Causes and solutions for the Great Atlantic Sargassum Belt

Background

Since 2011 the appearance of large amounts of seaweed (various species of Sargassum) on the beaches of the Caribbean Sea has become an annual problem e.g. in Barbados and Trinidad and Tobago. Small amounts of Sargassum reaching shores has beneficial effects - fertilising plants that strengthen shorelines, providing food for local species, etc., but the recent massive landings damage local ecology and economies and is challenging to clear up.

On shore, as well as blocking beaches and discouraging swimmers, the Sargassum releases sulphurous odours as it decomposes. Removal is time-consuming, expensive and can damage the beaches. Incoming rafts smother sea grasses and coral reefs, while local fishermen struggle to get into the water, with the huge rafts of seaweed blocking their engines and fishing gear. There is also the risk to the sea turtle population, which comes ashore to nest. Nesting sites can be blocked by the Sargassum or damaged by removal work - and the turtles may become entangled and die.

The new source of Sargassum appears to be in the tropical central Atlantic (the Great Atlantic Sargassum Belt, GASB e.g. Wang et al., 2019). Some possible causes have been suggested, such as the increase in discharge of nutrients from the Amazon, due to deforestation and increased use of agricultural fertiliser, as well as changes in sea surface temperature (SST) due to global warming and the biochemical composition of the seawater. The Sargassum has been observed in satellite images (e.g. MODIS) to extend from Brazil to West Africa (Wang et al., 2019), and it is then transported NW by the Guiana Current along the Lesser Antilles Island arc and into the Caribbean Sea.

The Project

This project is an exciting opportunity to get involved in interdisciplinary research in marine science, exploring the connections between physics, biogeochemistry and ecology, to address a real problem in managing our oceans. It will equip you with important skills in remote sensing and modelling, as well as management of Big Data, with opportunities to learn about AI and machine learning.

Here we propose to examine the transport of Sargassum by a regional numerical model of the 3D baroclinic hydrodynamic circulation (using a regional NEMO model embedded in the global NEMO model: Wilson et al., 2019), modulated by the Stokes' drift caused by wind waves (from the WAVEWATCH III model, hereafter referred to as WW3, see Bricheno and Wolf, 2018). The model can capture the 3D circulation (driven by tides, winds, freshwater discharge from rivers and heat fluxes), as well as surface temperature and salinity, allowing us to explore the variability of the source conditions and transport and dispersion within the Atlantic Ocean and the Caribbean Sea. This may also provide a useful predictive tool for occurrence of the nuisance seaweed and help understand the mechanisms and impacts of options that may be applied to its management.

Tools and Methods

The methods to be used include:

Examination of satellite data from MODIS (to examine extent and variability of Sargassum distribution in space and time)
Examination of other satellite datasets e.g. satellite altimetry data from Sentinel-1 and Sentinel-2 for sea level and waves, SST (from AVHRR) and salinity (from SMOS), to validate the 3D baroclinic hydrodynamic-wave coupled model
Running the coupled NEMO-WW3 model of Caribbean Sea, with boundary forcing from global NEMO and WW3 models, model diagnostics, validation and scenario testing
The PARCELS (or other) particle-tracking methodology will be applied to track to dispersion of the Sargassum through the model.