What feeds the Sargassum belt in the Tropical Atlantic?

In the equatorial Atlantic, Sargassum accumulates seasonally under the Inter-Tropical Convergence Zone (ITCZ), and is stretched eastward along the ITCZ by the North Equatorial Counter Current. In the tropical Atlantic, it was found that Sargassum has diverse nutrient inputs, such as upward entrainment of nutrients as the mixed layer deepens seasonally, seasonal contact with the Amazon River plume, and positive Ekman pumping (open-ocean upwelling) due to the curl of the wind (Information and figure taken from Johns et al., 2020).

The figure below shows the monthly averages (2010-2018) of Sargassum density, along with the averages of the ITCZ position, the open-ocean upwelling due to the wind stress curl (positive Ekman pumping), and the chlorophyll >0.2 mg/m^3. Sargassum patches start aggregating in the region of convergence of the trade winds (i.e., the ITCZ) between about March and April. From May to September, Sargassum is “tightly packed” in a giant windrow along the ITCZ. The North Equatorial Counter Current (NECC) contributes to the eastward transport of Sargassum. This windrow moves north following the ITCZ in its seasonal migration. In October, when the wind convergence weakens and the ITCZ starts its southward migration, the Sargassum belt is left behind in the subtropical North Atlantic (large red ovals in the figure).  In December-March, remnant Sargassum patches are still observed around 10°N (red ovals). As the ITCZ starts to migrate north again in its following annual cycle, remnant Sargassum patches aggregate due to the wind convergence.

Positive Ekman pumping occurs in the middle of the Atlantic Ocean between June and October with an offset to the north of the ITCZ, coinciding with the accumulation of Sargassum along the ITCZ; this open-ocean upwelling would bring nutrients to the surface close to the areas where Sargassum accumulates.

The purple filled-in areas show regions of satellite-derived apparent chlorophyll concentrations > 0.2 mg/m^3, showing the typical position and extent of the Amazon and Orinoco River plumes, and of the upwelling plumes off northwest Africa and the equator, in relation to the seasonal Sargassum accumulation in the tropical Atlantic. Between March and May, the Sargassum accumulation area can be located several degrees of latitude to the south of the area of northwestward extension of the Amazon plume. In July and August, Sargassum can be found in the area of the Amazon plume, and by September and October, it can also be found east of the Amazon plume. As such, the Sargassum aggregation probably benefits most from Amazon River nutrients after June, when it crosses the latitude of the NECC. Once the Sargassum has reached the Caribbean, it likely benefits from additional nutrients provided by the Orinoco River plume as well. The trans-Atlantic Sargassum windrow forms south of the northwest Africa upwelling region and north of the equatorial upwelling; indicating that the Sargassum aggregations do not feed from nutrients upwelled at those regions.

The seasonal variability of the surface Mixed Layer Depth (MLD) under the Sargassum belt tends to be deeper than or similar to the nitrate nutricline. Also, nitrate averaged within the MLD was usually higher than the nitrate concentration averaged over the first 10 m of the surface. Therefore, the MLD can be deep enough to make nutrients available to the large accumulation of the Sargassum in the equatorial Atlantic.

Figure: Sargassum density derived using MODIS satellite data adapted from Wang et al. (2019a, 2019b). The ITCZ position and the Ekman pumping were calculated from the satellite wind CCMP product. Purple contours were traced from the monthly climatology (2002-2018) of the NASA MODIS chlorophyll concentration product derived from ocean color (source: Goddard Space Flight Center). Information and figure taken from Johns et al. (2020).

Johns et al., 2020. The establishment of a pelagic Sargassum population in the tropical Atlantic: biological consequences of a basin-scale long distance dispersal event. Progress in Oceanography, https://doi.org/10.1016/j.pocean.2020.102269

Wang et al., 2019a. The great Atlantic Sargassum belt. Science, 365 (6448) (2019), pp. 83-87. DOI: https://doi.org/10.1126/science.aaw7912

Wang et al., 2019b. Sargassum density and coverage using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data from 2001-01-01 to 2018-12-31 (NCEI Accession 0190272). NOAA National Centers for Environmental Information. https://accession.nodc.noaa.gov/0190272

In the equatorial Atlantic, Sargassum accumulates seasonally under the Inter-Tropical Convergence Zone (ITCZ), and is stretched eastward along the ITCZ by the North Equatorial Counter Current. In the tropical Atlantic, it was found that Sargassum has diverse nutrient inputs, such as upward entrainment of nutrients as the mixed layer deepens seasonally, seasonal contact with the Amazon River plume, and positive Ekman pumping (open-ocean upwelling) due to the curl of the wind (Johns et al., 2020, https://doi.org/10.1016/j.pocean.2020.102269).