Evaluation of the effects of Caulerpa cylindracea on Posidonia oceanica through the analysis of primary production and morphometric characteristics

Today, biological invasions represent a threat to endemic animal and plant communities and a major cause of biodiversity loss worldwide. In the Mediterranean Sea, there are about 100 species of macrophytes, introduced intentionally or accidentally, most of which are highly invasive. Among these, the macroalga Caulerpa cylindracea Sonder, 1845, entered in the Mediterranean basin since 1990 through the Suez Canal and now it is widespread along the Italian coasts. This species is able to colonize a high number of coastal substrates and it can affect the density of some seagrasses, such as Cymodocea nodosa (Ucria) Ascherson, 1870 and Posidonia oceanica (L.) Delile, 1813. Its colonization ability is enhanced in environments with a high concentration of nutrients and its growth can modify the redox potential of the substrate making it unsuitable for the establishment of other seagrasses and algae. This work aimed to analyse and describe the potential interaction between the C. racemosa and P. oceanica in the coastal area of Civitavecchia. The potential effects of this interaction were studied inside of two different P. oceanica patches, located at a depth of 3-5 m and characterized by the presence/absence of the invasive alga, through the morphostructural analysis of the two species. In particular, the seagrass growth and primary production were analysed using some direct and indirect techniques (phenology and lepidochronology), while for the alga were analysed the phenological characteristics and the percentage of coverage of the substrate. The sampling campaigns were carried out in two different months of the same year, June and October 2019, in order to observe both the growth phase and the maximum bloom phase of the C. racemosa.