Standardization proposal for the mapping of Caulerpa taxifolia expansion in the Mediterranean sea

Fourteen years after the first observation of Caulerpa taxifolia (Valh) C. Agardh in the northwestern Mediterranean Sea, this green alga of tropical origin is now present in five countries (Spain, France, Monaco, Italy and Croatia). By the end of 1997, more than 46 km(2), at depths of between 0 and 50 m, were found to be affected by this expansion. A standardization of the cartographic procedure is proposed here. Such a standardization is necessary to compare maps produced by various organizations from different countries. For a given independent station, it is based on the definition of three levels of colonization: Level I refers to a station where one or several colonies less than 100 m apart cover a total surface area inferior to 1000 m(2): Level II refers to a station where several colonies less than 250 m apart totally cover more than 1000 m(2) with fragments and small colonies dispersed over a surface area inferior to 10 ha; Level III is attained when several large colonies are dispersed over a surface area superior to 10 ha, with a total covered surface of more than 1000 m(2), impossible to map with precision due to its size. For each of these colonization levels, descriptive parameters can be used to precisely describe the situation: covered surface area, dispersion surface (i.e. "affected" zone), estimation of very large affected zones (i.e. "concerned" zones) and the portions of the coastline along which an affected or concerned zone can be found. (C) Elsevier, Paris / Ifremer / Cnrs / Ird.