Effects of in situ CO2 enrichment on epibiont settlement on artificial substrata within a Posidonia oceanica meadow

Abstract

International audienceAlterations to colonization or early post-settlement stages may cause the reorganization of communities under future ocean acidification conditions. Yet, this hypothesis has been little tested by in situ pH manipulation. A Free Ocean Carbon Dioxide Enrichment (FOCE) system was used to lower pH by a ~ 0.3 unit offset within a partially enclosed portion (1.7 m3) of a Posidonia oceanica seagrass meadow (11 m depth) between 21 June and 3 November 2014. Epibiont colonization and early post settlement stages were assessed within the FOCE setup, as part of the larger community-level study, to better understand the outcome for a multispecies assemblage and the ecological processes that result in reported community shifts under altered carbonate chemistry. Two types of artificial collectors (tiles and scourers) were placed within three treatments: a pH-manipulated enclosure, an un-manipulated control enclosure, and an open plot in the ambient meadow. Tiles and scourers were collected after one to four months. Additionally, to see whether the outcome differed for communities in a later successional stage, previously settled scourer-collectors were also placed in the same three treatments. Enclosures acted to reduce settlement and migrant colonization. Scourers deployed for one to four months within the open-plot contained a community assemblage that could be distinguished from the assemblages within the enclosures. However, a comparison of enclosure assemblages on tiles showed evidence of a pH effect. There was lowered coverage of crustose coralline algae and fewer calcareous tube-forming polychaetes (Spirorbis sp. and Spirobranchus sp.) on tiles placed in the pH-manipulated enclosure compared to the un-manipulated enclosure. For assemblages in scourer collectors, shared and common taxa, in all treatments, were invertebrate polychaetes Psamathe fusca, Sphaerosyllis sp., Chrysopetalum sp., arthropods Harpacticoida, and Amphipoda, and the juvenile bivalve Lyonsia sp. Similar organism composition and abundance, as well as taxonomic richness and evenness, were found in scourers from both enclosures. Pre-settled scourers contained greater numbers of individuals and more calcified members, but the assemblage, as well as the growth rate of a juvenile bivalve Lyonsia sp., appeared unaffected by a two-month exposure to lowered pH and calcium carbonate saturation state. Results from this case study support the hypothesis that early stages of specific calcifiers (crustose coralline algae and calcareous tube-forming polychaetes) are sensitive to near future ocean acidification conditions yet suggest that negative effects on sessile micro-invertebrate assemblages will be minimal

Coralline Algae in a Changing Mediterranean Sea: How Can We Predict Their Future, if We Do Not Know Their Present?

“…The ability of some thalli to continue growing in lower pH suggests acclimation/adaptation to low pH conditions and ability to provide recruits for populations adapted to survive in lower pH environment in the future. Through in situ pH manipulation using a FOCE system in a Posidonia oceanica meadow, Cox et al (2017a) also found that early stages of CCA are sensitive to decreased pH, with lower coverage of CCA on recruitment tiles placed in a pHmanipulated enclosure (−0.3 pH unit offset) compared to an un-manipulated enclosure (ambient pH). Although previous studies suggested post-settlement competition between fleshy and calcareous algae (Porzio et al, 2011;Kroeker et al, 2012;Kamenos et al, 2016), Cox et al (2017a) suggested that losses of CCA were driven by taxa sensitivity, because the other taxa were also reduced by the lower pH conditions and there was still bare space available for colonization.…”

“…The responses of Mediterranean corallines to climate change and ocean acidification have been the subject of several studies that documented their tolerance or sensitivity to elevated temperatures and CO 2 . The effects of elevated CO 2 on Mediterranean coralline algae were studied on single species (Martin and Gattuso, 2009;Martin et al, 2013a,b), in association with a small group of taxa (Asnaghi et al, 2013) and at the community scale (Porzio et al, 2011;Kroeker et al, 2012;Cox et al, 2015Cox et al, , 2017aMarchini et al, 2019). These investigations have focused on a few species, mainly restricted to the group of (non-geniculate) crustose coralline algae (CCA), including epiphytic (on seagrass leaves) and engineering (coralligenous bio-constructors) species.…”

The Impacts of Ocean Acidification on Marine Ecosystems and Reliant Human Communities

“…Additionally, seagrasses are sensitive to water quality and benthic light levels, so acidification effects on plankton dynamics may also play a role (138). While epiphytes that produce calcium carbonate structures are expected to decrease with acidification (93,137), enhanced seagrass production may protect some calcareous species very close to the seagrass tissues in low flow environments (139). In contrast, fleshy epiphytic algae are largely expected to benefit from high pCO 2 (140).…”

Ocean acidification affects calcareous tube growth in adult stage and reared offspring of serpulid polychaetes

“…Also, the density of epiphytic Spirorbis sp. on P. oceanica seagrass leaves appeared to be unaffected in both enclosures, leading to the conclusion that the chemically mediated boundary layer around the leaf provided an ocean acidification refuge (Cox et al, 2017). In contrast, Spirorbis sp.…”

“…Observational studies along CO 2 vents have found a reduction in the number of serpulid individuals nearer to the vent source, citing both direct effects on calcification and decreased competitive ability with moretolerant species as an explanation for this (Cigliano et al, 2010;Donnarumma et al, 2014;Martin et al, 2008). Species of Spirorbis that settle on photosynthesizing organisms can be somewhat buffered from ocean acidification effects by the chemically mediated environment provided by their host (Cox et al, 2017;Hendriks et al, 2014;Saderne and Wahl, 2013). However, to be buffered, the benefits during the day need to be great enough to offset the negative effects of respiration at night.…”