What is Peat and Why is it Collapsing?

Written by Benjamin J. Wilson

For a background refresher on the impacts of sea level rise in South Florida, see my previous blog post.

Figure 1. Feedbacks that lead to the creation of wetland peat soil.

Peat soil is the backbone of many wetlands. It is full of organic-rich carbon that is formed when plants perform photosynthesis: they suck up carbon dioxide (CO₂) from the atmosphere and use it to make roots. These roots go into the soil, and because decomposition is slow in wetlands, these carbon-rich roots are stored in the soil for very long periods of time, allowing the marsh to grow in elevation (Fig. 1).

Under freshwater conditions, this carbon can be stored in the soils for thousands of years. This leads to a landscape full of pristine marshes that provide habitat for wildlife and are very efficient at filtering contaminants out of water (Fig. 2a). However, in some brackish marshes, especially in the Everglades, an exciting feature appears. From above, you can see the landscape is dotted with open-water ponds (Fig. 2b). I kept thinking, what could be driving this pattern? So, I decided to get out there and see it for myself.

Figure 2a Healthy marsh from above                                     Figure 2b Degraded marsh from above

 

Figure 3. "Peat collapse" at a brackish marsh in the Everglades.

When I got out there to see it for myself, I came across an astonishing sight! At ground level, something bizarre was happening to the plants. I saw that a lot of the sawgrass plants, the dominant vegetation for much of the Everglades, had their roots exposed! (Fig. 3) Plants usually grow with their roots IN the soil, not OUT of. Something weird was going on here. The peat soil was collapsing around healthy-looking plants! It was at that exact moment I knew what my dissertation research had to be about: what is causing peat collapse in the coastal Everglades?

Our hypotheses of why peat collapse was occurring revolved around saltwater intrusion (Fig. 4). As saltwater intrudes into freshwater wetlands, a series of events happen that causes plants to become stressed and soil biogeochemical processes to change. This “domino effect” of change leads to peat collapse, which converts the marsh into open water ponds, like those seen in Fig. 2b.

Figure 4. Our hypothesis for peat collapse. Borrowed from S. Davis and the Everglades Foundation

Figure 5. My hypotheses, but more in-depth to reflect changes
 in carbon cycling

Specifically, I think that saltwater intrusion was changing the cycling of carbon through the marsh. As plants become stressed, they cannot perform photosynthesis effectively, therefore leading to fewer roots being produced (Fig. 5). Because peat soils are composed mostly of roots (as much as 80-90%!), if you lose this input, the structural integrity of the soil may become compromised, leading to collapse.

But these are only my hypotheses. I tested these using both field-based and mesocosm experimental studies. Stay tuned for blog posts about each of my chapters and what I found.