Sargassum inundation and impacts on ST. Thomas, usvi coral world ocean and reef initiative, INC. (cwori) lead-PI: DR

Samuel j. Eliades, CO-PI: MS. Jessica michael total proposed cost: $438,000 budget period: 09/1/202409/31/2025 sargassum is a large, holopelagic brown seaweed that floats in island-like masses creating critical habitat for pelagic species in the open ocean. Once sargassum reaches shorelines around the atlantic however, IT no longer supports many of those pelagic species and can be damaging to coastal environments and human health. Sargassum inundation has been shown to negatively impact nearshore marine species including endangered corals and sea turtles and may even be responsible for large fish DIE-offs. Although sargassum is harmless to humans in water, on land IT decomposes and releases hydrogen sulfide gas and ammonia which are toxic to humans. The usvi has been heavily impacted by sargassum annually since 2011 with some years resulting in mats of seaweed stretching over 100 feet from shore in bays around ST. Thomas. In july 2022, governor bryan declared a state of emergency due to the impacts of sargassum inundation on a desalinization plant on ST. Croix, triggering a response by the U.S. Federal emergency management agency. Despite clear problems with nearshore sargassum in the territory, detailed environmental, economic, and health impacts are under-documented locally. For instance, IT is largely unknown when exactly sargassum shifts from critical habitat for pelagic species to empty rafts of algae smothering inshore habitats. Monitoring coastal zones before, during, and after sargassum influx in the usvi is needed to understand the impact sargassum has on territorial ecosystems, the many endangered, threatened, and near-threatened species found locally, and the residents that live in the territory. This project aims to better understand and address local sargassum influx in the usvi through three primary objectives: 1) quantifying the volume of sargassum arriving at popular bays and beaches throughout a year, 2) assessing the impact of sargassum inundation on nearshore habitats, particularly coral reefs, and 3) studying points at which sargassum mats shift from critical habitat to nuisance macroalgae. To calculate the amount of sargassum arriving in the usvi, aerial drones will be used to collect photos and videos of rafts and approximate sizes will be determined using multiple image identification software packages. In assessing the impacts of sargassum on local marine habitats, experimental plots will be established on reefs commonly affected by sargassum. These sites will be monitored regularly via divers on scuba to quantify coral cover and diversity before, during, and after sargassum mat arrival using orthomosaic software. Finally, key transitional points at which sargassum rafts become nuisances will be determined with visual and digital surveys to document species diversity presence and shifts as rafts approach local beaches. While doing this, endangered and threatened species like stony corals and sea turtles associated with sargassum mats will be monitored closely and local and federal regulatory bodies will be consulted to determine if wildlife rescue or relocation actions may be appropriate. Addressing these goals will provide necessary baselines to further improve the usvi response to nearshore sargassum influxes immediately and into the future.