Episodes of massive proliferation of macro-algae on European coasts and beaches have been reported in the scientific literature as early as 1905 on the Irish coasts. For the past forty years, this phenomenon has continued to grow. The proliferation of these algae is mainly linked to the presence of nitrates, brought by human activities (agriculture in particular), in the waters and to the topographic configuration of the coast (bays).
Once stranded on the beaches, these massive deposits of algae lead, during their putrefaction, to significant releases of gases, in particular hydrogen sulphide (H2S) which can be the source of olfactory nuisances and health problems for walkers and residents of beaches. In order to remedy this situation, certain beaches are subject to collection, the algae then being treated on the land. These different steps expose workers to these gases.
Alongside the question of the risks linked to these massive strandings for the health of the exposed populations, the question of the risks linked to the presence of these algae when swimming or consuming shellfish is also raised. In order to provide the health authorities with some answers, ANSES has carried out an inventory of available data on the dangers and possible exposures associated with swimming and the consumption of shellfish in areas affected by green algae blooms. .
The Agency’s work
An internal expert appraisal was carried out on the basis of the scientific literature and available monitoring data (in May 2011) in order to assess the relevance and feasibility of conducting such a risk assessment.
The main objective of this prospective work was to collect information on the micro-organisms and chemical substances associated with the massive presence of macro-algae in order to identify the possible dangers to which swimmers and consumers of shells collected at proximity.
The species involved in the Breton green tides are overwhelmingly Ulva Armorica et Ulva rounded. The algae of this family, Ulva spp., are part of the genera of macro-algae used in human food and referenced at European level as cosmetic ingredients.
Regarding microbiological hazards, the bibliography reports the presence of bacteria on macro-algae or in decomposing clusters of algae, but without making it possible to establish a typical profile of the bacterial populations that develop there. The available data are rare and do not make it possible to estimate the level of concentration of pathogenic bacteria in these different cases. Moreover, specific data from the French coasts are rare.
Regarding chemical hazards, data was collected on the composition of algae in various metallic trace elements, on contamination by persistent organic pollutants and pesticides, as well as on potentially toxic substances produced by certain algae (for example dimethyl sulphide, acrylic acid, dopamine). Again, specific data from the French coasts are rare and none relates to an episode of massive grounding of green algae. So, it is proving difficult to identify and characterize the chemical hazards associated with the occurrence of green tides in France.
From an epidemiological point of view, none of the publications analyzed in the context of this work and relating to microbiological or chemical hazards linked to an episode of proliferation of macro-algae, mentioned any symptoms in humans, after a bathing activity or the consumption of shells collected in these areas (e.g. toxic infections, irritations, etc.).
Estimating the exposure of populations to the identified hazards proved impossible due to a lack of suitable data, in particular concerning their concentration during massive strandings of green algae on the French coasts. The rare data available concerning the French coasts are those from the monitoring of bathing waters, monitoring of the contamination of shellfish and reports of green tides.
Before a possible quantitative assessment of health risks, the collection of epidemiological data could make it possible to objectify a possible health signal. If necessary, the acquisition of the knowledge needed to quantify the risks could be part of research work.