Policies aiming to preserve vegetated coastal ecosystems (VCE; tidal marshes, mangroves and seagrasses) to mitigate greenhouse gas emissions require national assessments of blue carbon resources. Here, we present organic carbon (C) storage in VCE across Australian climate regions and estimate potential annual CO2 emission benefits of VCE conservation and restoration. Australia contributes 5–11% of the C stored in VCE globally (70–185 Tg C in aboveground biomass, and 1,055–1,540 Tg C in the upper 1 m of soils). Potential CO2 emissions from current VCE losses are estimated at 2.1–3.1 Tg CO2-e yr-1, increasing annual CO2 emissions from land use change in Australia by 12–21%. This assessment, the most comprehensive for any nation to-date, demonstrates the potential of conservation and restoration of VCE to underpin national policy development for reducing greenhouse gas emissions. Policies aiming to preserve vegetated coastal ecosystems (VCE) to mitigate greenhouse gas emissions require national assessments of blue carbon resources. Here the authors assessed organic carbon storage in VCE across Australian and the potential annual CO2 emission benefits of VCE conservation and find that Australia contributes substantially the carbon stored in VCE globally.

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Australian vegetated coastal ecosystems as global hotspots for climate change mitigation

The term Blue Carbon (BC) was first coined a decade ago to describe the disproportionately large contribution of coastal vegetated ecosystems to global carbon sequestration. The role of BC in climate change mitigation and adaptation has now reached international prominence. To help prioritise future research, we assembled leading experts in the field to agree upon the top-ten pending questions in BC science. Understanding how climate change affects carbon accumulation in mature BC ecosystems and during their restoration was a high priority. Controversial questions included the role of carbonate and macroalgae in BC cycling, and the degree to which greenhouse gases are released following disturbance of BC ecosystems. Scientists seek improved precision of the extent of BC ecosystems; techniques to determine BC provenance; understanding of the factors that influence sequestration in BC ecosystems, with the corresponding value of BC; and the management actions that are effective in enhancing this value. Overall this overview provides a comprehensive road map for the coming decades on future research in BC science.

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The future of Blue Carbon science

Seagrass ecosystems contain globally significant organic carbon (C) stocks. However, climate change and increasing frequency of extreme events threaten their preservation. Shark Bay, Western Australia, has the largest C stock reported for a seagrass ecosystem, containing up to 1.3% of the total C stored within the top metre of seagrass sediments worldwide. On the basis of field studies and satellite imagery, we estimate that 36% of Shark Bay’s seagrass meadows were damaged following a marine heatwave in 2010/2011. Assuming that 10 to 50% of the seagrass sediment C stock was exposed to oxic conditions after disturbance, between 2 and 9 Tg CO2 could have been released to the atmosphere during the following three years, increasing emissions from land-use change in Australia by 4–21% per annum. With heatwaves predicted to increase with further climate warming, conservation of seagrass ecosystems is essential to avoid adverse feedbacks on the climate system. Marine ecosystems and their stored carbon are threatened by warming and marine heatwaves. During a 2010–2011 heatwave, around a third of a Western Australian seagrass ecosystem suffered damage, potentially releasing 2–9 Tg CO2 in the following years.

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A marine heatwave drives massive losses from the world’s largest seagrass carbon stocks

The Paris agreement target of limiting global surface warming to 15-2°C compared to pre-industrial levels by 2100 will heavily impact the ocean While ambitious mitigation and adaptation are both needed, the ocean provides major opportunities for action to reduce climate change globally and its impacts on vital ecosystems and ecosystem services A comprehensive and systematic assessment of 13 global- and local-scale, ocean-based measures was performed to help steer the development and implementation of technologies and actions towards a sustainable outcome We show that (1) all measures have tradeoffs and multiple criteria must be used for a comprehensive assessment of their potential, (2) greatest benefit is derived by combining global and local solutions, some of which could be implemented or scaled-up immediately, (3) some measures are too uncertain to be recommended yet, (4) political consistency must be achieved through effective cross-scale governance mechanisms, (5) scientific effort must focus on effectiveness, co-benefits, disbenefits, and costs of poorly tested as well as new and emerging measures

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Ocean solutions to address climate change and its effects on marine ecosystems

Summary
Seagrass meadows are sites of high rates of carbon sequestration and they potentially support ‘blue carbon’ strategies to mitigate anthropogenic CO2 emissions. Current uncertainties on the fate of carbon stocks following the loss or revegetation of seagrass meadows prevent the deployment of ‘blue carbon’ strategies.
Here, we reconstruct the trajectories of carbon stocks associated with one of the longest monitored seagrass restoration projects globally. We demonstrate that sediment carbon stocks erode following seagrass loss and that revegetation projects effectively restore seagrass carbon sequestration capacity. We combine carbon chronosequences with 210Pb dating of seagrass sediments in a meadow that experienced losses until the end of 1980s and subsequent serial revegetation efforts.
Inventories of excess 210Pb in seagrass sediments revealed that its accumulation, and thus sediments, coincided with the presence of seagrass vegetation. They also showed that the upper sediments eroded in areas that remained devoid of vegetation after seagrass loss. Seagrass revegetation enhanced autochthonous and allochthonous carbon deposition and burial. Carbon burial rates increased with the age of the restored sites, and 18 years after planting, they were similar to that in continuously vegetated meadows (26.4 ± 0.8 gCorg m−2 year−1).
Synthesis. The results presented here demonstrate that loss of seagrass triggers the erosion of historic carbon deposits and that revegetation effectively restores seagrass carbon sequestration capacity. Thus, conservation and restoration of seagrass meadows are effective strategies for climate change mitigation.

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2745.12370

Impact of seagrass loss and subsequent revegetation on carbon sequestration and stocks

In March 2012 we collected 3-replicated sediment cores (9 cm diameter and 12-15 cm long) per restored site along the planting chronosequence (i.e. years 1994, 1997, 2003, 2004, 2006). Similarly, we collected 3 sediment cores in two bare sites, previously colonized by seagrasses, and in a large seagrass patch that survived the disturbances in the second half of the 20th Century that we considered a mature and thus reference meadow. We measured sediment bulk density and organic matter content and carbon stocks along all sediment cores sliced at 1 cm interval. Organic carbon content (% Corg) was estimated from loss of ignition (% LOI) at 550oC for 5 h using the empirically fitted equation for Oyster Harbour sediments,Log % Corg = -0.62 + (1.33 * Log % LOI) 
SEintercept = 0.01, SEslope =0.10, N = 55, R2 = 0.77, P<0.0001
We analyzed the ƌ3C of the organic carbon in the top 3 cm sediment layer (ƌ13Csediment) along the chronosequence to estimate the fraction of seagrass (X) and sestonic (1-X) deposition as 
ƌ13Csediment = [X . Log % Corg = -0.62 + (1.33 * Log % LOI) 
SEintercept = 0.01, SEslope =0.10, N = 55, R2 = 0.77, P<0.0001
We analyzed the ƌ3C of the organic carbon in the top 3 cm sediment layer (ƌ13Csediment) along the chronosequence to estimate the fraction of seagrass (X) and sestonic (1-X) deposition as 
ƌ13Csediment = [X . ƌ13Cseagrass] + [(1-X) . ƌ13Cseston]
being ƌ13Cseagrass -9.65 ‰ and ƌ13Cseston -22 ‰ (Dauby 1989).

Geoff R. Bastyan

Papers

Impact of seagrass loss and subsequent revegetation on carbon sequestration and stocks

Carbon stocks and sources in Oyster Harbor (W Australia) seagrass sediments