Re-evaluation of the extent of Caulerpa taxifolia development in the northern Mediterranean using airborne spectrographic sensing

Abstract

There has been significant concern over development of the tropical green alga Caulerpa taxifolia in the Mediterranean Sea. Reports dating back to 1991 predicted that the species would expand exponentially on all types of substrata and displace major components of the coastal benthic flora, in particular the dominant native seagrass Posidonia oceanica. A pilot study of the Bay of Menton, SE France, where C. taxifolia has been established since at least 1989, indicated that these predictions might not be correct and that more sophisticated methods might be needed to gain a better estimate of the alga's abundance. We thus surveyed 44% of the total area reportedly colonized by C. taxifolia to a depth of 20 m on the south coast of France by compact airborne multispectral imaging and validated our maps by an extensive underwater survey. Our data indicate that the cover of C. taxifolia has been overestimated by at least 1 order of magnitude and that C. taxifolia has not substantially impacted the cover of P. oceanica. They also indicate that exponential expansion of the alga is only likely to have occurred on substrata situated in the vicinity of sewage outfalls and storm water drains, suggesting that it principally occupies partially vacant niches in stressed environments. In the light of these results, we consider that the risk to most endemic species should be considerably lower than formerly predicted.