The salt marshes, mud flats and eel grass meadows of temperate river estuaries are more effective than young coastal forests at capturing and storing carbon dioxide and may sequester this greenhouse gas for centuries, if not millennia, according to researchers from the University of Victoria (UVic). The amount of carbon sequestered by the Cowichan estuary salt marshes on Vancouver Island is roughly double that of an actively growing 20-year-old Pacific Northwest forest of the same area, reports [a study](https://www.frontiersin.org/articles/10.3389/fmars.2022.857586/full#B28) published in the journal *Frontiers in Marine Science*. The research was supported by UVic, Ocean Networks Canada, 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, preventing it from returning to the atmosphere,” said lead author Tristan Douglas, a UVic graduate student in 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 gigatonne 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 the carbon sink capacity of the 466-hectare Cowichan-Koksilah Estuary has been compromised by industrial and agricultural activity since the area was settled by European colonists. 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. This has reduced its natural capacity to sequester carbon by about 30 per cent, equivalent to putting 53 typical gasoline-powered motor vehicles back on the road.