Sink, Swim, or Get out of the Way: The Fate of Mangrove Forests.

Mangrove forests provide a range of valuable ecosystem services including sequestering organic carbon (OC) in their soils at rates greater than other forests on a per area basis. One of the fundamental mangrove-related climate change questions is whether their soils will continue to function as a globally significant OC sink. While changes to precipitation, temperature, cyclone activity, etc. may influence this sink capacity, the capability to keep pace with sea-level rise (SLR) is perhaps of greatest immediate concern.  Mangroves that receive minimal terrigenous sediments (such as those in South Florida) are largely dependent on the rate of OC accumulation as a key contributor to accretion.  In addition to the threat of SLR simply outpacing current accretion rates, there is the potential for this to be exacerbated in some areas by accelerating soil OC mineralization due to increasing salinity.  The increase in salinity supplies sulfate which functions as a terminal electron acceptor that soil microbes can utilize to enhance mineralization in the brackish ecotone regions of coastal wetlands.  To investigate these processes, we measured mangrove forest soil accretion and OC burial rates over the last 100 years (via ²¹⁰Pb dating) from the Gulf of Mexico to the upper freshwater reaches of the mangrove forest within Everglades National Park.  We compared accretion rates with the sea level tide gauge record at Key West, FL over the most recent 10, 50 and 100 year periods. Results indicate that accretion rates match (within error) the relatively modest average SLR over the 50 and 100-year periods for most of the system, but have not kept pace with the substantially higher SLR in the last decade.  The few locations that are not keeping pace with SLR over any of the three time scales also have substantially lower OC burial rates than the rest of the system and are located in the ecotone region most susceptible to enhanced OC mineralization.