Epiphytic hydroids on <i>Posidonia oceanica</i> seagrass meadows are winner organisms under future ocean acidification conditions: evidence from a CO₂ vent system (Ischia Island, Italy)

Effects of ocean acidification (OA) on the plant phenology and colonization/settlement pattern hydrozoan epibiont community leaves seagrass Posidonia oceanica have been studied at volcanic CO2 vents off Ischia (Italy). The study was conducted in shallow stands (2.5–3.5 m depth), three stations north south sides vent’s area (Castello Aragonese vents), distributed along a pH gradient. At each station, 10–15 P. shoots were collected every months for one-year cycle (Sept 2009–2010). shoot density beds most acidified gradient (pH < 7.4) significantly higher than that control = 8.10). On other hand, we recorded lower leaf lengths widths whole year observations, compared to those stations. However, overall surface (Leaf Area Index) available epiphytes under conditions side both because OA conditions. canopy accounted seven species, which relatively abundant occurring all around (Sertularia perpusilla, Plumularia obliqua, Clytia hemisphaerica). All hydroids species showed clear tolerance low levels, including chitinous non-calcifying forms, likely favoured also by absence competition substratum with calcareous forms selected against OA.