Etude de la prolifération des algues vertes dans le bassin d'Arcachon

Introduction Massive proliferations of algae and, in particular green macroalgae, have been known all over the continents for several decades. They affect various littoral sites and belong to diverse species (Ulva, Enteromorpha and especially Cladophora). In 1990, on the North Brittany shore, more than 80 000 m3 ulvas have been mechanically removed by affected communities. Up until recently, the Arcachon basin escaped from such phenomenon, even if the diversity and quantity of algae found there had already undergone increasing periods in the past. However, the first algae proliferations only started in the 80's, which scale was large enough to be a nuisance to all activities taking place in the basin. At first (starting in1982), those proliferations were green stringy algae such as Enteromorpha growing profusely at spring time and reaching maximal biomass from July to September. So called locally "lige" or "lime", Enteromorpha clathrata became from that time on a hindrance for professional or recreational fishing and a disturbance for bathers. From 1988-1989, another type of proliferation appeared: oyster farmers and fishermen reported another "cabbage" (name commonly used for ulvas) which not only grows at spring time, but lasts through fall and winter and still represents a significant amount of algae. Identified as the ulvale Monostroma obscurum, this algae "blew up" in 1990 and 1991, causing the same nuisance as the previous one but on a larger scale. Whatever the site or vegetal specie, whether fresh or marine shore waters, micro or macroalgae, those excessive vegetative growth phenomena are often caused by nitrogeneous and/or phosphoric components enrichment in the aquatic environment, nutriments. This situation is called eutrophication. This enrichment can originate from various sources among which industrial urban or agricultural discharges are mainly found. This study only covered the green algae Monostroma obscurum which development was the most worrisome. Faced with the weight and complexity of such a study during such a short time, it would have been presumptuous and inefficient to attempt covering all proliferations found in the Arcachon basin. However, the amount of observations gathered during this study, as well as the results of the work done concerning proliferations of other species on other sites, will allow us to better understand the occurrence and development mechanisms of those phenomena through the example of Monostroma obscurum.