Effects of in situ CO<sub>2</sub> enrichment on structural characteristics, photosynthesis, and growth of the Mediterranean seagrass <i>Posidonia oceanica</i>

Abstract

Abstract. Seagrass is expected to benefit from increased carbon availability under future ocean acidification. This hypothesis has been little tested by in situ manipulation. To test for ocean acidification effects on seagrass meadows under controlled CO2/pH conditions, we used a Free Ocean Carbon Dioxide Enrichment (FOCE) system which allows for the manipulation of pH as continuous offset from ambient. It was deployed in a Posidonia oceanica meadow at 11 m depth in the Northwestern Mediterranean Sea. It consisted of two benthic enclosures, an experimental and a control unit both 1.7 m3, and an additional reference plot in the ambient environment (2 m2) to account for structural artifacts. The meadow was monitored from April to November 2014. The pH of the experimental enclosure was lowered by 0.26 pH units for the second half of the 8-month study. The greatest magnitude of change in P. oceanica leaf biometrics, photosynthesis, and leaf growth accompanied seasonal changes recorded in the environment and values were similar between the two enclosures. Leaf thickness may change in response to lower pH but this requires further testing. Results are congruent with other short-term and natural studies that have investigated the response of P. oceanica over a wide range of pH. They suggest any benefit from ocean acidification, over the next century (at a pH of ∼ 7.7 on the total scale), on Posidonia physiology and growth may be minimal and difficult to detect without increased replication or longer experimental duration. The limited stimulation, which did not surpass any enclosure or seasonal effect, casts doubts on speculations that elevated CO2 would confer resistance to thermal stress and increase the buffering capacity of meadows.

High Net Primary Production of Mediterranean Seagrass (Posidonia oceanica) Meadows Determined With Aquatic Eddy Covariance

“…Light saturation at the nearshore meadow (77 ± 15 µmol photons m −2 s −1 ) and at the open-water meadow (113 ± 21 µmol photons m −2 s −1 ) occurred at less than half of peak irradiance. These saturating irradiances substantially exceed those determined in other studies using rapid light curves to determine electron transport rates on P. oceanica leaves (Dattolo et al, 2014;Cox et al, 2016). However, our results match saturation irradiances of P. oceanica leaf segments in some laboratory incubations (Pirc, 1986;Ruiz and Romero, 2003), and are well within the range reported in the literature (Drew, 1979;Alcoverro et al, 1998Alcoverro et al, , 2001b).…”

Concomitant ocean acidification and increasing total alkalinity at a coastal site in the NW Mediterranean Sea (2007-2015)

“…Anthropogenic forcing of CO 2 in seawater necessitates long-term monitoring, in order to understand and study future ecological change in the coastal environment. In the coastal NW Mediterranean Sea at EOL, near Point B, high-frequency pH data validated that weekly, morning sampling at Point B was sufficient to capture water mass changes that were independent from benthic, diel pH variability in seagrass beds inside the bay (Cox et al, 2016 observed changes in carbonate chemistry at this location. The increase in C T was the dominant driver of pH T change over the study period.…”

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

“…In the present study, the impact of ocean acidification on epibiont colonization and recruitment was assessed on artificial substrata in the framework of an in situ pH-manipulation experiment (European Free Ocean Carbon Dioxide Enrichment; eFOCE). This community-level experiment was designed to assess the impacts of ocean acidification on a P. oceanica community in the Northwestern Mediterranean Sea (Cox et al, 2016). The specific study presented here aimed at testing for the effects of ocean acidification on the micro-invertebrate assemblage colonizing artificial surfaces within a P. oceanica meadow and it tested the prediction that the sensitivity of early life history stages alters community formation.…”

“…This study used the European FOCE (eFOCE) system which allows for the in situ manipulation of pH in benthic enclosures as an offset from ambient pH (Cox et al, 2016). It was deployed in the Bay of Villefranche, approximately 300 m from the Laboratoire d'Océanographie de Villefranche (NW Mediterranean Sea, France; 43°40.73′N, 07°19.39′E).…”