Remarkable Sites and Stranding of Sargassum : the dangerous Door of Hell (Porte d’Enfer)

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)

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

Sargassum collection at Marie-Galante : July 2019

Map #1 : showing location of Marie-Galante and Capesterre city in Guadeloupe archipelago

Sargassum arrivals in late July 2019 have always been very common on the east coast of Marie-Galante island. The town of Capesterre is the most impacted by stranding, stagnation and decomposition, due to its direct exposure to Atlantic ocean currents and the trade winds. The coral reef located 500m from the coast line complicates water flow. The shallow waters surrounding the urban coastal strip [see map #1 & #2] remain warm with little cooling by the oceanic masses that are difficult to pass through the coral-drawn channels. The most favourable periods renew water masses of this lagoon intervene during strong swell, where there is large quantity of water penetrate by overflow (breaking waves).

Map #2 : Showing 7-day average location of detected rafts of sargassum in the western tropical Atlantic

These conditions give waters that are often poorly oxygenated and sensitive to pollution. The presence of a large quantity of organic matter promotes anoxic decomposition (fermentation) and the production of toxic gases such as hydrogen sulphide (H2S), ammonia (NH3) or Thiols. Rainwater discharges and the malfunctioning of the water treatment plants cause eutrophication characterized by the presence of numerous green algae and seasonal pollution. In the case of sargassum stranding, it is essential to quickly collect dead algae in order to limit the risk of atmospheric and aquatic pollution and to avoid exposing the population to H2S or NH3 levels, which is higher than the sub-chronic exposure threshold respectively of 0.02 ppm for H2S and 0.71 ppm for NH3 [HCSP 08/06/2018 ] for more than 15 to 30 days.

This pickup of algae, although important and urgent, can not be done in bad conditions do anything and without respect the rules. On July 24th, 2019 we had worrying example of bad practices that are going on (see picture 1 and picture 2)

  • lack of delimitation of the work area
  • lack of signalling of associated risks
  • use of unsuitable machinery in hazardous conditions
  • Poor choice of pickup locations, etc.

TCGNRG can contribute to the implementation of collection procedures respectful of natural sites, environmental conditions, safety and public and workers health. Do not hesitate to contact our services

contact@tcgnrg.com

Sargassum on West coast of Martinique

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

Brown seaweed bio-degradation goes up to decomposition sludge

The biodegradation of brown seaweed is a complex micro-biological process that is highly dependent on environmental conditions. Decomposition in a human activity area can be smelly and even become toxic depending on emissions of hydrogen sulfide, ammonia and mercaptan among others. Variations in force and direction of wind, atmospheric pressure at the sea level, air and water temperature can rapidly change the biogas concentrations for varying periods. The eolian transport can advect gases over long distances and intoxicate a large number of people.

This biodegradation which occurs most often in aquatic environment (or highly wet) can go through fermentation and putrefaction phases that modify water quality with more or less localized effects. In a port or semi-enclosed area with low water mass exchange variations of acidity and dissolved oxygen concentration can impact the biotop and remove all traces of life. It is therefore essential to monitor the evolution of water quality and degradation processes.

Vinaigrerie multi-purpose port
Picture of the multi-purpose port of La Vinaigrerie of Petit-Bourg, Guadeloupe, FWI, partially covered by  decomposition sludge of brown seaweed (Sargassum Fluitans)

TCGNRG can help you to manage these atmospheric and aquatic pollutions by monitoring, simulating and forcasting emission and propagation of those gas and associated waste. We also propose training in management and maintenance of port, beach and coastal water. Feel free to contact us!