Wetlands offer us a myriad of benefits, including flood protection, habitat for much-loved wildlife, and beautiful scenery for the public to enjoy. But did you know they also potentially offer significant carbon sequestration services?
Carbon sequestration occurs when carbon-containing greenhouse gases — specifically carbon dioxide (CO2) and methane (CH4) — are removed from the atmosphere and the carbon they contain is stored in the ground or ocean. The carbon may be converted into solid, immobile forms that stay in the soil or in biomass for long periods of time. This carbon is therefore no longer in the atmosphere contributing to the greenhouse effect. If an ecosystem absorbs more carbon than it emits, it is called a net mitigator of climate change. (It is important to note that methane is more than 25 times more potent of a greenhouse gas than carbon dioxide over a century).
Wetlands, with their high biological productivity, could play an important role in sequestering carbon, as we progress towards a future with net-zero or even net-negative carbon emissions. And, if this sequestration can be accurately quantified, it could make wetland restoration projects eligible for financial reimbursement through carbon credits, bringing in additional funds for these efforts. But what impact do wetlands have on a large scale, and how do factors such as water salinity and the plant species palette influence the ability of wetlands to sequester carbon?
Scientists are busy studying these issues through field experiments and computer models. Several local researchers presented their findings in December at the 2015 American Geophysical Union (AGU) Fall Meeting in San Francisco. Here are summaries of three relevant projects:
Water salinity and carbon sequestration
United States Geological Survey (USGS) Biologist Frank Anderson and his colleagues measured the fluxes of methane and carbon dioxide from a brackish marsh in Suisun Marsh, located in the northern part of the San Francisco Estuary. Brackish water is semi-salty, and exists where freshwater from rivers and deltas meets ocean water. The researchers found that saltier waters reduce methane emissions from the wetlands growing there. A class of chemicals present in seawater, called sulfates, is responsible for this inhibition.
However, freshwater wetlands are known to contain a higher density of biomass compared with salty water marshes. More biomass means more carbon dioxide pulled down from the atmosphere to form that biomass.
Therefore, further study is needed to determine if a “sweet spot” exists where the water is salty enough to inhibit methane emissions, but fresh enough to allow for high-biomass species. One factor to consider is the ability of tidal flows to change a marsh’s salinity, and therefore its emissions, on a short-term timescale. Learning more about these issues could help influence where and how to restore wetlands around the Bay for maximum carbon sequestration benefits.
Estimating the impact of restored wetlands
UC Berkeley graduate student Sara Knox and her collaborators tested the validity of using satellite data and photography to measure vegetative productivity in wetlands. Part of the motivation for this work is that in order to facilitate carbon credits for wetland restoration projects, reliable yet affordable ways to quantify ecosystem gas fluxes are needed. Accurate data can currently be obtained from towers installed in wetlands that have wind speed and gas analyzer instruments. However, they can cost twenty- to seventy-thousand dollars apiece, and may not be able to be deployed on the widespread scale needed.
By contrast, taking pictures on-site and analyzing publicly available satellite data is much lower-cost. The researchers took photographs of two restored wetlands in the Sacramento-San Joaquin Delta, and obtained land cover satellite data. They plugged this data into models to estimate biomass creation, also known as gross primary production (GPP). For both methods, their GPP estimates closely correlated with actual figures obtained from towers in the field. Since plants uptake carbon to create biomass, this suggests that relatively inexpensive techniques may be available to estimate wetland carbon sequestration.
Additionally, they found that of the two wetlands, the more recently restored one was actually a net greenhouse gas emitter, while the more established one was beneficial from a greenhouse gas standpoint in two of three study years. This raises the possibility that greenhouse gas emissions from wetlands decrease over time.
Greenhouse gas variability in restored wetlands
UC Berkeley graduate student Gavin McNicol and his collaborators studied the exchange of greenhouse gases at a restored marsh at Sherman Island, located in the Sacramento-San Joaquin Delta. They were interested in how and why emissions of methane, carbon dioxide, and a third greenhouse gas called nitrous oxide vary temporally and spatially in the wetland.
The researchers calculated the emissions coming from both vegetated and open-water areas. Vegetated areas emit gases through pores in plant tissue. Open-water emissions occur through diffusion, where gases move up gradually through soil and water towards the surface, and ebullition, where accumulating sediment gases form bubbles and eventually burst through the soil to the surface. For this analysis, they used water samples and computer models, and also collected data on gas concentrations in the air above the marsh and in trapped gas bubbles.
The researchers found that vegetated areas had a smaller net greenhouse effect stemming from their emissions than did open-water areas. This is because vegetated areas’ higher uptake of carbon dioxide counteracted their higher methane emissions, whereas open-water areas mainly released carbon dioxide. They also found significant seasonal variation in emissions, suggesting that changing biological productivity, temperature, and other factors make wetlands a dynamic and variable player when it comes to mitigating climate change.
These results could help inform how much vegetated versus open-water area restored wetlands should aim for. They also provide insight into what factors control the underlying chemical reactions which create and consume greenhouse gases in wetlands.
Save The Bay works to restore wetland habitats around the Bay, increasing our regional ecosystems’ ability to mitigate climate change and its impacts. We rely on volunteers to make our projects possible.