If you're a coastal saltmarsh, your live plants remove carbon dioxide from the air through photosynthesis. Your dead bits get stored underground, keeping a whole lot of CO2 captured and locked safely away from the atmosphere. You're also a bit nervous that sea level rise will spell your doom. Well, fear not. Recent work by a team from Duke University, lead by PhD candidate Katherine Ratliff, with Dr. Marco Marani and PhD candidate Anna Braswell, was published in Proceedings of the National Academy of Sciences and has some potential good news for you. I spoke with Katherine and Dr. Marani.
(Dr. Marco Marani)The work that Katherine led had to do with, what is the effect of increased CO2 on the ability of marshes to persist in the face of sea level rise? Because these are very low environments, they are very exposed to changes in the rate at which sea level changes. An increased CO2 concentration causes a greater carbon uptake by marshes, and this translates into a faster rate of organic soil production, which allows the marsh to grow in elevation more quickly. (Katherine Ratliff)These increased levels of CO2 are going to cause greater amounts of photosynthesis and also increased water use efficiency in a whole variety of plants. Not only does it increase below-ground biomass and inorganic production, but it also increases the leaves and the shoots and all the things that you see above ground from these plants.
The increase in plant production with carbon dioxide is called CO2 fertilization.
(Katherine Ratliff) As we increase the rates of CO2 in the atmosphere, that will, in turn, increase the rates of photosynthesis and plant development and growth in the marshes, which then causes greater amounts of carbon storage. So, there is this stabilizing feedback that we see with the increased rates of CO2 in the atmosphere and the amounts of carbon storage in the marshes themselves.
CO2 fertilization causes more plants to grow in the marsh. More live plants lead to more dead plants, which leads to more soil, and a rise in marsh elevation. It also leads to a greater capacity for carbon storage. So, will this stabilizing feedback save us from global warming?
(Katherine Ratliff) Unfortunately, I don't think it is necessarily a catchall and going to be the thing that saves us. Obviously, CO2 in the atmosphere is something that is distributed across the globe, so marshes and different terrestrial landforms all across the globe are going to be experiencing this effect, so it is certainly an important thing that we need to keep in mind, especially as there is a greater push to restore marshes and manage marshes and use all the ecosystem services that marshes provide.
The increase in biomass will help capture sediment from rivers, adding to the overall system. But with this comes a load of nutrients, which are increased by human activity.
(Dr. Marco Marani) We've realized that we don't know how marshes are responding to highly changed levels of nutrients in coastal waters. How will the nutrient effects, combined with CO2 fertilization, combined with enhanced rates of sea level rise determine the overall response of these important coastal systems? (Katherine Ratliff) There's been some talk of marshes being so threatened that it's not even worth putting in the time to try and restore them, or in various areas that sealed rise rates are just so high that they're going to just overwhelm marshes in certain locations. I think that our work goes to show that there is still some hope for some of these marshes, and that the increased CO2 that we tend to think of as such a negative thing for us, and it is for sure, but there is a positive effect for the marshes that we haven't been taking into account.