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

Abstract

Effects of ocean acidification (OA) on the plant phenology and colonization/settlement pattern of the hydrozoan epibiont community of the leaves of the seagrass Posidonia oceanica have been studied at volcanic CO 2 vents off Ischia (Italy). The study was conducted in shallow Posidonia stands (2.5-3.5 m depth), in three stations on the north and three on the south sides of the vent's area (Castello Aragonese vents), distributed along a pH gradient. At each station, 10-15 P. oceanica shoots were collected every three months for one-year cycle (Sept 2009-2010). The shoot density of Posidonia beds in the most acidified stations along the gradient (pH < 7.4) was significantly higher than that in the control area (pH = 8.10). On the other hand, we recorded lower leaf lengths and widths in the acidified stations in the whole year of observations, compared to those in the control stations. However, the overall leaf surface (Leaf Area Index) available for epiphytes under ocean acidification conditions was higher on the south side and on both the most acidified stations because of the higher shoot density under OA conditions. The hydrozoan epibiont community on the leaf canopy accounted for seven species, three of which were relatively abundant and occurring all year around (Sertularia perpusilla, Plumularia obliqua, Clytia hemisphaerica). All hydroids species showed a clear tolerance to low pH levels, including chitinous and non-calcifying forms, likely favoured also by the absence of competition for substratum with the calcareous forms of epiphytes selected against OA.

The role of epiphytes in seagrass productivity under ocean acidification

“…Several studies showed that encrusting red algae (Corallinales) are especially vulnerable to acidification due to the sensitivity of their carbonate skeleton 25 , 34 , 40 . On the other side, hydrozoans show a higher tolerance to reduced seawater pH and can, therefore, outcompete more pH-sensitive species 41 . In contrast to Mecca et al (2020), we found that also bryozoans were negatively affected by low pH conditions.…”