Climate-sargassum interactions across scales in the tropical Atlantic

Abstract

The impacts of climate change on ecosystems are highly uncertain but potentially profound. One such impact may be the emergence of extensive mats of seaweed (macroalgae), due to the extraordinary proliferation of pelagic sargassum species, in the tropical Atlantic since 2011. Sargassum blooms are now an annual event and reached record levels across what is now known as the ‘Great Atlantic Sargassum Belt’ (GASB) in summer 2022. Monitoring across scales, from satellite surveillance to in-situ beach surveys, is bringing step changes in process-level understanding of sargassum. Physical and biogeochemical drivers of sargassum act at basin scale to shape the GASB, highly variable from year to year. In the local environment, sargassum is sensitive to ambient conditions, with new findings confirming that growth rates are temperature dependent. Multidecadal ocean warming may therefore be detrimental to sargassum, although projected changes in other drivers are uncertain. Emerging options for climate change action around sargassum include valorisation and carbon sequestration, although uncertainties are again considerable. In conclusion, the emergence of sargassum across the tropical Atlantic highlights interconnected systems that embrace physical, biogeochemical, and socioeconomic dimensions, with considerable scope for improved monitoring, process-level understanding and prediction.

Tracking and detecting sargassum pathways across the tropical Atlantic

“…In December, tracker 4 experiences lower temperatures, down to 26 °C, and tracker 5 down to 27 °C. Marsh et al (2023) suggest that sargassum grows optimally at 26 °C and that mortality increases above 28 °C, suggesting that the waters in which the trackers drift through August, September and October are too warm for maximum sargassum growth. However, it has been shown that different sargassum morphotypes have different growth rates in different temperatures (Corbin and Oxenford 2023, Magana-Gallegos et al 2023).…”

“…It can be observed in figure 5 that throughout its journey in September and October this tracker stays in water that is 29°C-29.5°C, and in November 28.5°C-29°C. Following Marsh et al (2023) this would suggest that sargassum is experiencing a growth factor close to 90% of peak in September and October, and close to peak in November. Where the aggregation grows in early September, we must consider that there are other variables supporting its growth, such as nutrients, or that a specific morphotype is growing faster (as shown by Magana-Gallegos et al 2023).…”

“…In 2018 it was estimated that there was over 20 million metric tons of sargassum biomass covering an area of almost 3000 km 2 , subsequently coined the 'Great Atlantic Sargassum Belt' (Wang et al 2019). There is uncertainty in both the initial trigger of blooms in 2011 and the subsequent drivers of bloom proliferation in subsequent years (Marsh et al 2023). Regarding the latter, hypotheses invoke roles for ocean-surface circulation patterns, wind associated with the inter-tropical convergence zone, and nutrient enrichment (Johns et al 2020, Lapointe et al 2021, Skliris et al 2022.…”

“…Most satellite based monitoring of floating sargassum is done at the basin-scale using optical data collected by Moderate Resolution Imaging Spectroradiometer (MODIS), Sentinel-3 Ocean and Land Colour Instrument (OLCI), Visible Infrared Imaging Radiometer Suite (VIIRS) and Medium Resolution Imaging Spectrometer (MERIS) at spatial resolutions of 250-1 km. Optical satellite data collected at higher spatial resolution satellite of 0.5 m-30 m tend to be used to fill in 'gaps' on local to near-shore scales (Marsh et al 2023). Forecasting models that integrate satellite-derived sargassum data products have also been developed, see examples including Maréchal et al (2017), Johnson et al (2020) and Marsh et al (2021).…”

Changes in holopelagic Sargassum spp. biomass composition across an unusual year

“…Since 2011, annual stranding of millions of tons of sargassum biomass has become a new normal in the Caribbean and West Africa (4). Several hypotheses have been suggested to explain the emergence and recurrence of these sargassum events, including physical (5) and chemical (6) drivers associated with climate change and variability across the Tropical Atlantic (7).…”