Quantifying Early Warning Signs of Mangrove Stress and Peat Collapse in Tampa Bay

woman sitting in mangrove roots

An FWRI coastal wetlands technician collects a sample of porewater from a mangrove forest.

In July 2017, the Fish and Wildlife Research Institute’s Coastal Wetlands Research Program began a study to identify mangrove locations in Tampa Bay that show early signs of stress due to blocked tidal flow and document evidence of vegetative stress and early signs of peat collapse. The red mangrove (Rhizophora mangle), black mangrove (Avicennia germinans), and white mangrove (Laguncularia racemosa) are the species integral to this study. The identification of stressed mangroves allows for early intervention, in the form of targeted restoration to restore proper hydrology before extensive mangrove die-offs occur.

Tampa Bay is the largest estuary in Florida, encompassing nearly 400 square miles and bordering three counties. While large, Tampa Bay is not immune to the threats posed by an expanding urban population of over 3 million people, the second largest metropolitan area in the state.

Mangroves serve several important roles in the Tampa Bay ecosystem: they stabilize and protect shorelines from damaging storms and hurricanes; they help maintain water quality, filtering pollutants and trapping sediments; and they serve as a valuable nursery for a wide range of fish, shrimp, mollusks and crustaceans, including snook, mangrove (gray) snapper, red drum, sheepshead and others. Mangroves in Tampa Bay support some of the largest nesting waterbird colonies in North America, hosting around 40,000 pairs of birds, ranging from white ibis, blue heron and reddish egret – the rarest heron in Florida. Partly because of the fish and wildlife of Tampa Bay, as well as barrier island beaches, the area attracts nearly 5 million visitors a year – giving financial incentive for restoration and conservation of this important estuary.

A comparison of a peat core from a healthy mangrove forest (top) and low-density peat in a core from a stressed mangrove forest (bottom).

Mangrove forests in Tampa Bay are particularly susceptible to altered water flow due widespread mosquito ditching in remaining coastal wetlands. The mangrove forests in this highly urbanized area also face altered hydrology because of urban development, road construction, concentrated stormwater runoff and dredging. Mangroves become stressed if roads or blocked mosquito ditches prevent the flow of water in the outgoing tide, resulting in pools of stagnant water with low amounts of dissolved oxygen that can suffocate roots. Stress in mangrove ecosystems due to blocked tidal flow is reversible if proper water flow is restored. This type of intervention is most successful if signs of ecosystem stress and potential peat collapse are identified and acted upon before a mangrove die-off occurs.

map showing sampling locations

Locations of stressed and control sites sampled in mangrove forest across Tampa Bay, Florida.

Ten sites with evidence of mangrove stress were selected around Tampa Bay in Pinellas and Hillsborough counties. These locations were chosen based on the presence of stagnant or discolored water, black or white mangroves with adventitious root growth (roots growing directly from the trunk), or visual signs of mangrove mortality on satellite imagery. At each site, 15-meter transects were extended into the mangrove forest from the edge of the water. Metrics of water quality (dissolved oxygen, salinity, pH, temperature and depth), soil characteristics (pH, density and carbon content) and vegetative growth (tree species, density, trunk diameter and presence of adventitious roots) were surveyed along each transect at each site. Mesh bags of peat moss were also buried and left underground for seven months; retrieval and analysis enabled determination of rates of new root growth. Transects through adjacent healthy mangrove forests were also sampled following these same methods. Final sampling and collection of root bags will occur in September 2018, followed by data analysis and interpretation.

Stagnant water bodies in stressed forests were indeed observed to have low dissolved oxygen and extensive bacterial growth. Preliminary results show that mangrove forests that fit the paradigm of blocked tidal flow had significantly lower soil density and higher organic content. Preliminary analyses have shown that root bags in stressed areas had minimal root growth. The team, led by Drs. Kara Radabaugh and Ryan Moyer, plans to submit a manuscript to a peer-reviewed journal to publish results and transfer findings to local managers to prioritize and target restoration efforts. The results of this study will be provided to local restoration partners for their consideration in priority management and restoration planning.

This project is jointly funded by FWC’s Division of Habitat and Species Conservation’s MEHRMA program, and the Department of Environmental Protection’s Florida Coastal Management program.

Tampa Bay must balance the demands of an urban population with the conservation needs of an important natural estuary. A healthy mangrove environment is crucial for the future of Tampa Bay, both for the many people that call the bay home, and for the water quality, fish, wildlife and overall health of the Tampa Bay ecosystem, the largest estuary in Florida.