Article: Atwood, Trisha B., Rod M. Connolly, Euan G. Ritchie, Catherine E. Lovelock, Michael R. Heithaus, Graeme C. Hays, James W. Fourqurean, and Peter I. Macreadie. “Predators help protect carbon stocks in blue carbon ecosystems.” Nature Climate Change 5, no. 12 (2015): 1038-1045. DOI: 10.1038/NCLIMATE2763
What is blue carbon?
Blue carbon is a hot topic right now! You may even remember this recent post by Rebecca Flynn last month.
So what is blue carbon and why is it so buzz worthy?
Blue carbon is the umbrella term for carbon that is stored in vegetated coastal marine habitats such as salt marshes, seagrass beds, and mangrove forests. These coastal ecosystems are natural carbon storage bins which could help reduce the high atmospheric concentration of carbon dioxide.
Carbon dioxide is one of the main culprits responsible for climate change, so it is important to keep any place where carbon can be trapped and stored (called sequestration) healthy. Although coastal habitats make up a small fraction of the globe (Figure 1), they can uptake carbon as much as 40 times faster than rainforests and could even represent half of the carbon stored in the ocean.
Unfortunately, blue carbon ecosystems have declined by 25 to 50% over the last 50 years, which is bad news for carbon storage. This perspective brings to light another problem: the loss of predators.
Top Down: a trophic cascade
While many factors, such as primary production, can affect carbon storage capacity, new research suggests that changes in predator populations could have effects which cascade all the way down the food web to blue carbon ecosystems.
This is known in ecology as a trophic cascade.
Let’s use an example to illustrate this effect (Figure 2). In salt marshes, predatory crabs often control the abundance of snails that love to chomp on marsh grasses. So when blue crab populations decline, say from overfishing, snail populations can greatly increase. Now that this marsh is over-run by marsh grass eating snails, the amount of marsh grass and carbon storage might be reduced.
But the trophic cascade process is complicated since each habitat can have different pressures and food webs. This makes it challenging to understand how predator changes will affect carbon storage globally. For example, population reductions in the American alligator have increased blue crab abundances in some coastal ecosystems, which have actually increased the mangrove and salt marsh carbon storage capacity.
So, predators might indirectly affect carbon storage…and bad news…global predator populations could be reduced by 90% in the near future. For example, blue crab populations along the Eastern United States coast have already declined by 40-80%. Yikes!
One thing is clear: we need to know more!
On the case: indirect predator effects
The conventional thought is that carbon storage in coastal ecosystems is mostly controlled by the primary producers. However, this perspective by Atwood et al. brings to light the concept that predators should be considered as well!
Another example to illustrate this concept involves the sediments where carbon is stored on millennia timescales. The sediments in blue carbon ecosystems are often anoxic (no oxygen) which makes decomposition incredibly slow. Translation: long-term carbon storage.
Enter the bioturbators. Bioturbators are organisms such as worms and crabs that burrow and stir up sediments. This bioturbation process adds oxygen to the sediments, speeding up decomposition. In Cape Cod, over-fishing of predatory crabs and fish have led to large population increases of a herbivorous and bioturbing crab. This cascade has increased bioturbation, which increased the sedimentary oxygen concentration, which increased respiration, which ultimately decreased the carbon storage ability of this system.
The Carbon Market?
Blue carbon has also gained interest since it can have a monetary value.
In the United States, some business have joined the voluntary carbon market. Simply speaking, each business is allowed to “spend” a certain amount of carbon via emissions. If they need more, they must “buy” carbon credits. However, things like a salt marsh which would remove some carbon, can be used as an offset, allowing you to have a bigger carbon budget. This is a great incentive to preserve blue carbon environments.
On the current market, the social value of emitting one ton of carbon is $41. So let’s paint an example using some conservative estimates. If mangroves were globally reduced by 20%, and we assume that at least 10% of all mangroves are affected by predator losses, then we will lose ~9.5 million tonnes of CO2 sequestration per year. This translates to $390 million on the carbon market.
As carbon dioxide emissions continue to increase, the ability of blue carbon ecosystems to trap and store carbon is important. Healthy salt marshes, seagrass beds, and mangroves can sequester carbon for long time scales faster than most terrestrial ecosystems. Additionally, when we lose blue carbon habitats, not only do we reduce carbon storage capacity, but that trapped carbon is released back into the environment.
So what can we do? For starts, we need to conserve and protect these coastal ecosystems. This perspective uncovered another pressure that needs attention: protecting predators. With more research and better management and conservation practices, blue carbon can help us reduce our carbon footprint.
I received a Ph.D. in oceanography in 2014 from the Graduate School of Oceanography (URI) and am finishing up a post-doc at the University of Maryland Center for Environmental Science (Horn Point Laboratory). I am now the Research Coordinator for the Delaware National Estuarine Research Reserve.
Carbon is my favorite element and my past times include cooking new vegetarian foods, running, and dressing up my cat!