Climate-driven shifts in kelp forest composition reduce carbon sequestration potential

A systematic review of marine macroalgal degradation: Toward a better understanding of macroalgal carbon sequestration potential

Although macroalgae are gaining recognition for their potential role in marine carbon sequestration, critical knowledge gaps related to the fate of macroalgal carbon limit our capacity to quantify rates of macroalgal carbon sequestration. Understanding the degradation dynamics of macroalgal-derived biomaterials-including tissue/wrack, particulate organic matter/carbon (POM/POC), and dissolved organic carbon (DOC)-as well as the environmental drivers of decomposition are critical for assessing the longevity of macroalgal carbon and the potential storage capacity of macroalgae. Thus, a systematic literature review of macroalgal degradation studies was conducted to compile data, estimate the relative recalcitrance (i.e., relative stability) of macroalgal biomaterials, and elucidate key drivers of macroalgal decomposition dynamics. We found that macroalgal decay trajectories are highly variable and not always best described by the often-cited exponential decay models. Our analysis demonstrated that temperature was a notable driver of decomposition, with higher temperatures eliciting faster rates of decomposition. Furthermore, we found that brown algae had significantly higher proportions of recalcitrant biomaterials when compared to red algae. The impact of other factors, including biomaterial type, degradation environment, and tissue carbon and nitrogen content on macroalgal degradation, is variable across contexts, warranting further study. These results help to provide a foundation from which to plan and assess future studies on macroalgal degradation, which will improve our understanding of how macroalgae contribute to marine carbon cycles, trophic subsidies, and, potentially, marine carbon sequestration.

The impact of a marine heatwave on the productivity and carbon budget of a NW Mediterranean seaweed forest

Marine forests support coastal biodiversity and ecosystem functioning. Nonetheless, how their productivity and carbon uptake might be affected by extreme events, such as marine heatwaves (MHWs), is yet to be explored. We experimentally evaluated the changes in oxygen and carbon budgets of the benthic community formed by the fucoid Ericaria brachycarpa induced by the exposure to a MHW. Rocks colonized by E. brachycarpa and associated macroalgal and invertebrate assemblages were collected at Capraia Island (NW Mediterranean) and put into six 500 L tanks at 23 °C. After 10 days of acclimation, the seawater temperature in three randomly chosen tanks was gradually elevated to 30.5 °C and maintained for 5 days, to simulate a MHW predicted by the end of the century under the RCP 8.5 scenario. Oxygen and carbon metabolic rates of the whole community were evaluated under light and dark conditions, using transparent and black incubation chambers, respectively. The exposure to the MHW caused a reduction in Net Community Productivity (NCP) and increased Community Respiration (CR). There was a trend for MHW to enhance total DIC release through the reduction of calcification and the increase of respiration rates, thus shifting the community metabolism to net heterotrophic. Lower net productivity and carbon uptake suggest that the role of these forests in sustaining coastal food webs and mitigating CO2 emissions could be reduced under future climates. These results have implications for devising climate-proof strategies of conservation and restoration of macroalgal forests.

Shrinking suitable habitat of a sub-Arctic foundation kelp under future climate scenarios

Climate change has profound effects on the distribution of kelp forests in the Arctic and sub-Arctic. However, studies on the responses of kelps to climate change, particularly along the sub-Arctic regions of the Alaska coast, are limited. Eualaria fistulosa is a foundational kelp species in the Aleutian Islands, with an east-west distribution that extends from Japan to southern southwest Alaska. In this study, we utilized a species distribution model (SDM) to explore changes in the future habitat suitability of E. fistulosa under contrasting Shared Socioeconomic Pathway (SSP) scenarios. Our model exhibited relatively high predictive performance, validating our SDM predictions. Notably, the SDM results indicate that minimum sea surface temperature, annual range in sea surface temperatures, and annual mean current velocities are the three most important predictor variables determining E. fistulosa's distribution. Furthermore, the projected geographic distribution of Eualaria is generally consistent with its observed occurrence records. However, under high emission scenarios (SSP5-8.5), E. fistulosa is predicted to contract its distribution range by 9.0% by 2100, with widespread disappearance along the southeast Alaskan coast and limited northward migration to Kamchatka Krai in Russia and Bristol Bay in Alaska. These findings contribute valuable insights for conservation strategies via addressing climate-induced alterations in sub-Arctic kelp distribution.

Projected loss of brown macroalgae and seagrasses with global environmental change

Although many studies predict extensive future biodiversity loss and redistribution in the terrestrial realm, future changes in marine biodiversity remain relatively unexplored. In this work, we model global shifts in one of the most important marine functional groups-ecosystem-structuring macrophytes-and predict substantial end-of-century change. By modelling the future distribution of 207 brown macroalgae and seagrass species at high temporal and spatial resolution under different climate-change projections, we estimate that by 2100, local macrophyte diversity will decline by 3-4% on average, with 17 to 22% of localities losing at least 10% of their macrophyte species. The current range of macrophytes will be eroded by 5-6%, and highly suitable macrophyte habitat will be substantially reduced globally (78-96%). Global macrophyte habitat will shift among marine regions, with a high potential for expansion in polar regions.

Adverse Environmental Perturbations May Threaten Kelp Farming Sustainability by Exacerbating Enterobacterales Diseases

Globally kelp farming is gaining attention to mitigate land-use pressures and achieve carbon neutrality. However, the influence of environmental perturbations on kelp farming remains largely unknown. Recently, a severe disease outbreak caused extensive kelp mortality in Sanggou Bay, China, one of the world's largest high-density kelp farming areas. Here, through in situ investigations and simulation experiments, we find indications that an anomalously dramatic increase in elevated coastal seawater light penetration may have contributed to dysbiosis in the kelp Saccharina japonica's microbiome. This dysbiosis promoted the proliferation of opportunistic pathogenic Enterobacterales, mainly including the genera Colwellia and Pseudoalteromonas. Using transcriptomic analyses, we revealed that high-light conditions likely induced oxidative stress in kelp, potentially facilitating opportunistic bacterial Enterobacterales attack that activates a terrestrial plant-like pattern recognition receptor system in kelp. Furthermore, we uncover crucial genotypic determinants of Enterobacterales dominance and pathogenicity within kelp tissue, including pathogen-associated molecular patterns, potential membrane-damaging toxins, and alginate and mannitol lysis capability. Finally, through analysis of kelp-associated microbiome data sets under the influence of ocean warming and acidification, we conclude that such Enterobacterales favoring microbiome shifts are likely to become more prevalent in future environmental conditions. Our study highlights the need for understanding complex environmental influences on kelp health and associated microbiomes for the sustainable development of seaweed farming.

Marine heatwave duration and intensity interact to reduce physiological tipping points of kelp species with contrasting thermal affinities

BACKGROUND AND AIMS

Marine heatwaves (MHWs) are widely recognized as pervasive drivers of ecosystem change, yet our understanding of how different MHW properties mediate ecological responses remains largely unexplored. Understanding MHW impacts on foundation species is particularly important, given their structural role in communities and ecosystems.

METHODS

We simulated a series of realistic MHWs with different levels of intensity (Control: 14 °C, Moderate: 18 °C, Extreme: 22 °C) and duration (14 or 28 d) and examined responses of two habitat-forming kelp species in the southwest UK. Here, Laminaria digitata reaches its trailing edge and is undergoing a range contraction, whereas Laminaria ochroleuca reaches its leading edge and is undergoing a range expansion.

KEY RESULTS

For both species, sub-lethal stress responses induced by moderate-intensity MHWs were exacerbated by longer duration. Extreme-intensity MHWs caused dramatic declines in growth and photosynthetic performance, and elevated bleaching, which were again exacerbated by longer MHW duration. Stress responses were most pronounced in L. ochroleuca, where almost complete tissue necrosis was observed by the end of the long-duration MHW. This was unexpected given the greater thermal safety margins assumed with leading edge populations. It is likely that prolonged exposure to sub-lethal thermal stress exceeded a physiological tipping point for L. ochroleuca, presumably due to depletion of internal reserves.

CONCLUSIONS

Overall, our study showed that exposure to MHW profiles projected to occur in the region in the coming decades can have significant deleterious effects on foundation kelp species, regardless of their thermal affinities and location within respective latitudinal ranges, which would probably have consequences for entire communities and ecosystems.

Temperature sensitivity of detrital photosynthesis

BACKGROUND AND AIMS

Kelp forests are increasingly considered blue carbon habitats for ocean-based biological carbon dioxide removal, but knowledge gaps remain in our understanding of their carbon cycle. Of particular interest is the remineralization of detritus, which can remain photosynthetically active. Here, we study a widespread, thermotolerant kelp (Ecklonia radiata) to explore detrital photosynthesis as a mechanism underlying temperature and light as two key drivers of remineralization.

METHODS

We used meta-analysis to constrain the thermal optimum (Topt) of E. radiata. Temperature and light were subsequently controlled over a 119-day ex situ decomposition experiment. Flow-through experimental tanks were kept in darkness at 15 °C or under a subcompensating maximal irradiance of 8 µmol photons m-2 s-1 at 15, 20 or 25 °C. Photosynthesis of laterals (analogues to leaves) was estimated using closed-chamber oxygen evolution in darkness and under a saturating irradiance of 420 µmol photons m-2 s-1.

KEY RESULTS

T opt of E. radiata is 18 °C across performance variables (photosynthesis, growth, abundance, size, mass and fertility), life stages (gametophyte and sporophyte) and populations. Our models predict that a temperature of >15 °C reduces the potential for E. radiata detritus to be photosynthetically viable, hence detrital Topt ≤ 15 °C. Detritus is viable under subcompensating irradiance, where it performs better than in darkness. Comparison of net and gross photosynthesis indicates that elevated temperature primarily decreases detrital photosynthesis, whereas darkness primarily increases detrital respiration compared with optimal experimental conditions, in which detrital photosynthesis can persist for ≥119 days.

CONCLUSIONS

T opt of kelp detritus is ≥3 °C colder than that of the intact plant. Given that E. radiata is one of the most temperature-tolerant kelps, this suggests that photosynthesis is generally more thermosensitive in the detrital phase, which partly explains the enhancing effect of temperature on remineralization. In contrast to darkness, even subcompensating irradiance maintains detrital viability, elucidating the accelerating effect of depth and its concomitant light reduction on remineralization to some extent. Detrital photosynthesis is a meaningful mechanism underlying at least two drivers of remineralization, even below the photoenvironment inhabited by the attached alga.

Air-sea carbon dioxide equilibrium: Will it be possible to use seaweeds for carbon removal offsets?

To limit global warming below 2°C by 2100, we must drastically reduce greenhouse gas emissions and additionally remove ~100-900 Gt CO2 from the atmosphere (carbon dioxide removal, CDR) to compensate for unavoidable emissions. Seaweeds (marine macroalgae) naturally grow in coastal regions worldwide where they are crucial for primary production and carbon cycling. They are being considered as a biological method for CDR and for use in carbon trading schemes as offsets. To use seaweeds in carbon trading schemes requires verification that seaweed photosynthesis that fixes CO2 into organic carbon results in CDR, along with the safe and secure storage of the carbon removed from the atmosphere for more than 100 years (sequestration). There is much ongoing research into the magnitude of seaweed carbon storage pools (e.g., as living biomass and as particulate and dissolved organic carbon in sediments and the deep ocean), but these pools do not equate to CDR unless the amount of CO2 removed from the atmosphere as a result of seaweed primary production can be quantified and verified. The draw-down of atmospheric CO2 into seawater is via air-sea CO2 equilibrium, which operates on time scales of weeks to years depending upon the ecosystem considered. Here, we explain why quantifying air-sea CO2 equilibrium and linking this process to seaweed carbon storage pools is the critical step needed to verify CDR by discrete seaweed beds and nearshore and open ocean aquaculture systems prior to their use in carbon trading.

Impacts of Climate Change on Marine Foundation Species

Marine foundation species are the biotic basis for many of the world's coastal ecosystems, providing structural habitat, food, and protection for myriad plants and animals as well as many ecosystem services. However, climate change poses a significant threat to foundation species and the ecosystems they support. We review the impacts of climate change on common marine foundation species, including corals, kelps, seagrasses, salt marsh plants, mangroves, and bivalves. It is evident that marine foundation species have already been severely impacted by several climate change drivers, often through interactive effects with other human stressors, such as pollution, overfishing, and coastal development. Despite considerable variation in geographical, environmental, and ecological contexts, direct and indirect effects of gradual warming and subsequent heatwaves have emerged as the most pervasive drivers of observed impact and potent threat across all marine foundation species, but effects from sea level rise, ocean acidification, and increased storminess are expected to increase. Documented impacts include changes in the genetic structures, physiology, abundance, and distribution of the foundation species themselves and changes to their interactions with other species, with flow-on effects to associated communities, biodiversity, and ecosystem functioning. We discuss strategies to support marine foundation species into the Anthropocene, in order to increase their resilience and ensure the persistence of the ecosystem services they provide.

Carbon sequestration and climate change mitigation using macroalgae: a state of knowledge review

The conservation, restoration, and improved management of terrestrial forests significantly contributes to mitigate climate change and its impacts, as well as providing numerous co-benefits. The pressing need to reduce emissions and increase carbon removal from the atmosphere is now also leading to the development of natural climate solutions in the ocean. Interest in the carbon sequestration potential of underwater macroalgal forests is growing rapidly among policy, conservation, and corporate sectors. Yet, our understanding of whether carbon sequestration from macroalgal forests can lead to tangible climate change mitigation remains severely limited, hampering their inclusion in international policy or carbon finance frameworks. Here, we examine the results of over 180 publications to synthesise evidence regarding macroalgal forest carbon sequestration potential. We show that research efforts on macroalgae carbon sequestration are heavily skewed towards particulate organic carbon (POC) pathways (77% of data publications), and that carbon fixation is the most studied flux (55%). Fluxes leading directly to carbon sequestration (e.g. carbon export or burial in marine sediments) remain poorly resolved, likely hindering regional or country-level assessments of carbon sequestration potential, which are only available from 17 of the 150 countries where macroalgal forests occur. To solve this issue, we present a framework to categorize coastlines according to their carbon sequestration potential. Finally, we review the multiple avenues through which this sequestration can translate into climate change mitigation capacity, which largely depends on whether management interventions can increase carbon removal above a natural baseline or avoid further carbon emissions. We find that conservation, restoration and afforestation interventions on macroalgal forests can potentially lead to carbon removal in the order of 10's of Tg C globally. Although this is lower than current estimates of natural sequestration value of all macroalgal habitats (61-268 Tg C year-1 ), it suggests that macroalgal forests could add to the total mitigation potential of coastal blue carbon ecosystems, and offer valuable mitigation opportunities in polar and temperate areas where blue carbon mitigation is currently low. Operationalizing that potential will necessitate the development of models that reliably estimate the proportion of production sequestered, improvements in macroalgae carbon fingerprinting techniques, and a rethinking of carbon accounting methodologies. The ocean provides major opportunities to mitigate and adapt to climate change, and the largest coastal vegetated habitat on Earth should not be ignored simply because it does not fit into existing frameworks.

Impact of kelp forest on seawater chemistry - A review

Kelp forests, globally distributed in cool temperate and polar waters, are renowned for their pivotal role in supporting species diversity and fostering macroalgae productivity. These high-canopy algal ecosystems dynamically influence their surroundings, particularly by altering the physicochemical properties of seawater. This review article aims to underscore the significance of kelp forests in modifying water masses. By serving as effective carbon sinks through the absorption of bicarbonate (HCO3-) and carbon dioxide (CO2) for photosynthesis, kelp forests mitigate nearby acidity levels while enhancing dissolved oxygen concentrations, essential for sustaining diverse marine communities. Additionally, kelp beds have exhibited the need to use inorganic ions (NO3-, NO2-, PO43-) from seawater in order to grow, albeit with associated increases in NH4+ concentrations. Specific examples and findings from relevant studies will be presented to illustrate the profound impact of kelp forests on seawater chemistry, emphasizing their vital role in marine ecosystems.

Standing Crop, Turnover, and Production Dynamics of Macrocystis pyrifera and Understory Species Hedophyllum nigripes and Neoagarum fimbriatum in High Latitude Giant Kelp Forests

Production rates reported for canopy-forming kelps have highlighted the potential contributions of these foundational macroalgal species to carbon cycling and sequestration on a globally relevant scale. Yet, the production dynamics of many kelp species remain poorly resolved. For example, productivity estimates for the widely distributed giant kelp Macrocystis pyrifera are based on a few studies from the center of this species' range. To address this geospatial bias, we surveyed giant kelp beds in their high latitude fringe habitat in southeast Alaska to quantify foliar standing crop, growth and loss rates, and productivity of M. pyrifera and co-occurring understory kelps Hedophyllum nigripes and Neoagarum fimbriatum. We found that giant kelp beds at the poleward edge of their range produce ~150 g C · m-2 · year-1 from a standing biomass that turns over an estimated 2.1 times per year, substantially lower rates than have been observed at lower latitudes. Although the productivity of high latitude M. pyrifera dwarfs production by associated understory kelps in both winter and summer seasons, phenological differences in growth and relative carbon and nitrogen content among the three kelp species suggests their complementary value as nutritional resources to consumers. This work represents the highest latitude consideration of M. pyrifera forest production to date, providing a valuable quantification of kelp carbon cycling in this highly seasonal environment.