Assessing the Impacts of Hurricane Irma

By Dr. Ryan P. Moyer

The Coastal Wetlands research group at the Florida Fish and Wildlife Research Institute (FWRI) received a grant from the National Fish and Wildlife Federation (NFWF) to assess the impacts of Hurricane Irma to coastal wetland habitats of southwest Florida. Since 2014, the FWRI Coastal Wetlands group along with partner organizations, has been working in coastal marshes and mangroves across Southwest Florida, including Tampa Bay, Charlotte Harbor, Ten Thousand Islands, Biscayne Bay, and the lower Florida Keys. All pre-existing field sites were located within 50 km of Hurricane Irma’s eye path, with a few sites in the lower Florida Keys and Naples/Ten Thousand Islands region suffering direct eyewall hits. Since all locations include active field sites, a wealth of pre-storm data exists, and these locations are uniquely positioned to evaluate and quantify post-hurricane damage to standing biomass and ecosystem services across a wide geographic area.

Map showing the location of field sites in proximity to the path of Hurricane Irma. Pre-existing study sites are given for each area in the inset boxes at right.

Initial Post-Irma field assessments focused on qualitatively assessing and photographically documenting damage (e.g. defoliation, downed trees, redistribution of sediments, etc.). Upon identification of the habitats that experienced the most damage, storm impacts were then categorized as low-, moderate-, and severe-impacts based upon physical habitat damage and distance from eyewall path, height of storm surge, and maximum wind speed experienced during the hurricane. Quantification and monitoring of aboveground damage included measurements of indicators of defoliation (canopy coverage), mortality (recently felled trees and branches), plant community structure (tree diameter or height), and recovery (seedling percent coverage or density). Recovery of mangrove forest was assessed by subsequent visits to long-term monitoring plots in the six months following Hurricane Irma. Sedimentary impacts were also examined and included elevation change, shoreline erosion, and geochemical characterization of storm-derived sedimentary deposits.

Example of severe damage to a mangrove forest in the Ten Thousand Island due to Hurricane Irma. This site was directly under Irma’s eye path as Category 3 storm and was found to have complete loss of leaves in the canopy (defoliation) and numerous downed trees. Canopy cover is typically 80-100% in non-impacted forests.

Preliminary findings indicate a reduction in mangrove canopy cover from 70-90% pre-storm, to 30-50% post-Irma, and a reduction in tree height of approximately 1.2 m. Although signs of forest recovery and shows signs of slow regrowth, mangrove seedling density has significantly increased in the six months post-Irma. A sedimentary layer of fine carbonate mud up to 10-cm thick was imported into the mangroves of the lower Florida Keys, Biscayne Bay, and the Ten Thousand Islands. A siliciclastic mud layer up to 5-cm thick was observed in the marshes of Charlotte Harbor. All sites had imported tidal wrack consisting of a mixed seagrass and mangrove leaf litter, with some deposits as thick as 6 cm. In areas with newly opened canopy, a microbial layer was coating the surface of the imported wrack layer. Overwash and shoreline erosion were also documented at two sites in the lower Keys and Biscayne Bay and will be monitored for change and recovery over the next few years, pending subsequent funding. Due to changes in intensity along the storm path, direct comparisons of damage metrics can be made to environmental setting, wind speed, storm surge, and distance to eyewall. This information will help provide direct evidence of hurricane impact and recovery trajectories in coastal wetland ecosystems in Florida. The data will be shared with coastal ecosystem managers in order to enhance management and response planning for large natural disasters in Florida such as hurricanes.

Project partners included the U.S. Geological Survey, University of South Florida Saint Petersburg, Rutgers University, Nanyang Technical University, the University of Rhode Island, Tufts University, University of South Florida College of Marine Science, and the US National Park Service.