Can the Marine Ecosystem of a Posidonia oceanica Back-reef React and Defend Itself against the Spread of Caulerpa racemosa var. cylindracea?

Abstract

A back-reef of Posidonia oceanica (Santa Liberata, Orbetello, Italy) subject to degradation lost its typical mixed meadow of Cymodocea nodosa , Nanozostera noltii and Caulerpa prolifera and was colonised by the invasive chlorophycea Caulerpa racemosa var. cylindracea ( C. racemosa ) between 2003 and 2004. When the submerged flora behind the P. oceanica barrier reef was studied between 2005 and 2006, C. racemosa constituted 25% of the macroalgal biomass and showed high cover (>50%). Residual dead patches of the mixed meadow had been colonized by Penicillus capitatus . By 2011, C. racemosa had fallen to about 3% of total algal mass and its cover had also dropped (5-25%), while other typical species predominated: P. oceanica dead mattes were covered by thin dense mats of photophile species dominated by Jania rubens and Cladophora sp. Periodically, the latter produced balls that floated freely on the bottom. We compared the lists of phytobenthic flora for 2005, 2006 and 2011 by explorative correspondence analysis. The 2011 list showed a 27% increase in autochthonous species. The results suggest that the invasion of allochthonous fast-spreading C. racemosa may occur in degraded ecosystems after events that altered the original community. The community can recover through a succession of vegetation changes.