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

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Abstract

Coastal ecosystems can efficiently remove carbon dioxide (CO₂) from the atmosphere and are thus promoted for nature-based climate change mitigation. However, natural methane (CH₄) emissions from these ecosystems may counterbalance atmospheric CO₂ uptake. 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. This study shows 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 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.

Introduction

Climate change mitigation demands reduced anthropogenic emissions of greenhouse gases (GHGs) and effective removal of excess carbon dioxide (CO₂) and methane (CH₄) from the atmosphere. Shallow-water coastal ecosystems can absorb and store large amounts of carbon from the atmosphere through vegetation buildup and long-term sediment burial. This ecosystem function has raised worldwide interest in the scientific community, conservation organizations, and governmental bodies about the potential of these ecosystems in short-term climate mitigation.

Restoring the carbon sequestration capacity of coastal ecosystems and improving their global management could result in an annual uptake of 841 Tg CO₂-equivalents (CO₂-eq.) per year, representing a significant GHG sink in the global carbon budget. However, some of this organic carbon is metabolized and returned to the atmosphere as CH₄. While aquatic CH₄ emissions can partly offset the GHG sink estimate of the terrestrial landscape and of some vegetated coastal ecosystems, such as mangroves, the magnitude of CH₄ fluxes and their contribution to the net atmospheric GHG exchange remains unknown for the majority of coastal environments and challenges our ability to develop informed climate mitigation strategies for these ecosystems.

Sustainable Development Goals (SDGs) Emphasis

• SDG 13: Climate Action: The study emphasizes the importance of accurately accounting for both CO₂ and CH₄ emissions in coastal ecosystems to develop effective climate action strategies.
• SDG 14: Life Below Water: Understanding the carbon dynamics in coastal ecosystems contributes to better management and conservation practices, ensuring sustainable use of marine resources.
• SDG 15: Life on Land: The findings highlight the interconnectedness of terrestrial and aquatic ecosystems in global carbon cycling, promoting integrated ecosystem management approaches.

Results and Discussion

Dynamic CH₄ and CO₂ Sea-Air Gas Exchange Across Coastal Ecosystems

The study revealed that all habitats were net sources of CH₄ to the atmosphere during all study periods, with high variability in fluxes across habitats and seasons. In situ CH₄ sea-air fluxes ranged from 0.1 ± 0.0 to 1.8 ± 0.1 mg CH₄ m^-2 d^-1. Daily mean net sea-air CO₂ fluxes ranged from -763 ± 99 mg CO₂ m^-2 d^-1 (sink) to 390 ± 35 mg CO₂ m^-2 d^-1 (source), with significant differences across habitats and seasons.

Methane Emissions Offset Carbon Sink Capacity Attributed to Atmospheric CO₂ Uptake

The simultaneous measurement approach of CH₄ and CO₂ sea-air fluxes is viable for directly comparing the direction and magnitude of carbon-based sea-air gas exchange. Over an annual cycle, CO₂-eq. CH₄ fluxes substantially offset the carbon sink capacity attributed to net atmospheric CO₂ uptake by 28% in macroalgae habitats, 35% in mixed vegetation habitats, and augmented positive net sea-air CO₂ fluxes by 57% in bare sediments.

Sustainable Development Goals (SDGs) Emphasis Continued

• SDG 13: Climate Action:
  1. The study underscores the need for comprehensive GHG accounting in coastal ecosystems to inform climate policies.
  2. The findings support efforts to enhance carbon sequestration while mitigating methane emissions.
• SDG 14: Life Below Water:
  1. The research highlights the role of marine vegetation in carbon cycling, promoting conservation initiatives.
  2. The data can guide restoration projects aimed at enhancing blue carbon habitats.
• SDG 15: Life on Land:
  1. The interconnectedness of land-sea carbon dynamics emphasizes integrated ecosystem management approaches.
  2. The study provides insights into managing coastal areas to maximize their climate mitigation potential.

Sediment Biogeochemical and Microbial Characteristics

The study found distinct microbial community structures across different habitats, with methanogenic archaea detected at all sites. The presence of methanogens in organic matter-rich anoxic sediments suggests local CH₄ production within these habitats. Additionally, floating filamentous algal and/or organic matter debris associated with dense stands of macroalgae were identified as potential non

Analysis of the Article

1. Which SDGs are addressed or connected to the issues highlighted in the article?

1. SDG 13: Climate Action
2. SDG 14: Life Below Water

2. What specific targets under those SDGs can be identified based on the article’s content?

1. SDG 13: Climate Action
   • Target 13.1: Strengthen resilience and adaptive capacity to climate-related hazards and natural disasters in all countries.
   • Target 13.2: Integrate climate change measures into national policies, strategies, and planning.
2. SDG 14: Life Below Water
   • Target 14.2: Sustainably manage and protect marine and coastal ecosystems to avoid significant adverse impacts, including by strengthening their resilience, and take action for their restoration in order to achieve healthy and productive oceans.
   • Target 14.3: Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

1. SDG 13: Climate Action
   • Indicator 13.1.1: Number of deaths, missing persons, and directly affected persons attributed to disasters per 100,000 population (Implied through the need for resilience and adaptive capacity).
   • Indicator 13.2.1: Number of countries that have communicated the establishment or operationalization of an integrated policy/strategy/plan which increases their ability to adapt to the adverse impacts of climate change (Implied through the need for integrating climate change measures).
2. SDG 14: Life Below Water
   • Indicator 14.2.1: Proportion of national exclusive economic zones managed using ecosystem-based approaches (Implied through the need for sustainable management and protection of marine ecosystems).
   • Indicator 14.3.1: Average marine acidity (pH) measured at agreed suite of representative sampling stations (Implied through the need to address ocean acidification).

4. Table with SDGs, Targets, and Indicators

The article discusses the role of coastal ecosystems in mitigating climate change by removing carbon dioxide from the atmosphere but also highlights the counterbalancing effect of methane emissions from these ecosystems. This connects directly to SDG 13 (Climate Action) by emphasizing the need for integrated climate strategies that account for both CO₂ uptake and CH₄ emissions. Additionally, it relates to SDG 14 (Life Below Water) by focusing on the sustainable management and protection of marine ecosystems to enhance their resilience against climate change impacts.

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Fuente: nature.com

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