Presented by Judith A. Rosentreter (Postdoctoral Researcher Centre for Coastal Biogeochemistry Southern Cross University, Lismore, Australia) on 25 September 2019 at Blue Carbon Regional Workshop, Merida, Yucatan.

1. METHANE in
COASTAL BLUE CARBON
ECOSYSTEMS
Judith A. Rosentreter
Postdoctoral Researcher
Centre for Coastal Biogeochemistry
Southern Cross University, Lismore,
Australia
judith.rosentreter@scu.edu.au

2. BLUE CARBON
Greenhouse gas concentrations
Climate change
Impacts
Responses
Mitigation
reduce emissions enhance sinks
forest restoration
natural carbon stores
Howard et al. 2017_Front.Ecol. Environ.
METHANE?
CH4

3. Methane has a global warming potential (GWP) 34 to 86 times more powerful when looking at
the 100 year and 20 year time horizons, respectively (including carbon-climate feedbacks)
Atmospheric concentrations have tripled since pre-industrial
times with a steady increase since 1984
Little or no growth between 2000-2006 “stabilization period”
New rapid growth phase “re-newed growth” → was not expected
in future greenhouse gas scenarios compliant with the targets of
the Paris Agreement
Urgent need to reduce methane emissions!
2000 2004 2008 2012 2014
strong rise since 2006
METHANE - the second most important greenhouse gas
Myhre et al. 2013/IPCC2013 Saunois et al. 2016_Earth Syst. Sci. Data
Nisbet et al.2016_Global Biogeochem. Cycles

4. Anaerobic Methane Production
is the terminal step in organic matter mineralization by methanogenic archaea, in marine
sediments depleted of electron acceptors for microbial respiration → anoxic sediments
METHANE – Production and Oxidation
Diffusion Ebullition
Three Major Pathways:
1) hydrogenotrophic methanogenesis using CO2 and H2,
2) acetoclastic methanogenesis using acetate (CH3COOH)
3) methylotrophic methanogenesis using methylated compounds such as methanol,
methylamines or methylsulfides (dimethyl sulfide (DMS), dimethylsulfoniopropanate (DMSP))
Modified from Carmichael et al. 2014_Biogeochemistry Thauer 2011_Curr.Opin.Microbiol.
Methane Oxidation = Methane Consumption
by methanotrophic bacteria/archaea under aerobic or anaerobic conditions
Aerobic Methane Production
from plants/plants biomass (whole plant, parts of
plants, root, stem, leaf)
Uncertainties exists due to the fundamental lack of
understanding mechanistic pathways involved in
aerobic methane production
Carmichael et al. 2014_Biogeochemistry

5. CO2 recycled
Export of
POC, DOC, DIC
Carbon Burial
Atmospheric CO2 uptake
CH4 produced
Wood production
Root production
Litter fall
Net Primary Production
CARBON CYCLING IN MANGROVES
Offsets?
Exports?

6. METHANE EMISSIONS FROM MANGROVE ECOSYSTEMS
METHANE ??
time (24 hours)
Methaneconcentration(µM)
Tidalheight(m)
Example: Burdekin Mangrove Time Series
Methane emissions from Australian
mangroves combined with other studies
Sediment-air fluxes (low tide)
Water-air fluxes (high tide)
Global Mangrove Methane Emissions
0 – 1.7 Tg CH4 yr-1
Average 0.3 Tg CH4 yr-1
“Tidal pumping”, porewater exchange
High and low salinities
Bunting et al. 2018_Remote Sens.
Rosentreter et al. 2019_Science Advances

7. MANGROVE CARBON BURIAL OFFSETS
Methane emissions have the potential to
offset BLUE CARBON burial in
mangrove sediments on average by 20%
Rosentreter et al. 2018_Science Advances

8. METHANE PATHWAYS - from sediments to the atmosphere
Freshwater Wetlands Different Methane Pathways in Coastal Blue Carbon Ecosystems?
Tom Rayner/www.shutterstock.com
Global Seagrass Methane Emissions
0.09 – 2.7 Tg CH4 yr-1
Garcias-Bonet & Duarte 2017_Front. Mar. Sci.
Diffusion
Ebullition
Plant-mediated
Interface
Sediment-water
Sediment-air
Water-air
Tidal control
“Tidal pumping”
Seasonal variability – wet and dry season
Salinity gradient – low and high salinity
Need for further
studies!
Jeffrey et al. 2019a_ Limnol. Oceanogr

9. The Future of Coastal Methane?
Mangrove tree-stem emission may
account for 26% of the net ecosystem
CH4 flux
1) Trees were conduits for sed CH4
to the atmosphere
2) Dead trees were a source of CH4
3) Living trees oxidize sed CH4
therefore have lower fluxes
High methane fluxes from shrimp ponds
Yang et al. 2018_Sci. Total Environ.Jeffrey et al. 2019b_New Phytol.
Ottinger et al. 2017_Remote Sens.
Conversion from coastal habitats to aquaculture ponds
Major source of GHGs
Highest fluxes during the middle
stage of aquaculture
Yang et al. 2017_Sci. Total Environ.
10x higher
8x higher

10. Management Recommendations:
- Restoration of coastal wetlands to enhance carbon burial and reduce GHG emissions
- Tidal restoration can significantly reduce GHG emissions: removal or opening of dikes and tide gates
to renew tidal water exchange between coastal wetland and coastal ocean, thus restore natural
water level and salinity (Kroeger et al. 2017_Sci. Report)
- Salinities thresholds of 10-15‰ represent an important tipping point for biogeochemical processes in
wetlands (Wang et al. 2017_Wetlands)
- Important drivers: salinity, water table elevation, temperature, plant productivity
Challenges
- Large uncertainties remain with regard to latitudinal distribution, climatic and climate change drivers,
fluxes through different pathways (diffusion, ebullition, plant-mediated) in coastal wetlands
For further questions please contact: judith.rosentreter@scu.edu.au
- Although atmospheric concentrations are ~200 times lower than CO2, methane is a significant greenhouse gas with a global
warming portential 34-86 times more powerful than CO2, depending on the time horizon
- Methane has the potential to offset some of the carbon buried in Blue Carbon ecosystems
- Methane emissions will likely increase as a result from wetland loss, coastal wetland use (thinning, harvesting of trees), and
land use changes (rewetting, dredging, aquaculture)
SUMMARY
UN Environmental Protection Agency (EPA) estimated methane emission offset
carbon removal from the atmosphere by 30% (carbon burial 12.2 Mt CO2e per
year, methane emissions 3.5 Mt CO2e yr-1) over the 1990-2016 Inventroy period
(Crooks et al. 2018_Nature Climate Change)
Kroeger et al. 2017_Sci. Report

11. REFERENCES
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methane budget? Biogeochemistry 119: 1–24.
Crooks, S., A. E. Sutton-Grier, T. G. Troxler, N. Herold, B. Bernal, L. Schile-Beers, and T. Wirth. 2018. Coastal wetland management as a contribution to the US National Greenhouse Gas Inventory. Nat.
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Garcias-Bonet, N., and C. M. Duarte. 2017. Methane Production by Seagrass Ecosystems in the Red Sea. Front. Mar. Sci. 4: 340. doi:10.3389/fmars.2017.00340
Howard, J., A. Sutton-Grier, D. Herr, J. Kleypas, E. Landis, E. Mcleod, E. Pidgeon, and S. Simpson. 2017. Clarifying the role of coastal and marine systems in climate mitigation. Front. Ecol. Environ. 15:
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Jeffrey, L. C., D. T. Maher, S. G. Johnston, B. P. Kelaher, A. Steven, and D. R. Tait. 2019a. Wetland methane emissions dominated by plant-mediated fluxes: Contrasting emissions pathways and
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