Simulating transport pathways of pelagic Sargassum from the Equatorial Atlantic into the Caribbean Sea

Nathan F Putman,Gustavo J Goni,Lewis J Gramer,Chuanmin Hu,Elizabeth M Johns,Joaquin Trinanes,Mengqiu Wang

doi:10.1016/j.pocean.2018.06.009

Abstract

Since 2011, beach inundation of massive amounts of pelagic Sargassum algae has occurred around the Caribbean nations and islands. Previous studies have applied satellite ocean color to determine the origins of this phenomenon. These techniques, combined with complementary approaches, suggest that, rather than blooms originating in the Caribbean, they arrive from the Equatorial Atlantic. However, oceanographic context for these occurrences remains limited. Here, we present results from synthetic particle tracking experiments that characterize the interannual and seasonal dynamics of ocean currents and winds likely to influence the transport of Sargassum from the Equatorial Atlantic into the Caribbean Sea. Our findings suggest that Sargassum present in the western Equatorial Atlantic (west of longitude 50°W) has a high probability of entering the Caribbean Sea within a year’s time. Transport routes include the Guiana Current, North Brazil Current Rings, and the North Equatorial Current north of the North Brazil Current Retroflection. The amount of Sargassum following each route varies seasonally. This has important implications for the amount of time it takes Sargassum to reach the Caribbean Sea. By weighting particle transport predictions with Sargassum concentrations at release sites in the western Equatorial Atlantic, our simulations explain close to 90% of the annual variation in observed Sargassum abundance entering the Caribbean Sea. Additionally, results from our numerical experiments are in good agreement with observations of variability in the timing of Sargassum movement from the Equatorial Atlantic to the Caribbean, and observations of the spatial extent of Sargassum occurrence throughout the Caribbean. However, this work also highlights some areas of uncertainty that should be examined, in particular the effect of “windage” and other surface transport processes on the movement of Sargassum. Our results provide a useful launching point to predict Sargassum beaching events along the Caribbean islands well in advance of their occurrence and, more generally, to understand the movement ecology of a floating ecosystem that is essential habitat to numerous marine species.

Highlights

Pelagic Sargassum spp. aggregates to form “Lagrangian ecosystems” that drift across the ocean surface
In 2011 Sargassum beaching was reported across the eastern Caribbean, and in 2012 beaching was more restricted to the southeastern Caribbean. For both of these years, Hybrid Coordinate Ocean Model surface currents (HYCOM) simulations suggest that transport from the Equatorial Atlantic to the western Caribbean would be unlikely (Fig. 4A and C), whereas simulations including windage predicted higher than usual inputs to the western Caribbean (Fig. 4B and D)
One hypothesis is that when transport to the Caribbean Sea is reduced within the western Equatorial Atlantic, Sargassum and the nutrients that facilitate its growth might be allowed to concentrate in this region, eventually resulting in a major “bloom.” Given that oceanic transport from the western Equatorial Atlantic into the Caribbean Sea is, essentially, inevitable (e.g., synthetic particles (Fig. 5), drifters (Lumpkin and Garzoli, 2005), water masses (Kelly et al, 2000), nutrients and larval fish (Johns et al, 2014), and young sea turtles (Mansfield et al, 2017)), the initial reduction in transport would eventually result in an increase in Sargassum reaching the Caribbean Sea. Consistent with this possibility, we found a strong negative correlation between the annual percentage of synthetic particles transported into the Caribbean Sea and the Sargassum percent coverage (SPC) of the western Equatorial Atlantic in the following year (Spearman r = −0.789, p = 0.0008, n = 14)

Summary

Introduction

Pelagic Sargassum spp. aggregates to form “Lagrangian ecosystems” that drift across the ocean surface. A similar event occurred in 2012 (Marechal et al, 2017), followed by even greater Sargassum inundation events throughout the Caribbean in 2014 through 2015 (Hu et al, 2016; Wang and Hu, 2016, 2017). These anomalous events caused widespread negative impacts to the fishing and tourism economies throughout the region. These events have been the subject of considerable scientific investigation using remote-sensing observations from satellite (Gower et al, 2013; Hu et al, 2016; Wang and Hu, 2016, 2017) and morphological and genetic

Methods

Results

Conclusion