Sargassum brown tide impacts nearshore water quality and seagrasses in Jobos Bay, Puerto Rico

The Caribbean region is experiencing seasonal inundation of the shoreline by large mats of pelagic Sargassum spp. (Sargassum) leading to novel impacts to ecological communities. Where Sargassum becomes trapped along the shoreline, leachates turn the water a brown color, coined Sargassum Brown Tide (Sbt). We conducted monthly sampling at six sites along the offshore mangrove keys of Jobos Bay, PR between April 2022 to July 2023 to collect temperature, pH, salinity, dissolved oxygen, total nitrogen (TN), total phosphorus (TP), chlorophyll a (chl a), total suspended solids (TSS), and volatile suspended solids (VSS) at nearshore, midshore, and offshore zones along transects running perpendicular to the shoreline. We also collected data on submerged aquatic vegetation (SAV) community dynamics along transects at each site. We found significantly higher chl a and lower dissolved oxygen concentrations within the nearshore zone during Sbt events but the differences did not extend out to the midshore and offshore zones. Total suspended solids were also higher at nearshore zones compared to offshore zones when a Sbt event occurred. In addition, sites that experienced Sbt had higher turbidity and lower pH. Total percent cover of SAV was different between sites impacted by Sbt and control sites depending on transect zone, with higher SAV percent cover for control sites within the 5 m zone and often within the 15 m zone. Our data suggest that Sbt has significant impacts to nearshore water quality, chl a, and SAV percent cover; however, most impacts are not seen beyond 45 m in well flushed systems.