Methane emissions from macrophyte beach wrack on Baltic seashores

Abundance and mass of plastic litter on sandy shore: Contribution of stormy events

The accumulation of marine litter on beaches has a detrimental impact on the environment, human health, and recreational activities. A total of 116 monitoring surveys were conducted along the shore of the Kaliningrad region between 2019 and 2023. Sampling of anthropogenic and plastic litter (>0.5 cm) was carried out under various meteorological conditions on eight sandy beaches. The greatest abundance and mass of plastic marine litter (mean ± SE: 13.75 ± 8.61 items/m2 and 19.97 ± 5.92 gDW/m2, correspondingly) were observed in the aftermath of storms within beach-cast accumulation stains at the shoreline, where it was intermixed with organic debris. This is two orders of magnitude greater than the plastic litter contamination obtained using the OSPAR methodology at the same beach during fine weather (0.11 ± 0.01 items/m2, 0.33 ± 0.02 gDW/m2). The results suggest that the most effective strategy for beach cleaning is to implement it in the post-storm period.

High contribution of an invasive macroalgae species to beach wrack CO2 emissions

Accumulations of macroalgal wrack are important for adequate functioning of the beach ecosystem. However, the sudden beaching of seaweed masses smothers the coastline and forms decomposing piles on the shore, harming tourism-based economies, but also affecting the beach ecosystem metabolism. The decomposition of sudden pulses of wrack can modify the biogeochemistry of beach sands and increase greenhouse gas (GHG) emissions. The presence of invasive species in the wrack deposits can superimpose harmful effects on the beach functioning. We quantified the wrack biomass of Rugulopteryx okamurae, an invasive species of extreme impact, on five sandy beaches from the Atlantic coast of the Strait of Gibraltar (Spain), and we tested the effects on in situ respiratory CO2 fluxes using an infrared gas analyser. All the beaches showed massive accumulations of Rugulopteryx wrack deposits. However, the biomass changed significantly between beaches, ranging (mean ± SE) from 968.3 ± 287.7 kg m-1 to 9210 ± 1279.4 kg m-1 of wet weight. Wrack supported high respiration rates, with CO2 fluxes averaging (±SE) 19.15 ± 5.5 μmol CO2 m-2 s-1 across beaches, reaching astounding maximum peaks of 291 μmol CO2 m-2 s-1. The within-beach variability was related to the distance of the wrack deposits from the shoreline, as the average metabolic rates tended to increase significantly from the swash to the drift line. Thicker wrack and a more degraded algae stage showed significantly higher CO2 fluxes. We estimated that the annual CO2 flux of R. okamurae in our study area ranged between 0.39 (±0.01) and 23.30 (±11.33) kg C m-2 y-1. We suggest that massive amounts of beach wrack can become a globally significant contributor to GHG emissions that can offset any potential carbon-sink capacity of macroalgal forests. However, the piles of wrack located several meters above the drift line showed non-measurable CO2 efflux. Transferring beach wrack from swash to drier upper-beach areas, a common practice in many coastal regions suffering from massive wrack accumulations, might help reduce GHG emissions while removing the wrack stockpiles from the intertidal. However, this practice is not necessarily suitable for all beaches and can create ecological and conservation problems in the dune system. There is an urgent need to implement practical and sustainable management practices for massive wrack deposits capable of presenting various solutions to achieve a balance between conservation and recreation actions, answering the consequences of a problem that links both, environmental and economic issues.

Beach wrack: Discussing ecological roles, risks, and sustainable bioenergy and agricultural applications

The equilibrium of the marine ecosystem is currently threatened by several constraints, among which climate change and anthropogenic activities stand out. Indeed, these factors favour the growth of macroalgae, which sometimes end up stranded on the beaches at the end of their life cycle, forming what is known as beach wrack. Despite its undeniable important ecological role on beaches, as it is an important source of organic matter (OM), and provides food and habitat for several invertebrates, reptiles, small mammals, and shorebirds, the overaccumulation of beach wrack is often associated with the release of greenhouse gases, negatively impacting tourist activities, and generating economic expenses for its removal. Although currently beach wrack is mainly treated as a waste, it can be used for numerous potential applications in distinct areas. This review aimed at providing a solid point of view regarding the process of wrack formation, its spatiotemporal location, as well as its importance and risks. It also contains the current advances of the research regarding sustainable alternatives to valorise this organic biomass, that range from bioenergy production to the incorporation of wrack in agricultural soils, considering a circular economy concept. Although there are some concerns regarding wrack utilisation, from its variable availability to a possible soil contamination with salts and other contaminants, this review comprises the overall beneficial effects of the incorporation of this residue particularly in the organic agricultural model, strengthening the conversion of this wasted biomass into a valuable resource.

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.

Variable promotion of algae and macrophyte organic matter on methanogenesis in anaerobic lake sediment

Shallow lakes are an important natural source of atmospheric methane (CH4), and the input of autochthonous organic matter (OM) into their sediments encourages methanogenesis. Although algal- and macrophytic-originated OM in these lakes are expected to have different impacts on methanogenesis and methanogenic archaeal communities in lake sediments owing to their various properties, their specific influence and role in sediment remain unclear. In this study, a 148-day incubation was carried out by adding algal- and macrophytic-OM to the sediments of shallow eutrophic Lake Chaohu and Lake Taihu in China. CH4 was periodically monitored, while the methanogens were examined via qPCR and high-throughput sequencing at the end of incubation. Algal-OM stimulated CH4 production more than macrophytic-OM in both sediments, with the rates initially increasing and then decreasing before reaching a relative constant. Macrophytic-OM promoted CH4 production to a comparable extent in both lakes, while algal-OM promoted greater CH4 in Lake Chaohu than in Lake Taihu. However, algal-OM did not significantly increase mcrA gene copies, while macrophytic-OM did by 17.0-20.1-fold. Algal-OM potentially promoted the methylotrophic pathway in Lake Taihu but did not change the methanogenic structure in Lake Chaohu. Comparatively, macrophytic-OM promoted CH4 production mainly by acetoclastic methanogen proliferation in both lakes. More CH4 release with algal-OM compared to macrophytic-OM deserves further attention owing to the prevailing increasing algal blooms and the declining macrophyte population in lakes.

Methane emissions offset atmospheric carbon dioxide uptake in coastal macroalgae, mixed vegetation and sediment ecosystems

Coastal ecosystems can efficiently remove carbon dioxide (CO₂) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH₄) emissions from these ecosystems may counterbalance atmospheric CO₂ uptake. Still, knowledge of mechanisms sustaining such CH₄ emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH₄ in the range of 0.1 - 2.9 mg CH₄ m^-2 d^-1 to the atmosphere, revealing in situ CH₄ emissions from macroalgae that were sustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO₂-equivalent CH₄ emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO₂ uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO₂ release of unvegetated sediments by 57%. Accounting for CH₄ alongside CO₂ sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.