How to make coastal habitats sequester even more carbon

Managing nutrient pollution, bioturbation and freshwater flows to increase blue carbon; Optimizing mangroves

by | May 3, 2017  Anthropocene

Of all the carbon buried in the floors of Earth’s oceans, most of it is found in the narrow strip of tidal marshes, seagrass beds, and mangroves along their edge. Known as blue carbon ecosystems, these vegetated coastal habitats “occupy only 0.2% of the ocean surface, yet contribute 50% of the total amount of carbon buried in marine sediments,” write researchers, led by Deakin University ecologist Peter Macreadie, in the journal Frontiers in Ecology in the Environment. Meter for meter, they’re some of the most effective carbon storage systems we have. But could people make them even more effective?

Macreadie and colleagues pose that question, noting that present attentions are focused mostly on protection and restoration—which are unquestionably important, as roughly half of all vegetated coastal habitats have been lost to development, sea level rise, and extreme weather events. But even more is possible. Optimizing existing blue carbon ecosystems offers “the potential to profoundly alter carbon accumulation and retention,” write the researchers, “providing new and previously undervalued strategies for mitigating climate change.”

The researchers focus on three key environmental processes that control carbon breakdown and sequestration in these habitats. The first is nutrient pollution, especially nitrogen and phosphorus released from agricultural fertilizers and sewage, which leads to microbe and algae population shifts that reduce the amount of carbon stored by ecosystems….

The second process is bioturbation, or the disturbance of sediments by shrimp, worms, crabs, and other organisms. Bioturbation sets off a cascade of other processes, and when bioturbators are present at relatively low densities, they enhance carbon storage. But at high densities, the opposite is true….

Lastly the researchers discuss freshwater flows around the estuaries and coastal rivers where most vegetated coastal habitats are found. When unimpeded, these flows deliver carbon-rich inland sediments that are quickly buried by successive deposits, allowing sediment to rapidly lock up carbon.….

Macreadie et al. “Can we manage coastal ecosystems to sequester more blue carbon?Frontiers in Ecology and the Environment. 2017.

ABSTRACT: To promote the sequestration of blue carbon, resource managers rely on best-management practices that have historically included protecting and restoring vegetated coastal habitats (seagrasses, tidal marshes, and mangroves), but are now beginning to incorporate catchment-level approaches. Drawing upon knowledge from a broad range of environmental variables that influence blue carbon sequestration, including warming, carbon dioxide levels, water depth, nutrients, runoff, bioturbation, physical disturbances, and tidal exchange, we discuss three potential management strategies that hold promise for optimizing coastal blue carbon sequestration: (1) reducing anthropogenic nutrient inputs, (2) reinstating top-down control of bioturbator populations, and (3) restoring hydrology. By means of case studies, we explore how these three strategies can minimize blue carbon losses and maximize gains. A key research priority is to more accurately quantify the impacts of these strategies on atmospheric greenhouse-gas emissions in different settings at landscape scales.