After a year 2025 marked by a long period of strandings of holopelagic sargassum, already spanning nearly seven (7) months, the situation on the Guadeloupe coast is complicated. The solutions implemented (mechanical shovels, sea collection from a barge, and floating barriers to divert or block the algae) appear to be increasingly ineffective and are not adapted to the volumes and frequencies of strandings, nor even to the particularly high sea water temperatures. Concentrations of hydrogen sulfide (H₂S) and ammonia (NH₃) have been elevated for more than four months.
Dr. Jean-François was invited to this public conference. He was able to speak for a little over twenty minutes and answer questions from a relatively large audience, for a weekday meeting after 7 p.m. Dr. Dorville focused on the current limitations of dams and methods of reoxygenating water and Sargassum sludge to limit toxic gas emissions and mitigate risks to human health and the integrity of copper products, allowing time to better organize the management of stranded algae.
An attentive and numerous audience for a weekday (Photo credit: JF DORVILLE)
Sargassum (Sargassum fluitans and natans) have been the main species of visible pelagic floating algae since 2011, stranded on the Caribbean coast. Their stranding are strongly influenced by sea currents and configuration of the coastlines (human making included). Arrival of dense or scattered rafts causes ecological disturbances because of the quantity of organic matter (composed by carbon C, hydrogen H, Oxygen O, Nitrogen N, Phosphorus P and sulphur S) which it brings and the speed of stranding. Rafts of Sargassum move under effects of wind, waves, sea currents and tide. They are home to a wide variety of plankton, fish and shellfish colonies. Sargassum oxygen needs coupled with the poor quality of shallow coastal waters cause asphyxiation and anoxic decomposition -fermentation- generating toxic gases such as hydrogen sulphide (H2S), ammonia (NH3), thiols (R -SH) but also greenhouse gases such as methane (CH4).
These processes of decomposition without oxygen are the subject of many controversies, health, economic and ecological crises. The solutions adopted in the French West Indies are mechanized pickups and emergency dredging, causing extensive destruction of beaches and harbours. The concentrations of two of the most toxic gases (H2S & NH3) are controlled so that populations can be evacuated if there is a risk. Despite measured rates of more than 10ppm H2S (Limit Value-Short-Term Exposure VLCT) no evacuation has been done since 2011.
Remarkable
sites
Many beaches are considered to be
remarkable sites. They attract many visitors and constitute the
tourism potential of our developing countries. The presence of
sargassum causes degradation of bathing water quality, landscape
potential and air quality. These sites, which are highlighted and
exploited by the tourism industry, offer significant profitability
due to the lack of maintenance required to date.
The management of these sites is mostly
complex and is pooling of many stakeholders, but this can not in any
case justify their long-term pollution.
Porte-d-Enfer
Beach
Map describing the configuration of Porte-d’Enfer beach and creek
The
range of Porte-d-Enfer (16.48 ° N, 61.44 ° W) in Anse-Bertrand
(known as the Trou à Man Coco) has been known for the last years of
numerous massive sargassum stranding massive (see photo) provoking
the fact of its geographical configuration the partial or complete
blockage of the creek and a thick layer (more than 15 cm in height)
of decomposed compacted algae. Under favourable conditions the
production of bio-gases takes place on the whole column of wet
Sargassum. It can be observed by the presence of bubbles (under
columns shape) or by the dispersion of biofilms of sulpho-reducing
bacteria greyish colour on the surface.
In these cases of mass stranding (example September 2018, January and July 2019) swimming is impossible and dangerous for health. The presence in the zone (up to 500m) of the basin is risky because the production of biogas is important, non-homogeneous and highly variable. The temperature of the water favours the production of hydrogen sulphide (H2S) and ammonia (NH3) during the sunniest hours of the day (9am-5pm) and therefore the risk during periods of affluence. Time variations such as wind failure or pressure drops favour high concentrations.
Cleaning – mitigating the effects of stranding
The site is naturally cleaned during
the phases of strong increase of swell which warranty a ebb of the
organic matter and the re-oxygenation of the basin.
The rainy season lets in the mouth
which encourages the ebb and feeding of the sand beach.
The mechanized methods of collection
are put in place by the municipality and the services of the French
State are only for small quantity stranded algae. An attempt to set
up a floating dam was a failure. Collection with crawler excavator
help to strongly damaged the beach and nearby roads as well as
pollution from petroleum products (see image of a tractor-excavator).
Manual cleaning stay the more efficient
and ecologically respectful of the site, but unfortunately it is not
set up.
Protection
of populations
There have been 22 surveys of H2S and NH3 since 2018 with 15-minute portable measurements. No measures have indicated overtaking of the limit values for exposure, the maximum values recorded are 1.9 ppm for H2S (24/04/2018) and 36 ppm for NH3 (28/05/2018). We can question the validity of these measures when we know that the potential area of biogas production in case of total coverage is 14,000m2 while stranding areas producing biogas is rarely greater than 5,000m2 in Guadeloupe (case of the fishing port of Capesterre-Belle-Eau). Comments made by users in the area confirm this question.
Nevertheless, these measurements are much higher than the chronic and sub-chronic exposure limits of 0.02 ppm for H2S and 0.714 ppm for NH3 [HCSP 08/06/2018]. The sub-chronic exhibition is an exhibition of one to several months which was the case between March and July 2018 according to the statements published by the ARS Guadeloupe (i.e., Regional Health Agency).
Two last values of H2S measured Thursday, July 18, 2019 and Monday, August 12th 2019 reach respectively 5.2 and 4.2 pm which are alert values (>5 ppm) for workers in French Labour Law.
No signboard is visible to alert passers-by (and tourists) of the health risks during periods of beaching and fumes of toxic gas. Without knowledge of the site, many passers-by, children and the elderly, admire the stranding without taking into account the serious health risk.
In
the case of the crisis of green algae (Ulva aka sea-lettuce) of
Brittany it took nearly ten years, dead wild boars, horses, dogs and
several joggers before appearing on the beaches of official
information boards.
It
is therefore urgent to recognize that human health is worth more than
the reputation (or image) of a tourist site. It is essential that the
authorities protect people and tourists by intelligently informing
about the non-permanent risk of stranding sargassum. This is
particularly true for territories that rely on sustainable tourism
development.
The Caribbean Geophysics and Numerical Research Group (TCGNRG) remains at your disposal to assist you (communities, collection companies and individuals) to secure the best places taking into account environmental conditions.
Last update (9/07/2019)
Municipal decree 8/20/2019
Display area of the decree
Since 20 August 2019 a municipal decree (see photo above) prohibits all nautical activities. This first plain decision of common sense is to be commended. But despite all the poor visibility of the display and the lack of pictogram for people who do not read the French prevents to really warn the risks associated with the presence of anoxic decomposing (fermentation) seaweed for more than two months.
At
the moment the photos were taken an under water spear fisherman was
coming out of the water without noticing the posters
Illustration 1 : Oceanographic and Meteorological data for beginning of June 2019
June
1st and 2nd 2019 beaching of sargassum were observed on the
Caribbean coast of Martinique (Fort-de-France, Schoelcher). This type
of beaching has been rare since 2011 and the over-abundance of
drifting sargassum rafts in the North Atlantic and the Caribbean Sea.
Each year one or two beaching are observed on this coast from Fort-de-France to the Prêcheur. They take place only under favourable oceanographic and meteorological conditions.
The breakdown of the Trade Wind felt after the passage of a tropical wave favoured the action of the Southwest currents. Sargassum beds generally extend northward along the Caribbean coast pushed by the East wind and sea currents. This Sunday June 2nd pelagic algae were trapped by the shallow waters of creeks and bays.
In two
days of presence, the decomposition of algae forced their collection
on the small artificialized Cove of Batelière at Schoelcher (500 on
200m), due to emanation of di-oxyde of sulphide and ammonia (SO2,
NH3). This rapid decomposition can obviously be explained by the age
of the rafts that went aground. But also by relatively high water
temperatures (27.9-28.5 ° C) which decreases dissolved oxygen
concentrations in the water of the order of 1 to 2 mg per litre and
therefore anoxic decomposition (i.e., without oxygen ).
The quality of the water is another element to take into account. Only the quality for bathing is continuously monitoring. The last map, published by the Regional Office of Health (ARS), available from (illustration 2) indicates for the beach of Batelière a ‘good’ quality. This measure is misleading because it indicates only the frequency of the presence of certain bacteria of faecal origin but not the capacity of the ecosystem to withstand anthropogenic or natural pollution.
Illustration 2 : Map of Water Quality for Martinique’s bathing of 2018
The
runoff water related to this tropical wave (give a name) to degrade
the quality of the water, on the biological and physiochemical point,
making this area more sensitive to the presence of large amount of
living organism.
It is
essential to take better account of bathing water quality in the
determination of areas exposed to the risk of decomposition pollution
of seaweed-type algae. To better prepare the pickup procedures and
necessary equipment.
TCGNRG can help you design better risk maps for over-concentration of seaweed based on environmental conditions and better organised your organic matter collection and your recovery procedures. Do not hesitate to contact us
