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Mediterranean Bioconstructions Along the Italian Coast 101

sensitivity to high summer temperatures may increase tissue mortality in
A. calycularis, which in turn makes the corals more susceptible to ocean
acidification affecting net calcification.
Today’s surface ocean is saturated with respect to calcium carbonate,
but increasing atmospheric carbon dioxide concentrations are reducing
ocean pH and carbonate ion concentrations, thus reducing the level of cal-
cium carbonate saturation (Gattuso et al., 2015). If these trends will continue,
key marine bioconstructors, such as corals, shellfish and calcified algae, will be
threatened. Direct physiological effects of decreased pH on bioconstructors
typically include changes in survival, calcification, growth, development,
reproduction and abundance (Kroeker et al., 2013). A recent work revealed
that pH reduction impairs the recruitment success of the reef-building gastro-
pod D. cristatum (as D. petraeum), likely causing shell dissolution of the recruits
and altering their shell mineralogy (Milazzo et al., 2014).
The coralline algae are vulnerable to ocean acidification (Fabricius et al.,
2015; McCoy and Kamenos, 2015) due to their high-Mg calcite skeletons
that are more soluble than other forms of calcium carbonate (Andersson
et al., 2008); hence, ocean acidification will impact the coralline algae
(Cornwall et al., 2017). Martin and Gattuso (2009) concluded that, on
the long term, a combination of increased pCO 2 and elevated temperatures
may lead to dissolution exceeding calcification in Lithophyllum cabiochiae
(Boudouresque & Verlaque) Athanasiadis, one of the main bioconstructor
coralline algae in the coralligenous. These authors, however, reported that
the intensity of this effect is seasonally variable. Differences in sensitivity to
acidification can also be expected in relation to habitat and depth; for
instance, slow-flow habitats (such as deep coralligenous habitats) might
become refugia from ocean acidification for calcifying organisms (Hurd,
2015). However, the areas of dense water formation are preferential sites
for atmospheric carbon dioxide absorption and through them the ocean
acidification process can quickly propagate into the deep layers. In the Gulf
of Lions and in the Adriatic Sea, the two most active deep convection area of
the Mediterranean Sea (Boero, 2015), the ocean acidification of the deep
layer can be particularly strong (Ingrosso et al., 2017b; Touratier et al.,
2016) and this may negatively affect CWCs in the near future (Gori
et al., 2016).
Scaling up the direct effects of ocean acidification from single species to
ecosystem-level parameters, such as habitat complexity or density of bio-
constructions, is difficult (Sunday et al., 2017). However, an overall reduction
of structural complexity has been detected in the biogenic habitat that rely on
calcification for their structure (Kroeker et al., 2013), which in turn may