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

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

Many cities impacted by brown seaweed stranding (Sargassum Fluitans and Sargassum Natans) try to react to the various consequences of this presence of organic matter, living or dead, in large quantity. The effects, although many times reproduced, remain poorly understood and uncontrolled. They range from the simple presence of living floating algae which will modify the landscape, prevent swimming and / or navigation to the decomposition of packed algae on a beach or rip rap in anoxia or anaerobic (lack or total absence of oxygen). This latter case causes multiplication of micro-organisms specialized in usage of nitrogen or sulphur to produce energy. It causes the emission of biogas such as hydrogen sulphide (H2S) and ammonia (NH3).

Figure 1 [english] — Fig. 1: View of the port of Goyave from one of the docks

These two gases are only a small part of the gases emitted during the anaerobic decomposition of these algae, wrongly considered as toxic. This pseudo-toxicity only depends on the fermentation conditions caused by human activities. The risks related to exposure over long periods to concentrations below the alert threshold, 5 ppm for H2S revised to 1 ppm by the High Council for Public Health (HCSP) on 08/06/2018, is undoubtedly dangerous for respiratory tract and general health. This is for people living or working near these emanations from few meters to several kilometres.

Collection, cleaning or bio-redemption of sargassum algae or their waste can’t be imagined using conventional mechanized methods due to the nature of the seaweed: small; charged with sea water and friable in the ageing phase. Beaching area, coastline, mangrove swamp, bay are at the scales of Caribbean island always complex and ecologically sensitive to modifications. Usage of a power shovel on an un-stabilized soils compresses them and modifies water flows rendering these soils at best unusable for a short period and at worst subject to erosion.

There is no point picking up algae that will leave with the next tide or to move sand, rock and concrete to shift algae from an area. The preparation of any cleaning-collection-rehabilitation operations is essential. It must include the adaptation of harvesting methods, the respect of the areas to be treated, the ability to move, store and process the organic or mineral matter collected, while ensuring the safety and health of the people involved and the residents.

The case of the municipality of Goyave (in Guadeloupe, FWI, [16.13N, 61.57W]) is a good example. Subject for several years (2014-2018) to arrivals of seaweed it remains unprepared for pickup operations in its small port . The collection area is a small fisherman port located in a highly anthropized mangrove swamp area which is used as outlet for the wastewater and rainfall water network (Fig. 1). The objective here is not to throw the stone at a particular town, because few communities in the world are truly prepared to respond to non-permanent solicitations such as water, air or soil pollution.

Figure 2 [english] — Fig. 2: Photograph of the mini hydraulic excavator used in the port of Goyave on August 8-9, 2018 with a modified bucket damaged

The recent pickup operations of August 8th & 9th 2018 shows that well. The hydraulic crawler excavator used although its smallness is not adapted to the site and causes damage (Fig. 2). The lack of sargassum on the water surface, in phase of aging these pelagic algae lose their floats (pneumatophore) and sink, pushed to clean the bottom of the port basin. The non-adaptation of the bucket to scratch the rocky bottom damaged it (Fig. 2). The mud a mixture of clay, organic and plastic materials as well as hydrocarbons is stored without precaution on a mangrove swamp area (Fig. 3) and in an unsupervised area initially reserved for sargassum spreading (Fig. 4) . The lack of markup on the building site and the lack of information boards pose questions for the safety of workers and passerby. Presence of biofouling, sargassum decomposition sludge and non-dry mud can emit biogas and locally increase the atmospheric concentrations of H2S or NH3 (Fig. 5).

Figure 3 [english] — Fig. 3: Photograph of a pile of mud stored on old mangrove swamp area near the port of Goyave

This clean-up operation certainly had no effect on air pollution levels, main biogas production areas was not in this area. But it will allow a better quality of the waters of the port and therefore reduce impact of algae decomposition. The environmental cost: degradation of public goods, soil and water pollution is prohibitive compared to the benefits.

Figure 4 [english] — Fig. 4: Photograph of Goyave port mud spreaded on the backside of the beach of Viard

Stranding of sargassum algae should push for better organization and management of environmental work. The complexity of the places and systems concerned, their bio-diversity level imposes a great rigour on the actions that will be conducted there. TCGNRG remains at your disposal to help you set up sustainable devices adapted to the management and treatment of beaching of algae, organic matter or plastic.

Figure 5 [english] — Fig. 5: Photograph showing soiling on one of the docks of the port of Goyave after cleaning operation

Category: coastal engineering