Impacts of Sea Level Rise on the Everglades Mangrove Forest

Mangroves are highly productive ecosystems that serve as an estuarine habitat for a multitude of fish, bird, mammal and other species in Everglades National Park (1). The three species of mangrove that are common to intertidal zones of South Florida, the Caribbean, and the Gulf of Mexico are black mangrove (Avicennia germinans), white mangrove (Laguncularia racemosa), and red mangrove (Rhizophora mangle) (2). All three species are present in the Everglades National Park mangrove forest, which is one of the largest continuous mangrove forests in United States (10).

Mangrove forests help reduce the impacts of coastal erosion and storm surge from extreme weather events such as hurricanes and tsunamis (17). They are biogeochemically important to protection of other Everglades ecosystems and to the surrounding region due to their capacity to accumulate sediment, nutrients and contaminants and their ability to capture and store carbon (24). Mangroves serve as critical breeding habitat and nursery grounds for numerous crustaceans, fish, birds, reptiles (1, 14).

Multiple studies have indicated that sea level rise and warming waters during the late Holocene (period of last 11,700 years, including the present) have effectively drowned mangroves living directly on the shoreline (1, 6, 8, 12, 13, 20). The same studies also found that mangrove forests have expanded in size and migrated inland, with other types of estuarine wetland covering areas that were previously inhabited by mangroves (1, 2, 3, 6, 7, 10, 14, 15, 18, 20, 23, 24, 28). This was clearly shown in a time series Landsat study of Everglades National Park from 1985-2017 where there was a total cumulative increase of mangrove coverage of 10.2%, with coastal decreases in mangrove coverage in years of hurricanes (10).

Since mangroves are halophytes, they are able to tolerate changes in salinity and tidal stress, making them more readily able to adapt to changing habitat conditions caused by sea level rise and salt water intrusion (2). Mangroves have been observed to be able to build up peat soil with some levels of sea level rise, managing additional saltwater intrusion for a time; as sea level rise accelerates, salt water overwhelms both the mangroves and the ecosystems they protect (22, 1). As sea levels continue to rise over time, saturation occurs and salinity conditions increase in coastal areas, resulting in mangroves to migrate inland into areas of the Everglades that were previously freshwater marsh ecosystems and other types of wetlands habitats.

The recent Comprehensive Everglades Restoration Plan notes that the Everglades are especially vulnerable to sea level rise since its flat landscape is only about a 5 cm increase in elevation per linear kilometer inland (23). With this topography and changes in salinity that occur with sea level rise, different types of wetland habitat, such as sawgrass marsh, have been observed to migrate seaward as mangroves have migrated inland (7, 23, 27). It is estimated that the mangrove forest within the Everglades has moved inland at least 3 km on average due to sea level rise since the 1940s (23). Studies that modeled future sea level rise impacts using GIS data showed that under continued sea level rise conditions, freshwater marsh habitat will continue to decrease by 20-30% and mangroves will migrate approximately 15 km inland (6).

Research provides evidence for the overall resilience of tidal wetlands that can adapt with saline fluctuations in response to sea level rise, including mangrove forests, and the resulting landward migration of mangroves may be how tidal wetland habitats are adapting in order to continue to persist in the Everglades (3). Studies further indicate that other variables such as hurricanes, temperature, increased atmospheric CO2 and precipitation have exacerbated the impacts of mangrove migration due to sea level rise (1, 10, 13, 14, 23). Mangrove forests and other wetland ecosystems of the Everglades National Park have also been impacted by other external stressors such as hydrological modifications for flood control and agricultural runoff impacts. (11, 22)

An in-depth literature review was conducted to examine the impacts of sea level rise on Everglades National Park, Southern Florida, and other areas with similar topographic and climate conditions with large mangrove forests. To analyze the impacts of sea level rise on the mangrove forest of Everglades National Park over time, Landsat 5 datasets from 2006 and Landsat 7 datasets from 2011 were downloaded from from USGS Earth Explorer and clipped to the Everglades National Park boundary (25, 26, 5). Landsat data from 2006 and 2011 were selected for comparison due to availability of data with similar seasonality to limit variability in precipitation and vegetation health. Landsat data and NOAA Sea Level Rise Viewer data were used to perform various analyses in ArcGIS Pro 2.7 to evaluate migration and health of mangrove habitat over time in comparison to sea level rise, including:

• Vegetation Analysis Combination Banding
• Unsupervised and Supervised Classifications
• NDVI
• NPS Mangrove Habitat layer comparison (4)
• Raster Calculator to Generate Supervised Classification and NDVI Difference Images
• Reclassification of Difference Images to Generate Change/No Change Images

Differences between land use classes were calculated using image attribute tables and the raster calculator to visualize the difference in a change/no change image. The areas in red show land cover that changed in class between 2006-2011, and areas in yellow show land cover that had no change.

To compare the change of land cover with the overall change mangrove habitat over time, the current mangrove habitat layer from Esri was added (in green with 50% transparency) to the supervised classification change/no change layer. Though some of the areas near the coast overlap with areas of no change (yellow), many of the areas that are inland overlap with areas of change (red).

To better interpret the calculated difference between the 2006 and 2011 NDVI analyses, the NDVI change image was reclassified to show only change or no change in vegetation health. There was a significant amount of change in vegetation health across the entire Everglades area, including the mangrove habitat area.

When comparing the current mangrove habitat layer (50% transparency in green) to the NDVI change/no change image, it appears that current mangrove habitat is located in areas that did not experience a change in vegetation health.

Maps were created in ArcGIS Pro to compare and contrast results.

Similar to the results presented in previous research, results from this study indicate that sea level rise impacts the mangrove forest within the Everglades National Park. According to NOAA sea level rise data, areas of mangrove forest habitat are impacted by sea level rise and marsh migration. Between the years of 2006 to 2011, the areas of mangrove land cover increased based on both unsupervised and supervised classification analyses. Mangrove land cover also migrated inland based on these same analyses.

As mangrove habitat within the park boundary expanded inland over time, mangroves appear to be moving into areas historically inhabited by other types of vegetation. Land cover change comparisons with the current mangrove forest area indicate that the mangrove forest exists in areas that have changed land cover class over time, and continues to move. Similarly, in coastal areas once inhabited by mangroves, other wetland marsh habitats such as sawgrass have increased , confirming previous research from other studies. From 2006-2011, wetland areas of vegetation decreased within the Everglades, while barren land cover areas increased. Based on previous research, both are a likely result of sea level rise impacts and salt water intrusion, or other hydrological impacts from recent flood control modifications.

Mangrove forest land cover increased during the study period, and the health of the mangroves seemed to remain consistent over time as indicated by NDVI analyses. Despite mangroves maintaining health during the study period, health of other wetland communities including sawgrass marsh declined. Inland vegetation declined the greatest along with an increase in barren land cover. NDVI analyses indicated a decrease in overall health of Everglades vegetation during the study period, likely due to sea level rise and other variables, as was described in previous research. When changes in vegetation health were compared to the current mangrove forest, mangrove areas corresponded primarily with areas of no change over time, confirming their adaptability to changing conditions within the Everglades despite migration and sea level rise impacts.

This study only compared two years of Landsat data (although a study in the literature review with Landsat data from 1985-2017 and had similar conclusions) (10). The analysis was limited to consideration of sea level rise and vegetation health, and did not consider other variables that could impact mangroves including hurricanes, precipitation, temperature and atmospheric CO2. Sampling error in supervised classifications should also be considered, along with the acknowledgment that there may have been classes that were not identified or included.

As sea levels rise, mitigation efforts should be taken to prevent further flooding of coastal mangrove forests to avoid further inundation from saltwater intrusion. Since mangroves serve as a coastal buffer to erosion and storm surge, and they absorb nutrients and contaminants that otherwise would filter into the Everglades ecosystem, it is crucial that the loss of coastal mangrove forest cover is attenuated as sea level continues to rise. It is also important to consider as overall vegetation health of the Everglades National Park has declined over time, with salt water intrusion from sea level rise being a key factor.

As the mangrove forests migrate, preservation efforts should continue and adapt as the mangrove forest area adapts with sea level rise over time. Research has shown that the mangrove forest of Everglades National Park has migrated inland in response to sea level rise, as this study also indicates. Other types of plant ecosystems have migrated seaward, like sawgrass marsh. Both have implications for plant and animal species living in those ecosystems, and for the Everglades ecosystem as a whole.

Sea level rise and the migration of mangroves also has significant consequences on the freshwater balance of the Everglades. Environmental managers of the park will need to consider sea level rise and climate change impacts in their management decisions for restoration, conservation, and preservation efforts. The current Comprehensive Everglades Restoration Plan includes plans to improve bringing freshwater distribution to the Everglades region for habitat restoration purpose, with considerations to combat saltwater intrusion from sea level rise (23).

Previous research indicated that the impacts of sea level rise could be further exacerbated by variables such as temperature, hurricanes, precipitation and atmospheric CO2 (1, 10, 13, 14, 23). Impacts of climate change will likely cause fluctuations in many of these variables that should be considered along with sea level rise to fully understand the adaptability of the Everglades mangrove forest to sea level rise and other climate impacts. With the importance of the mangrove as buffer, filter, and habitat, further determining the stability and adaptability of the mangrove forest of Everglades National Park to the impacts of climate change may prove critical to mitigating the impacts of sea level rise for its ecosystem and the surrounding region.