2.5 Threats to Coral Reefs
39
to adsorb metals (Crist et al. 1988; Wilde et al. 2006). Hence, the effect of pH on
the speciation of metal-organic complexes in the marine environment is not as well
characterized as the inorganic ligands due to their heterogeneous composition and
the unknown structure of the organic ligands (Millero et al. 2009; Zeng et al. 2015).
Fluctuations in pH also affect the oxidation and reduction reaction rates of metals
and their photochemical processes (Millero et al. 2009; Zeng et al. 2015). In the ocean
surface waters, photochemical processes produce a number of free radicals that can
change the oxidation state of a number of metals (Millero et al. 2009). Acidification
usually increases the reduction rates more than the oxidation rates, as the latter is
less pH dependent (Zeng et al. 2015). For example, the half-life of Fe (II) in seawater
increases from 1 min to 24 min when pH decreases from 8.1 to 7.4 (Millero et al.
2009).
The speciation of metals changes due to ocean acidification and a shift in speciation leads to changes in the solubility, bioavailability and toxicity of those metals
(Diaz and Rosenberg 2008; Millero et al. 2009; Marangoni et al. 2019).
A lowered pH causes a reduction in the concentrations of OH
− and CO 3
2− , which
forms strong bonds in ocean water with divalent and trivalent cations (Zeng et al.
2015). Metals forming strong complexes with OH
− and CO 3
2− have a higher fraction
in their free forms at lower pH (Millero et al. 2009) and thus ocean acidification could
transform heavy metals into a specification more toxic to biota (Zeng et al. 2015).
Sediments become the ultimate reservoir for pollutants such as metals discharged
into the marine environment and hence metals become unavailable. Metals in the
sediment reside in different geochemical fractions, namely exchangeable and water
soluble, carbonate-, organic-, Fe–Mn oxide-bound and residual and shows a high
variation in changing factors such as pH, granulometry, temperature (Wang et al.
2015). Under reduced pH conditions, metals desorb from the sediments and organic
ligands causing an elevated flux of dissolved metals into the water column (de Orte
et al. 2014a, b) thus causing toxicity. Reactions such as adsorption/desorption, dissolution of carbonates, sulphides and iron oxyhydroxide occur at the sediment-water
interface.
Hence, the coexistence of ocean acidification and pollution in many coastal regions
may have combined effects on marine ecosystems in the forseeable future (Zeng
et al. 2015). Bielmyer-Fraser et al. (2018) highlighted that both metal pollution and
ocean acidification have shown to cause deleterious effects in aquatic organisms
individually; however, the problem of dual exposure may be exacerbated because
lower pH (increased acidification) causes changes in metal speciation, resulting in a
shift to more toxic ionic metal species. Negri and Hoogenboom (2011), Fonseca et al.
(2017) and Banc-Prandi and Fine (2019) deliberated that in the context of an era of
climate change, the toxicity of metals is expected to be enhanced due to increasing
seawater temperature and acidification, thus resulting in more severe impacts to coral
reefs. Ocean acidification is a global phenomenon; with its impacts expressed at local
levels (Lebrec et al. 2019).
39
to adsorb metals (Crist et al. 1988; Wilde et al. 2006). Hence, the effect of pH on
the speciation of metal-organic complexes in the marine environment is not as well
characterized as the inorganic ligands due to their heterogeneous composition and
the unknown structure of the organic ligands (Millero et al. 2009; Zeng et al. 2015).
Fluctuations in pH also affect the oxidation and reduction reaction rates of metals
and their photochemical processes (Millero et al. 2009; Zeng et al. 2015). In the ocean
surface waters, photochemical processes produce a number of free radicals that can
change the oxidation state of a number of metals (Millero et al. 2009). Acidification
usually increases the reduction rates more than the oxidation rates, as the latter is
less pH dependent (Zeng et al. 2015). For example, the half-life of Fe (II) in seawater
increases from 1 min to 24 min when pH decreases from 8.1 to 7.4 (Millero et al.
2009).
The speciation of metals changes due to ocean acidification and a shift in speciation leads to changes in the solubility, bioavailability and toxicity of those metals
(Diaz and Rosenberg 2008; Millero et al. 2009; Marangoni et al. 2019).
A lowered pH causes a reduction in the concentrations of OH
− and CO 3
2− , which
forms strong bonds in ocean water with divalent and trivalent cations (Zeng et al.
2015). Metals forming strong complexes with OH
− and CO 3
2− have a higher fraction
in their free forms at lower pH (Millero et al. 2009) and thus ocean acidification could
transform heavy metals into a specification more toxic to biota (Zeng et al. 2015).
Sediments become the ultimate reservoir for pollutants such as metals discharged
into the marine environment and hence metals become unavailable. Metals in the
sediment reside in different geochemical fractions, namely exchangeable and water
soluble, carbonate-, organic-, Fe–Mn oxide-bound and residual and shows a high
variation in changing factors such as pH, granulometry, temperature (Wang et al.
2015). Under reduced pH conditions, metals desorb from the sediments and organic
ligands causing an elevated flux of dissolved metals into the water column (de Orte
et al. 2014a, b) thus causing toxicity. Reactions such as adsorption/desorption, dissolution of carbonates, sulphides and iron oxyhydroxide occur at the sediment-water
interface.
Hence, the coexistence of ocean acidification and pollution in many coastal regions
may have combined effects on marine ecosystems in the forseeable future (Zeng
et al. 2015). Bielmyer-Fraser et al. (2018) highlighted that both metal pollution and
ocean acidification have shown to cause deleterious effects in aquatic organisms
individually; however, the problem of dual exposure may be exacerbated because
lower pH (increased acidification) causes changes in metal speciation, resulting in a
shift to more toxic ionic metal species. Negri and Hoogenboom (2011), Fonseca et al.
(2017) and Banc-Prandi and Fine (2019) deliberated that in the context of an era of
climate change, the toxicity of metals is expected to be enhanced due to increasing
seawater temperature and acidification, thus resulting in more severe impacts to coral
reefs. Ocean acidification is a global phenomenon; with its impacts expressed at local
levels (Lebrec et al. 2019).