted cover for both current and future projections. Notwithstanding model-specific variation, it is evident that kelps are widespread throughout the area and likely contribute significantly to the functioning of current Arctic ecosystems. Our results emphasize the importance of kelp in Arctic ecosystems and the underestimation of their potential distribution there.

The production and fate of seaweed detritus is a major unknown in the global C-budget. Knowing the quantity of detritus produced, the form it takes (size) and its timing of delivery are key to understanding its role as a resource subsidy to secondary production and/or its potential contribution to C-sequestration. We quantified the production and release of detritus from 10 Laminaria hyperborea sites in northern Norway (69.6° N). Kelp biomass averaged 770 ± 100 g C m−2 while net production reached 499 ± 50 g C m−2 year−1, with most taking place in spring when new blades were formed. Production of biomass was balanced by a similar formation of detritus (478 ± 41 g C m−2 year−1), and both were unrelated to wave exposure when compared across sites. Distal blade erosion accounted for 23% of the total detritus production and was highest during autumn and winter, while dislodgment of whole individuals and/or whole blades corresponded to 24% of the detritus production. Detachment of old blades constituted the largest source of kelp detritus, accounting for > 50% of the total detrital production. Almost 80% of the detritus from L. hyperborea was thus in the form of whole plants or blades and > 60% of that was delivered as a large pulse within 1–2 months in spring. The discrete nature of the delivery suggests that the detritus cannot be retained and consumed locally and that some is exported to adjacent deep areas where it may subsidize secondary production or become buried into deep marine sediments as blue carbon.

15 pages, 12 figures, supplementary data https://doi.org/10.1016/j.dsr.2020.103433, Kelp forests produce large amounts of macroalgal detritus, ranging from whole plants to small particles (1 mm). The role of this kelp detritus in fueling deep-sea communities adjacent to healthy kelp forests was investigated in a region in the north of Norway by comparing the community structure and biodiversity of meio-, macro-, and megafauna in two deep (450 m) areas with different expected input of kelp detritus: a deep fjord basin surrounded by kelp forests and the adjacent continental shelf 15 km offshore from the kelp forests. The results showed that, although the fjord received a significantly higher amount of large kelp detritus (i.e. blades) than the shelf area, the amount of small kelp detritus available on the sediment was similar in both areas. There were significant differences in the multidimensional scaling analyses on the community structure for meio-, macro-, and megafauna between the fjord and the shelf. Significant differences were also found in biomass, abundance and biodiversity indices for some groups. However, no clear pattern emerged in the community structure and biodiversity between the fjord and the shelf, and the observed differences could not be linked directly to kelp detritus availability. The similar amounts of small particles of kelp detritus in the fjord and shelf area suggest that kelp detritus can provide organic matter to ecosystems further away than initially hypothesized, thus potentially shaping the structure and functioning of deep benthic communities distant from the kelp forests. Yet, the direct (trophic) links of kelp detritus and the studied benthic fauna need to be further analysed. The results are discussed in relation to current global changes in kelp forest, including regime shifts from healthy kelp reefs to turfs or barren areas, which reduce drastically the amount of macroalgal detritus produced and exported, This study was funded by the Norwegian Research Council through the KELPEX project (NRC Grant no. 255085/E40). With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

Funding Funding for this work was provided by the Australian Research Council (TW: FT110100174, TW, KFD: DP190100058), the University of Western Australia Research Collaboration Awards (TW, TdB, JCL, DD, JNF, HC), the France-Australia Science Innovation Collaboration 2014 program (TdB, TW, JCL, DD), Metabomer and Corsaire Core Facility (CL), the French Government run by the National Research Agency with regards to the investment expenditure programme IDEALG ANR-10-BTBR-04 (JCL, DD) and the Norwegian Research Council (TW, MFP, KFD, KELPEX 255085/E40). As the environment is getting warmer and species are redistributed, consumers can be forced to adjust their interactions with available prey, and this could have cascading effects within food webs. To better understand the capacity for foraging flexibility, our study aimed to determine the diet variability of an ectotherm omnivore inhabiting kelp forests, the sea urchin Echinus esculentus, along its entire latitudinal distribution in the northeast Atlantic. Using a combination of gut content and stable isotope analyses, we determined the diet and trophic position of sea urchins at sites in Portugal (42° N), France (49° N), southern Norway (63° N), and northern Norway (70° N), and related these results to the local abundance and distribution of putative food items. With mean estimated trophic levels ranging from 2.4 to 4.6, omnivory and diet varied substantially within and between sites but not across latitudes. Diet composition generally reflected prey availability within epiphyte or understorey assemblages, with local affinities demonstrating that the sea urchin adjusts its foraging to match the small-scale distribution of food items. A net “preference” for epiphytic food sources was found in northern Norway, where understorey food was limited compared to other regions. We conclude that diet change may occur in response to food source redistribution at multiple spatial scales (microhabitats, sites, regions). Across these scales, the way that key consumers alter their foraging in response to food availability can have important implication for food web dynamics and ecosystem functions along current and future environmental gradients. info:eu-repo/semantics/publishedVersion

Special issue: Marine ecosystem restoration: theory, practice and future perspectives.-- This research is part of POLARCSIC activities.-- 8 figures, 1 table, supporting information https://doi.org/10.1111/rec.13327.-- This is the pre-peer reviewed version of the following article: Daniel Vilas, Marta Coll, Torstein Pedersen, Xavier Corrales, Karen Filbee-Dexter, Thomas Wernberg; Future trajectories of change for an Arctic deep-sea ecosystem connected to coastal kelp forests; Restoration Ecology 29(S2): e13327 (2021), which has been published in final form at https://doi.org/10.1111/rec.13327. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions, Environmental stressors related to climate change and other anthropogenic activities are impacting Arctic marine ecosystems at exceptional rates. Within this context, predicting future scenarios of deep-sea ecosystems and their consequences linked with the fate of coastal areas is a growing need and challenge. We used an existing food-web model developed to represent the outer basin of the Malangen fjord, a northern Norwegian deep-sea ecosystem, to assess the potential effects of plausible future trajectories of change for major drivers in the area, including links to coastal kelp forests. We considered four major drivers (kelp particulate organic matter [POM] production entering the deep sea, fishing effort, king crab invasion, and ocean warming) to project 12 future scenarios using the temporal dynamic module of Ecopath with Ecosim approach. Overall, we found that the impact of warming on the deep-sea ecosystem structure and functioning, as well as on ecosystem services, are predicted to be greater than changes in kelp forest dynamics and their POM production entering the deep sea and the king crab invasion. Yet, the cumulative impacts are predicted to be more important than noncumulative since some stressors acted synergistically. These results illustrate the vulnerability of sub-Arctic and Arctic marine ecosystems to climate change and consequently call for conservation, restoration, and adaptation measures in deep-sea and adjacent ecosystems. Results also highlight the importance of considering additional stressors affecting deep-sea communities to predict cumulative impacts in an ecosystem-based management and global change context and the interlinkages between coastal and deep-sea environments, This work was funded by the Norwegian Research Council through the KELPEX project (NRC Grant no. 255085/E40). [...] MC acknowledges partial funding by the European Union´s Horizon research program grant agreement No 689518 for the MERCES project, With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

Este artículo contiene 14 páginas, 6 figuras, 1 tabla., Humans are rapidly transforming the structural configuration of the planet's eco systems, but these changes and their ecological consequences remain poorly quantified in underwater habitats. Here, we show that the loss of forest-forming seaweeds and the rise of ground-covering ‘turfs’ across four continents consistently resulted in the miniaturization of underwater habitat structure, with seascapes converging towards flattened habitats with smaller habitable spaces. Globally, turf seascapes occupied a smaller architectural trait space and were structurally more similar across regions than marine forests, evidencing habitat homogenization. Surprisingly, such habitat convergence occurred despite turf seascapes consisting of vastly different species richness and with different taxa providing habitat archi tecture, as well as across disparate drivers of marine forest decline. Turf seascapes contained high sediment loads, with the miniaturization of habitat across 100s of km in mid-Western Australia resulting in reefs retaining an additional ~242 million tons of sediment (four orders of magnitude more than the sediments delivered flu vially annually). Together, this work demonstrates that the replacement of marine forests by turfs is a generalizable phenomenon that has profound consequences for the ecology of temperate reefs., Holsworth Wildlife Research Endowment; Ministerio de Economía, Industria y Competitividad, Gobierno de España, Grant/Award Number: CTM2017-86695- C3-1-R; Australian Research Council, Grant/Award Number: DE190100692 and DP190100058; UWA; Australian Government International Research Training Program

Extreme climatic events including marine heatwaves (MHWs) are becoming more frequent and severe in the Anthropocene. However, our understanding of how these events affect population dynamics of ecologically important species is limited, in part because extreme events are rare and difficult to predict. Here, we quantified the occurrence and severity of MHWs over 60 years in warm range edge kelp forests on both sides of the North Atlantic. The cumulative annual intensity of MHWs increased two- to four-fold during this period, coinciding with the disappearance of kelps. We experimentally demonstrated a relationship between strong and severe 2018 heatwaves and high kelp mortality in both regions. Patterns of kelp mortality were strongly linked to maximum temperature anomalies, which crossed lethal thresholds in both regions. Translocation and tagging experiments revealed similar kelp mortality rates on reefs dominated by healthy kelp forests and degraded sediment-laden algal ‘turfs’, indicating equal vulnerability to extreme events. These results suggest a mechanistic link between MHWs and broad-scale kelp loss, and highlight how warming can make ecosystem boundaries unstable, forcing shifts to undesirable ecosystem states under episodically extreme climatic conditions.

Marine plant communities such as kelp forests produce significant amounts of detritus, most of which is exported to areas where it can constitute an important trophic subsidy or potentially be sequestered in marine sediments. Knowing the vertical transport speed of detrital particles is critical to understanding the potential magnitude and spatial extent of these linkages. We measured sinking speeds for Laminaria hyperborea detritus ranging from whole plants to small fragments and sea urchin faecal pellets, capturing the entire range of particulate organic matter produced by kelp forests. Under typical current conditions, we determined that this organic material can be transported 10 s of m to 10 s of km. We show how the conversion of kelp fragments to sea urchin faeces, one of the most pervasive processes in kelp forests globally, increases the dispersal potential of detritus by 1 to 2 orders of magnitude. Kelp detritus sinking speeds were also faster than equivalent phytoplankton, highlighting its potential for rapid delivery of carbon to deep areas. Our findings support arguments for a significant contribution from kelp forests to subsidizing deep sea communities and the global carbon sink.

Recognition of the potential for vegetated coastal ecosystems to store and sequester carbon has led to their increasing inclusion into global carbon budgets and carbon offset schemes. However, kelp forests have been overlooked in evaluations of this 'blue carbon', which have been limited to tidal marshes, mangrove forests, and seagrass beds. We determined the continental-scale contribution to blue carbon from kelp forests in Australia using areal extent, biomass, and productivity measures from across the entire Great Southern Reef. We reveal that these kelp forests represent 10.3-22.7 Tg C and contribute 1.3-2.8 Tg C year

This research is part of POLARCSIC activities.-- 15 pages, 9 figures, 3 tables, supplementary data https://doi.org/10.1016/j.jmarsys.2019.103268, The flow of non-living carbon (detritus) is considered an important process because it connects ecosystems and fuels benthic communities. In Norwegian kelp forests, 90% of the kelp production is exported to adjacent ecosystems where it can play a significant role in shaping benthic communities. We quantified the major structural and functional traits of an Arctic deep-sea ecosystem associated with kelp exports and assessed the ecological role of kelp export into the deep-sea system. We first developed a food-web model using the Ecopath with Ecosim (EwE) approach to represent the state of the deep (450 m) ecosystem of the Malangen fjord (Northern Norway) in 2017. Subsequently, we used the temporal dynamic model Ecosim to explore the structure and functioning traits of a theoretical deep-sea ecosystem projecting a decrease of kelp detritus biomass reaching the deep-sea ecosystem. Overall, our findings reveal that kelp detritus from shallow coastal areas has a small but noticeable role structuring the deep-sea ecosystem of Malangen. The temporal simulations show important differences depending on the application of mediating effects, which allow considering the detritus as a mediating group in prey-predator interaction, in addition to its direct role in trophic relationships. When mediating effects are applied, biomass increases for benthopelagic shrimps and suprabenthos groups and decreases for rays and skates, velvet belly, rabbitfish and other commercial demersal fishes under the low kelp detritus scenarios. Biomass-based and trophic-based indicators reveal a noticeable impact on the deep-sea ecosystem structure due to depletion of kelp detritus. To further assess future changes of the Arctic deep-sea ecosystems, dependencies with adjacent ecosystems, such as kelp detritus production, should be included, This work was funded by MARINFORSK Norwegian Research Council through the KELPEX project (NRC Grant no. 255085/E40). [...] MC acknowledges partial funding by the European Union's Horizon research program grant agreement No 689518 for the MERCES project, With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

Understanding processes that drive sudden shifts in ecosystem structure and function has become an important research focus for coastal management. In kelp bed ecosystems, regime shifts occur when high densities of sea urchins destructively graze kelp and create coralline algal barrens. While the importance of predation and disease in mediating shifts between kelp beds and barrens on shallow rocky reefs has been well documented, little is known about the role of deep-living urchins in these alternative stable-state dynamics. In this study we test the hypothesis that deep-living urchins along the central Atlantic coast of Nova Scotia move onshore and trigger shifts from kelp beds to barrens on shallow rocky reefs. We documented urchin distribution and abundance using tow-camera surveys down to 140 m depth and spanning 140 km of coast and created a predictive species-distribution model using these observations and spatial data on environmental factors that likely delineate suitable habitat for urchins. We used a random forest model to generate our predictions, which correctly classified 91% of observations into a positive or negative occurrence of urchins. Sea urchins predominantly occurred within 1.5 km of shore, in depressions and flat habitats between 40 and 85 m depth. We found that shallow regions where destructive grazing fronts have been documented over the past 4 decades were closer to deep-living sea urchin habitats compared to regions that remained in a kelp bed state during the same period. This supports our prediction that deep-living urchins play an important role in driving shallow regime shift dynamics, and indicates that their distribution can help identify areas of coast that are most vulnerable to a collapse to barrens. This article is protected by copyright. All rights reserved.

Source at https://doi.org/10.1002/ece3.4963. Ongoing changes along the northeastern Atlantic coastline provide an opportunity to explore the influence of climate change and multitrophic interactions on the recovery of kelp. Here, vast areas of sea urchin‐dominated barren grounds have shifted back to kelp forests, in parallel with changes in sea temperature and predator abundances. We have compiled data from studies covering more than 1,500‐km coastline in northern Norway. The dataset has been used to identify regional patterns in kelp recovery and sea urchin recruitment, and to relate these to abiotic and biotic factors, including structurally complex substrates functioning as refuge for sea urchins. The study area covers a latitudinal gradient of temperature and different levels of predator pressure from the edible crab (Cancer pagurus) and the red king crab (Paralithodes camtschaticus). The population development of these two sea urchin predators and a possible predator on crabs, the coastal cod (Gadus morhua), were analyzed. In the southernmost and warmest region, kelp forests recovery and sea urchin recruitment are mainly low, although sea urchins might also be locally abundant. Further north, sea urchin barrens still dominate, and juvenile sea urchin densities are high. In the northernmost and cold region, kelp forests are recovering, despite high recruitment and densities of sea urchins. Here, sea urchins were found only in refuge habitats, whereas kelp recovery occurred mainly on open bedrock. The ocean warming, the increase in the abundance of edible crab in the south, and the increase in invasive red king crab in the north may explain the observed changes in kelp recovery and sea urchin distribution. The expansion of both crab species coincided with a population decline in the top‐predator coastal cod. The role of key species (sea urchins, kelp, cod, and crabs) and processes involved in structuring the community are hypothesized in a conceptual model, and the knowledge behind the suggested links and interactions is explored.

With the increasing imperative for societies to act to curb climate change by increasing carbon stores and sinks, it has become critical to understand how organic carbon is produced, released, transformed, transported, and sequestered within and across ecosystems. In freshwater and open-ocean systems, shredders play a significant and well-known role in transforming and mobilizing carbon, but their role in the carbon cycle of coastal ecosystems is largely unknown. Marine plants such as kelps produce vast amounts of detritus, which can be captured and consumed by shedders as it traverses the seafloor. We measured capture and consumption rates of kelp detritus by sea urchins across four sampling periods and over a range of kelp detritus production rates and sea urchin densities, in northern Norway. When sea urchin densities exceeded 4 m−2, the sea urchins captured and consumed a high percentage (ca. 80%) of kelp detritus on shallow reefs. We calculated that between 1.3 and 10.8 kg of kelp m−2 are shredded annually from these reefs. We used a hydrodynamic dispersal model to show that transformation of kelp blades to sea urchin feces increased its export distance fourfold. Our findings show that sea urchins can accelerate and extend the export of carbon to neighboring areas. This collector–shredder pathway could represent a significant flow of small particulate carbon from kelp forests to deep-sea areas, where it can subsidize benthic communities or contribute to the global carbon sink.

Kelp forests are extensive underwater habitats that range along 25% of the world’s coastlines, providing valuable resources, habitat, and services for coastal communities. They grow best in cold, nutrient-rich water, where they attain some of the highest rates of primary production of any natural ecosystem. Kelps exhibit a great diversity of growth forms and life strategies, with the largest individuals reaching lengths of > 30 m and biomasses of 42 kg. In the past half century, threats to kelp forests have increased in number and severity, leading to a global decline of kelp abundances of ~ 2% per year. Trajectories of change vary considerable across regions and include range contractions, range expansions, species replacements, establishment of invasive kelps, replacement by turf algae reefs, or regime shifts to sea urchin barrens. These changes will likely have significant impacts on marine biodiversity and ecosystem functioning because kelps are foundation species for a plethora of habitat-associated plants and animals, many of which are socioeconomically important. Some forms of management have been effective in restoring kelp forests, but in many cases the threats facing kelp forests in the future greatly exceed local conservation strategies, necessitating novel conservation solutions to protect and conserve these ecosystems. Although the diversity of changes to kelp forest globally make it challenging to generalize about their future, it seems almost certain that many kelp forests a few decades from now will differ substantially from what they are today.

The Arctic is one of the most rapidly changing places on Earth and it is a sentinel region for understanding the range and magnitude of planetary changes, and their impacts on ecosystems. However, our understanding of arctic coastal ecosystems remains limited, and the impacts of ongoing and future climate change on them are largely unexplored. Kelp forests are the dominant habitat along many rocky Arctic coastlines, providing structure and food for economically and ecologically important species. Here we synthesize existing information on the distribution and diversity of arctic kelp forests and assess how ongoing changes in environmental conditions could impact the extent, productivity, and resilience of these important ecosystems. We identify regions where the range and growth of arctic kelp are likely to undergo rapid short-term increase due to reduced sea ice cover, increased light, and warming. However, we also describe areas where kelps could be negatively impacted by rising freshwater input and coastal erosion due to receding sea ice and melting permafrost. In some regions, arctic kelp forests have undergone sudden regime shifts due to altered ecological interactions or changing environmental conditions. Key knowledge gaps for arctic kelp forests include measures of extent and diversity of kelp communities (especially northern Canada and northeastern Russia), the faunal communities supported by many of these habitats, and the role of arctic kelp forests in structuring nearby pelagic and benthic food webs. Filling in these gaps and strategically prioritizing research in areas of rapid environmental change will enable more effective management of these important habitats, and better predictions of future changes in the coastal ecosystems they support and the services that they provide.

Kelp forests are structurally complex habitats, which provide valuable services along 25% of the world's coastlines. Globally, many kelp forests have disappeared and been replaced by turf algae over the last decade. Evidence that environmental conditions are becoming less favorable for kelps, combined with a lack of observed recovery, raises concern that these changes represent persistent regime shifts. Here, we show that human activities mediate turf transitions through geographically disparate abiotic (warming and eutrophication) and biotic (herbivory and epiphytism) drivers of kelp loss. Evidence suggests kelp forests are pushed beyond tipping points where new, stabilizing feedback systems (sedimentation, competition, and Allee effects) reinforce turf dominance. Although these new locks on the degraded ecosystems are strong, a mechanistic understanding of feedback systems and interactions between global and local drivers of kelp loss will expose which processes are easier to control. This should provide management solutions to curb the pervasive trend of the flattening of kelp forests globally.

Resource subsidies in the form of allochthonous primary production drive secondary production in many ecosystems, often sustaining diversity and overall productivity. Despite their importance in structuring marine communities, there is little understanding of how subsidies move through juxtaposed habitats and into recipient communities. We investigated the transport of detritus from kelp forests to a deep Arctic fjord (northern Norway). We quantified the seasonal abundance and size structure of kelp detritus in shallow subtidal (0‒12 m), deep subtidal (12‒85 m), and deep fjord (400‒450 m) habitats using a combination of camera surveys, dive observations, and detritus collections over 1 year. Detritus formed dense accumulations in habitats adjacent to kelp forests, and the timing of depositions coincided with the discrete loss of whole kelp blades during spring. We tracked these blades through the deep subtidal and into the deep fjord, and showed they act as a short-term resource pulse transported over several weeks. In deep subtidal regions, detritus consisted mostly of fragments and its depth distribution was similar across seasons (50% of total observations). Tagged pieces of detritus moved slowly out of kelp forests (displaced 4‒50 m (mean 11.8 m ± 8.5 SD) in 11‒17 days, based on minimum estimates from recovered pieces), and most (75%) variability in the rate of export was related to wave exposure and substrate. Tight resource coupling between kelp forests and deep fjords indicate that changes in kelp abundance would propagate through to deep fjord ecosystems, with likely consequences for the ecosystem functioning and services they provide.

Highly productive kelp beds off Nova Scotia, Canada export detrital material to adja- cent sedimentary habitats in deeper waters. We evaluated the importance of this subsidy to sea urchins Strongylocentrotus droebachiensis in less productive habitats. Gonad index and gut con- tent of urchins on sediments at 60 m depth were measured monthly from September 2010 to May 2013 in a large semi-protected bay, and compared to concurrent measurements of sea urchins at 8 m depth off a nearby headland. Detrital algal cover in the bay was measured using a towed cam- era system. Seasonal patterns in algal deposition and gut contents of deep-living urchins indicated that detritus was the predominant food source from March to October. We observed lower gonad indices in deep-living sea urchins compared to those at 8 m, but deep-living urchins spawned out- side of the main spring season, suggesting periodic detrital subsidy enhances their reproductive condition. Food availability and temperature appear to control the timing of gametogenesis, and phytodetritus may cue gamete release in these deep habitats. To evaluate the response of deep- living urchins to detrital subsidy, and turnover time of this material, we deployed lines baited with kelp thalli at 45 and 60 m depth and measured rates of encounter and consumption of kelp detritus by sea urchins, using the towed camera. Sea urchins aggregated on kelp within hours of deploy- ment, and consumed it after 3 to 5 mo. Our findings suggest kelp detritus represents an important energy source that can support a high reproductive capacity in deep-living urchins.

Sea urchin barrens are benthic communities on rocky subtidal reefs that are dominated by urchins and coralline algae; in the absence of intense herbivory by urchins, these barrens support luxuriant seaweed communities such as kelp beds (or forests). Barrens can extend over 1000s of km of coastline or occur in small patches (10s to 100s of m) within a kelp bed. They are characterized by low primary productivity and low food-web complexity relative to kelp communities and are generally considered a collapsed state of the kelp ecosystem. To assess the stability of sea urchin barrens and potential for return to a kelp-dominated state, we document temporal and spatial patterns of occurrence of barrens along temperate and polar coasts. We examine the various drivers of phase (or regime) shifts in these areas, the threshold levels of urchin abundance that trigger abrupt changes in ecosystem state, and the feedback mechanisms that stabilize each state. Although longitudinal (decadal) studies are limited, we find evidence in several regions that transitions between barrens and kelp beds are characterized by discontinuous phase shifts, with different thresholds for forward (to barrens) and reverse (to kelp beds) shifts, in accordance with alternative stable-state dynamics. In other areas, barrens may reflect regime shifts associated with large-scale oceanographic changes. Accelerating climate change and increasing anthropogenic impacts play important roles in altering alternative stable-state dynamics and triggering phase shifts. Recovery of the kelp state may be possible through management or remediation measures, but this necessitates a clear understanding of the thresholds and stabilizing factors for a given system.

Severe storm events are important agents of disturbance that can transport large quantities of algal detritus from highly productive kelp beds (or forests) in shallow water to deeper, more food-limited areas. We measured canopy cover in shallow kelp beds (5 to 15 m depth) and the cover of detrital kelp in sedimentary habitats directly offshore of these beds (20 to 45 m) before and after Hurricane Earl, which struck the Atlantic coast of Nova Scotia in September 2010. The storm resulted in large losses of kelp canopy cover (from 71.0 to 38.7%, averaged across sites) and significantly increased the cover of detrital kelp deposits below the kelp beds (from 1.5 to 3.4%). Detrital deposits were more commonly found in a semi-protected bay than off an exposed headland and persisted in the bay for at least 6 wk. Sea urchins Strongylocentrotus droe- bachiensis were associated with detrital kelp deposits in offshore habitats. At sites with the great- est amount of detrital kelp, we estimated that sea urchins could consume this material within 2 mo, indicating that storm-generated detrital pulses may be an important form of trophic connectivity between adjacent ecosystems off this coast.