Center for Marine Biodiversity and Conservation

The ocean’s biological pump connects the surface ocean, where light-driven photosynthetic processes fix dissolved carbon dioxide (CO2), to the ocean’s mesopelagic zone (approx. 200 –1000 meters) and beyond. It is a process that depletes the ocean’s surface of CO2 relative to the CO2 in deep water through mechanisms such as the sinking of organic material to the deep Ocean. The Atlantic Ocean features high productivity of the macroalgae genus Sargassum floating on the ocean’s surface in the Sargasso Sea, and in recent years giant blooms of the brown algae Sargassum have been observed stretching from the west coast of Africa to the Gulf of Mexico, the largest macroalgae bloom that has ever been recorded. The sinking of macroalgae from surface waters to the seafloor is considered to be an important carbon sink, but one that is little understood. With the logistical challenges of accessing the deep sea, the record of Sargassum appearing on the seafloor remains limited.

This project utilized an archived exploratory dataset that is freely available to the public in order to make novel discoveries in previously unexplored areas. The following report documents the presence and distribution of Sargassum falls in the deep sea during six dives conducted by NOAA Ship Okeanos Explorer’s ROV Deep Discoverer off the Southeastern United States, in the Gulf of Mexico, and in the Caribbean. Sargassum was observed on each of the dives, in numbers ranging from 6 to 30 observations per dive, with Sargassum being observed an average of every 171 linear meters. This suggests that Sargassum does make its way to the deep sea, in potentially significant amounts.