LB 4-29 - Effect of seasonal pelagic Sargassum accumulation in areas of upside- down jellyfish (Genus Cassiopea) blooms across Jobos Bay National Estuarine Research Reserve (JBNERR

Massive accumulations of pelagic seaweed of the genus Sargassum cause local increases in temperature, turbidity, and organic matter. These changes can lead to eutrophication and low levels of oxygen in sediment environments that are harmful to many marine organisms such as fish, shellfish, corals and aquatic macrophytes. Sargassum accumulations were first noted in Caribbean coastal waters, including those of Puerto Rico in 2011. Accumulations are now common along the coasts of Puerto Rico especially after summer. This causes an island wide emergency because it affects benthic oxygenation, including in important recreational areas such as the bioluminescent bays and natural reserves. These events can affect many organisms that have important roles across ecosystems. In Jobos Bay, on the southeast coast, the upside-down jellyfish, Cassiopea, can be an important bioindicator. These jellyfish typically prefer shallow water with low turbidity, so enough sunlight is available for their algal symbionts. The purpose of this research is to understand how different factors such as Sargassum accumulations can alter sediment and water quality, and how changes can influence the spatial distribution of Cassiopea. Sediment samples were collected in areas of Cassiopea blooms and those that commonly have Sargassum accumulations. Sediment samples within water depths of 0.5–1m were collected for grain size and composition to compare areas that contained large amounts of Sargassum detritus and how this relates to the spatial distribution of Cassiopea. Preliminary data indicates that Sargassum accumulation may have a negative effect on Cassiopea densities. Cassiopea abundance decreased drastically (100+ individuals to 0) in sites with blooms in August 2022 that had accumulation of Sargassum. However, in February 2023 Cassiopea juveniles were spotted (3-4 individuals) in those areas. Sediment seems to be fine, silty sediment across both sites of Cassiopea blooms and Sargassum accumulations. Additional steps will include correlating benthic macrophyte composition associated with these areas of Sargassum accumulation and Cassiopea blooms.