Accelerating Rate Of Sea Level Rise And The Fate Of Coastal Ecosystems And Urbanized Areas

Naples, Florida coast (Credit: Pixabay)

During the past 3000 years (called the Late Holocene) the rate of sea level rise was less than 1 mm y-1 and coastal ecosystems such as barrier islands and coastal wetlands were stable, that is shorelines remained in the same location because sediment accumulation kept up with sea level rise. During the last century, the global rate of sea level rise accelerated to 3.4 mm yr-1 and our regional rate of 9 mm yr-1 in our study area in South Florida. This acceleration is the direct result of increased atmospheric CO2 associated with anthropogenic sources, increased global temperature and increased ice melt.

With the rate of sea level rise three or more times greater than during the Late Holocene shorelines are no longer stable. The resultant instability is creating changes in most coastal ecosystems in the form of coastal erosion, wetland retreat in response to salt water encroachment and inundation because of sediment accumulation deficit, a situation where wetland sediment accumulation is less than the rate of sea level rise. Our, a team of Florida International University scientists, study of how the SESE responded to historic changes in sea level rise provides insight on the effects future changes in sea level rise will incur.

In the Southeast Saline Everglades (SESE) we document: salt water encroachment and associated mangrove retreat (or landward migration) at the expense of interior fresh water marshes, overstep a situation where all communities cannot retreat at the same rate resulting in one community replacing another, and retreating coastline along exposed coastlines of Biscayne Bay. We found that the rate of salt water encroachment differed by a factor of 15 in the five coastal watersheds studied. These differences in encroachment rates were found to be related to Everglades water delivery changes. Salt water encroachment was lowest in the watershed receiving the most Everglades water, Taylor Slough. This suggests that increasing Everglades water delivery might prevent further or reverse salt water encroachment. However, there is 70 % less Everglades water available than historically because of consumptive uses and diversion. Therefore sufficient water to mediate salt water encroachment may not exist.

A marine transgressive stratigraphic sequence has developed as marine mangrove sediments develop over fresh water marl and peat with salt water encroachment because tides transport mangrove propagules into historic fresh water areas. We have termed this transgression the Anthropocene Marine Transgression because the transgression coincides with the increased rate of sea level rise during the last century in response to increased atmospheric CO2 resulting from anthropogenic activities such as burning fossil fuels.

We suggest that coastal wetlands with their biogenic sediments in this oligotrophic, micro-tidal, extremely low relief coast with no allochthonous sediment supply have reached their resilience threshold. Therefore, we expect coastal wetland loss to increase with continued salt water encroachment, overstep and inundation because sediment accumulation can no longer keep up with sea level rise. Loss of coastal ecosystems will cause major changes in coastal oceans and on the mainland because of lost storm protection, changing OC storage, decreased fresh water storage in the surficial aquifer and infringement into agricultural and urban areas. Such actions are already reported from Miami Beach flooding and sewers backing up to more frequent flooding of areas in Florida City and loss of fresh water storage.

These findings are described in the article entitled Salt water encroachment and prediction of future ecosystem response to the Anthropocene Marine Transgression, Southeast Saline Everglades, Florida, published in the journal Hydrobiologia. This work was led by John F. Meeder from Florida International University.

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