Alterations of the structure of <em>Posidonia oceanica</em> beds due to the introduced alga <em>Caulerpa taxifolia</em>

Abstract

ea 4228 eCoMers, laboratoire environnement Marin littoral, université de nice-sophia antipolis, Faculté des sciences, Parc Valrose, 06108 nice cedex 02, France. e-mail: thierry.thibaut@unice.fr suMMary: The impact of Caulerpa taxifolia on the structure of shallow Posidonia oceanica beds was studied in permanent quadrats from 1995 to 2005 at the invaded site of Cap Martin and the control site of Cap d'antibes (French riviera, France). The cover of C. taxifolia, shoot density, number of orthotropic and plagiotropic shoots and proportion of ramifications of P. oceanica were measured yearly. The cover of C. taxifolia in the invaded zone rapidly reached a maximum of infestation in 2000 with 93% of the quadrats covered by the alga. in 2001 an unexplained phenomenon led to a sharp decrease in the infestation and in the following years the colonisation remained low. Within the 10 years of the study, P. oceanica did not disappear from the permanent quadrats, but we observed a drastic change in the structure of the meadow invaded by C. taxifolia. between 1999 and 2000 a decrease in the shoot density observed at both sites was probably related to the warm temperature event recorded in 1999 (from 636 to 143 shoots m -2 at the invaded site and from 488 to 277 at the control site). at the invaded site, the seagrass never recovered its initial density even after a sharp decrease in C. taxifolia. The orthotropic/ plagiotropic shoot ratio was strongly modified at the invaded site, where plagiotropic shoots became dominant because of an increase in their ramification.Keywords: Caulerpa taxifolia, Posidonia oceanica, competition, ecological impact.resuMen: Alteraciones en la estructura de praderas de Posidonia oceanica debido al alga introducida caulerPa taxifolia. -el impacto de Caulerpa taxifolia sobre la estructura de la comunidad de Posidonia oceanica poco profunda se estudió entre 1995 y 2005 mediante parcelas permanentes en una zona invadida de Cap Martin y una zona control en Cap d'antibes (riviera Francesa, Francia). Para ello, se midieron anualmente la densidad de C. taxifolia, y la densidad de haces, el número de haces ortotrópicos y plagiotrópicos y la proporción de ramificaciones de P. oceanica. la densidad de C. taxifolia en la zona invadida alcanzó un máximo en 2000, con una cobertura del 93%, y a partir del año 2001 disminuyó drásticamente, dejando valores relativamente bajos en los años siguientes. durante los 10 años de estudio, la pradera de P. oceanica no desapareció en las zonas invadidas por C. taxifolia, aunque se observaron cambios drásticos en su estructura. en 2000, se observó una disminución en la densidad de haces de P. oceanica, coincidiendo con la máxima cobertura de C. taxifolia (de 636 a 143 haces m -2 en la zona invadida y de 488 a 277 en la zona control). entre 1999 y 2000 en los dos sitios se observó una disminución de la densidad probablemente relacionada con las altas temperaturas registradas en 1999. la densidad de haces de P. oceanica no recuperó los valores iniciales, incluso después de la di...

Does habitat complexity influence fish recruitment?

“…In the Caulerpa spp. case, the invading species act as exotic engineers and tend to homogenize the sea-scape and reduce the diversity of habitats and microhabitats available (Harmelin-Vivien et al, 2001;Molenaar et al, 2009). Our study suggests it is relevant to bring new information about the fish recruitment patterns that operate in these transformed systems, as well as about their causes.…”

“…For example, beach artificial nourishment is known to homogenize the mixed heterogeneous bottoms of pebbles, boulders and rocks, therefore reducing their habitat quality for Sparidae fish juveniles . Among macrophytes, seascape homogenization has been as well reported through the introduction and dominance of invasive habitat-forming species, such as Caulerpa taxifolia (Vahl) C. Agardh and C. cylindracea Sonder, two benthic macroalgae (Chlorophyta) that have been introduced into the Mediterranean (Levi & Francour, 2004;Longepierre et al, 2005;Klein & Verlaque, 2008;Francour et al, 2009;Molenaar et al, 2009;Box et al, 2010;Tomas et al, 2011). In many coastal sites, heterogeneous habitats such as rocky reefs (Cebrian et al, 2012), sandy bottoms, or seagrass meadows tend consequently to be replaced by homogenous Caulerpa spp.…”