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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:

  1. Examination of satellite data from MODIS (to examine extent and variability of Sargassum distribution in space and time)
  2. 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
  3. Running the coupled NEMO-WW3 model of Caribbean Sea, with boundary forcing from global NEMO and WW3 models, model diagnostics, validation and scenario testing
  4. The PARCELS (or other) particle-tracking methodology will be applied to track to dispersion of the Sargassum through the model.

We will explore the following questions:

  1. What is the likely cause of this recent phenomenon e.g. extra nutrients, increased SST?
  2. Can we explain the variability, due to inter-annual variation in freshwater discharge, heat flux, nutrients and transport, using a coupled hydrodynamic-wave model? Is this dominated by freshwater discharge, nutrients, temperature or wind-driven variability?
  3. Do we need also to use a biogeochemistry/ecosystem model like ERSEM or MEDUSA to explore the new source of Sargassumg. due to increased warming or discharge of nutrients?
  4. Can we develop appropriate source terms for growth and decay of Sargassum to implement into the particle-tracking tool?

The beaching phenomenon of Sargassum, and its impacts, will be examined. What are the potential management options for prevention of the beaching and/or uses of the Sargassum recovered from beaches or offshore? Mass harvesting of Sargassum at sea can have negative consequences for important marine species that use Sargassum mats as refuges, nurseries, and foraging areas – including several threatened species of turtle, blue fin tuna, and several other commercial fish species.

Potential uses of Sargassum (fertilizers, biofuels, sunscreens, food products, pharmaceuticals, cosmetics) have led to increased interest in development of suitable methods for harvesting the blooms. See for example http://sargassummonitoring.com/the-best-solution-to-stop-sargassum-seaweed/. Can these ideas be explored further with industry partners?

The Student

The studentship will be based at the National Oceanography Centre in Liverpool, although you will be registered at Leeds University. You may also be based at NOC HQ in Southampton for extended periods, while working on Earth Observation methods you and there will be visits to Leeds University to meet your co-supervisor. You will interact with other PhD candidates in the Universities of Southampton and Liverpool. You will need a good undergraduate/master’s degree in a numerate science discipline such as maths, physics, marine science or engineering. Experience with remote sensing data or numerical modelling is not essential, but could be desirable. You will receive training in research methods in general, as well as the specific tools to be used. An enquiring mind, good personal organisation and self-motivation are essential!

References

Bricheno, L.M. and Wolf, J. (2018) Future wave conditions of Europe, in response to high-end climate change scenarios. Journal of Geophysical Research: Oceans, 123, doi:10.1029/2018JC013866

Wang, M., Hu, C., Barnes, B.B., Mitchum, G., Lapointe, B. and Montoya, J.P. (2019) The great Atlantic Sargassum belt. Science, 365, 83–87.

Wilson, C., Harle, J. and Wakelin, S. (2019) Development of a regional ocean model for the Caribbean, including 3D dynamics, thermodynamics and full surface flux forcing. National Oceanography Centre Research and Consultancy Report, no. 65. National Oceanography Centre, UK, 40pp, http://nora.nerc.ac.uk/id/eprint/523606

Mean density of Sargassum July 2018 from MODIS, NASA Earth Observatory

Sargassum impacts Caribbean beaches
Sargassum on beach in Barbados, June 2018