Healthy wetlands could help stave off rising seas

Sea levels are rising, so what happens if the ground starts sinking?
 
A team of researchers led by FIU scientist Sean Charles set out to determine if and how saltwater intrusion can cause coastal wetlands to sink. Their findings demonstrate local actions can play a large role in the resilience of ecosystems to climate change.

When the researchers subjected coastal sawgrass marshes to elevated saltwater, they lost 2.8 centimeters of elevation in one year — almost 10 times the rate of sea level rise. Coastal sawgrass marshes without added saltwater maintained or increased elevation during the same time period. Due to their position in the landscape, coastal wetland survival is determined by the balance between changes in sea level and soil elevation. Therefore, the rapid elevation loss identified in this study could create a grim future for the Everglades and other coastal wetlands.

Coastal wetland plants remove carbon dioxide from the atmosphere through photosynthesis and over  time plant-derived carbon builds up in soils. This helps maintain and even increase soil elevation. However, freshwater and brackish coastal wetlands are experiencing saltwater intrusion in the Florida Everglades and globally due to both local (reduced freshwater flow from water extraction and diversion) and global (sea level rise) causes. The team showed  saltwater in Everglades sawgrass marshes can result in plant root die-off and the loss of soil carbon, leading to peat collapse (the rapid loss of soil elevation).

“The good news is that restoring freshwater flow to the Everglades as planned in the Comprehensive Everglades Restoration Plan will reduce saltwater intrusion and help maintain elevation. That would buy us a lot of time,” Charles said. “We all need to support Everglades restoration.”

The research team cut out large sections of peat soil with plants on top from an Everglades marsh and brought them to an outdoor experimental facility where they added elevated saltwater to one group but not the other. Aboveground, wetland plants treated with added saltwater seemed fine, but below the soil the scientists found dying roots. Since roots increase soil volume and stabilize sediments, their loss means a loss in elevation. The scientists were surprised to find a staggering 2.8-centimeter drop in elevation — or the equivalent of nearly a decade of sea level rise.

Aside from storing carbon, coastal wetlands filter water, provide crucial habitats for many species and protect coasts from flooding and storm damage. It’s in everyone’s best interests if they stay above water, the researchers say.

“Peat collapse not only leads to land loss, it makes our coastline more vulnerable to storm surge and saltwater intrusion,” said Stephen Davis, senior ecologist for the Everglades Foundation and co-author of the study. “It also represents an internal source of nitrogen and phosphorous pollution to coastal waters like Florida Bay.”

In areas with normal freshwater conditions, the scientists observed an actual increase in soil elevation that was rapid enough to outpace the current rate of sea level rise. This proves, under the right conditions, coastal marshes can be resilient.

The takeaway for the researchers is restoring freshwater makes for healthy plants. Healthy plants store carbon. Organic carbon helps maintain or even increase soil elevation. And the higher the soil elevation, the greater chance coastal wetlands will have to stave off rising seas.

“If you don’t improve water quality, you lose carbon. Storing carbon in wetland soils removes it from the atmosphere and helps wetlands survive,” Charles said. “Coastal wetlands face an uncertain future, but by providing conditions that promote carbon storage and elevation gain, we can give them a chance.”

The research, which was published in Estuaries and Coasts, is a collaboration with the Everglades Foundation, the South Florida Water Management District and Everglades National Park. The research is part of the Florida Coastal Everglades Long Term Ecological Research, a program funded by the National Science Foundation.