Estuaries’ vast potential for climate mitigation

The salt marshes, mud flats and eel grass meadows of temperate river estuaries are more effective at capturing and storing greenhouse gases than young coastal forests and may sequester carbon for centuries, if not millennia, according to researchers from the University of Victoria (UVic).

The amount of carbon captured and stored, known as sequestered, by the Cowichan estuary salt marsh on Vancouver Island is roughly double that of an actively growing 20-year-old Pacific Northwest forest of the same area, reports a new study published in the journal Frontiers in Marine Science. The research was supported by UVic, Ocean Networks Canada (ONC)—and the Cowichan Estuary Restoration and Conservation Association.

So-called blue carbon—carbon dioxide (CO2) captured from the atmosphere by marine plants and algae—collects as organic debris in estuary sediments where low-oxygen conditions prevent their decomposition.

“Oxygen is depleted very quickly from the surface of the sediment due to aerobic microbial processes. This prevents buried organic matter from being remineralized back into CO2 and returning to the atmosphere,” says lead author Tristan Douglas, a 2021 UVic graduate from the School of Earth and Ocean Sciences, who spent two years analyzing the physical and chemical properties of sediment cores collected from the Cowichan estuary.

That makes undisturbed estuaries a potent passive carbon storage system with the global potential to capture and store greenhouse gas (GHG) emissions at the gigatons scale. Intertidal ecosystems—especially those in the tropics—can be 20 to 60 times more effective than forests at capturing and storing carbon dioxide. However, compromised estuaries can and do release carbon on a similar scale, the authors warn.

Plant species like salt marsh grasses and sedges, mangrove forests and seagrasses are particularly efficient natural carbon sinks. They capture and store up to 70 per cent of the organic carbon resident in marine systems despite only occupying 0.2 per cent of the ocean surface.

The report shows that human activities have reduced the carbon capture and storage capacity of the 466-hectare Cowichan-Koksilah Estuary by about 30 per cent, equivalent to putting 53 gasoline-powered motor vehicles back on the road. Eel grass on about 129 hectares of the intertidal zone has been disturbed by log handling and storage, while about 100 hectares of salt marsh was drained for farming and cattle pasture since being settled by European colonists.

The researchers say policymakers should take note of the triple impact of undisturbed and restored estuaries. Wetlands act as a buffer against flood waters in the spring and as nurseries for aquatic species from crabs to salmon, in addition to reducing atmospheric GHG emissions.

We need to be looking at these kinds of blue carbon nature-based solutions to mitigate the impact of CO2 emissions on our climate.

- Co-author and ONC Chief Scientist and UVic Professor Kim Juniper.

The authors say restoration of the Cowichan-Koksilah Estuary’s carbon sequestration capacity can be pursued on two fronts. Preventing further disturbance by industrial activity and replanting eel grass could reap benefits in just a few years. Opening the dikes that prevent ocean water from flooding sea-level pastures would allow salt marshes to re-establish naturally and improve carbon sequestration capacity over decades and centuries.

This research benefitted from Indigenous knowledge of the pre-colonial condition of the estuary, based on interviews with local Elders in the report Proceedings of the Cowichan Tribes Estuary Workshop (2010).

This research was supported by the Cowichan Valley Regional District, the Cowichan Estuary Restoration and Conservation Association, Fisheries and Oceans Canada and ONC, an initiative of UVic.

Read the study here: Blue Carbon storage in a northern temperate estuary subject to habitat loss and chronic habitat disturbance: Cowichan Estuary, British Columbia, Canada

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