Abstract
SUMMARY: A two-year study focusing on the associated polychaete assemblages revealed that the degradation of the Posidonia oceanica seagrass meadows, together with the colonisation of rhizomes by invasive Caulerpa racemosa in the Balearic Islands, have produced important changes in the ecosystem functioning, and have therefore affected the benthic faunal communities. The highest abundance and number of species occurred in C. racemosa from August to December. The species composition of the polychaete assemblage in C. racemosa is similar to that of P. oceanica but abundances of shared species differ between Caulerpa and Posidonia. Abundance, number of species and diversity were positively correlated with C. racemosa biomass. The spatial complexity provided by the network of C. racemosa fronds and stolons seemed to combine with that of the remaining seagrass mat to support a well developed polychaete assemblage. Although invaded meadows harboured significantly modified polychaete assemblages when compared with living P. oceanica meadows, the main impacts seem to be non-harmful (i.e. increased abundance and number of species) and mainly affect their seasonal patterns, which is a direct consequence of the corresponding biomass seasonality of C. racemosa.
Highlights
The presence of alien species in the Mediterranean, currently about 662 (Gollasch, 2006), is an increasing problem that still needs to be studied to clarify its consequences
SUMMARY: A two-year study focusing on the associated polychaete assemblages revealed that the degradation of the Posidonia oceanica seagrass meadows, together with the colonisation of rhizomes by invasive Caulerpa racemosa in the Balearic Islands, have produced important changes in the ecosystem functioning, and have affected the benthic faunal communities
The species composition of the polychaete assemblage in C. racemosa is similar to that of P. oceanica but abundances of shared species differ between Caulerpa and Posidonia
Summary
The presence of alien species in the Mediterranean, currently about 662 (Gollasch, 2006), is an increasing problem that still needs to be studied to clarify its consequences. In the Mediterranean Sea, 84 introduced macrophytes have been reported to date (Boudouresque and Verlaque, 2002), and two of them belong to the green algae genus Caulerpa: C. taxifolia and C. racemosa (Ceccherelli and Sechi, 2002). Their presence induces drastic changes in faunistic composition and abundance of the affected systems, and they have become a serious problem in the western Mediterranean (Pandolfo and Chemello, 1995; Bellan-Santini et al, 1996; Argyrou et al, 1999; Buia et al, 2001). Caulerpa species produce anti-herbivorous secondary metabolites (Amade and Lemee, 1998; Jung et al, 2002), the main one being caulerpenyne, which negatively affect grazing organisms (Boudouresque et al, 1996; ParentMassin et al, 1996; Amade and Lemee, 1998; Pesando et al, 1998; Sureda et al, 2006; Sureda et al, 2008) and chemically change the environmental conditions of the habitat (Amade and Lemee, 1998; Sureda et al, 2006)
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