Ecological and functional consequences of coastal ocean acidification: Perspectives from the Baltic-Skagerrak System

. 2018 Dec 1;48(8):831–854. doi: 10.1007/s13280-018-1110-3

Box 3: Chemical consequences of ocean acidification in the Baltic-Skagerrak System
In the surface mixed layer of the oceans, the photosynthetic capture of light energy to combine CO₂, macro-nutrients (such as nitrate and phosphate), and micro-nutrients in the form of trace metals (Me²⁺, such as iron(II)) to create organic matter and oxygen can be formulated as $140 {CO}_{2} + {16 H}^{+} + {16 NO}_{3}^{-} + {HPO}_{4}^{2 -} + {Me}^{2 +} + {123 H}_{2} O \to {({CH}_{2} O)}_{91} {({CH}_{2})}_{16} {({NHCH}_{2} CO)}_{16} ({CHPO}_{4} Me) + {172 O}_{2}$ (1) Decomposition of sedimenting organic matter in deeper water runs in the opposite direction, releasing CO₂ and H⁺. In the following formulation, this CO₂ release is illustrated by balancing it to the bicarbonate ion, HCO₃⁻, the dominating form of dissolved inorganic carbon at typical seawater pH (Box 1). ${({CH}_{2} O)}_{91} {({CH}_{2})}_{16} {({NHCH}_{2} CO)}_{16} ({CHPO}_{4} Me) + {172 O}_{2} + {17 H}_{2} O \to 140 {HCO}_{3}^{-} + {156 H}^{+} + {16 NO}_{3}^{-} + {HPO}_{4}^{2 -} + {Me}^{2 +}$ (2) Thus decomposition produces hydrogen ions, and hence lowers pH. This reaction normally occurs deep in the water column or at the sediment surface. In waters with limited exchange, the decomposition process (reaction 2) can sometimes completely deplete the available oxygen, resulting in strongly hypoxic or anoxic bottom water. Under these circumstances, other “electron acceptors” are needed to replace oxygen in the decomposition process. The most energetically favourable electron acceptor after oxygen is nitrate, and hence in hypoxic and anoxic areas, decomposition leads to denitrification: ${({CH}_{2} O)}_{91} {({CH}_{2})}_{16} {({NHCH}_{2} CO)}_{16} C ({MeHPO}_{4}) + {112 NO}_{3}^{-} \to 140 {HCO}_{3}^{-} + {12 H}^{+} + {56 N}_{2} + {16 NH}_{4}^{+} + {HPO}_{4}^{2 -} + {Me}^{2 +} + {23 H}_{2} O$ (3) When comparing reactions (2) and (3), it can be seen that denitrification generates far fewer hydrogen ions per bicarbonate ion produced. If decomposition proceeds to deplete all the nitrate, then other electron acceptors step in. In seawater, these are (in order) manganese(IV), iron(III) and sulphate. When these are used as electron acceptors the following reactions (4–6) occur (here organic matter is simplified to “carbohydrates”; CH₂O_(org)): ${CH}_{2} O_{(org)} + 2 Mn (IV) O_{2} + {3 H}^{+} \to {HCO}_{3}^{-} + {2 H}_{2} O + 2 Mn {(II)}^{2 +}$ (4) ${CH}_{2} O_{(org)} + 4 Fe (III) OOH + {7 H}^{+} \to {HCO}_{3}^{-} + {6 H}_{2} O + 4 Fe {(II)}^{2 +}$ (5) ${CH}_{2} O_{(org)} + 0.5 {SO}_{4}^{2 -} \to {HCO}_{3}^{-} + 0.5 H^{+} + 0.5 {HS}^{-}$ (6) These reactions have very different impacts on pH as both manganese and iron reduction consume H⁺, whereas sulphate reduction produces H⁺. An important consequence of this is that the sulphide bottom-water that occurs in the Baltic Proper has close to constant pH, even if the sulphide concentration increases with depth (Fig. 5). Note that temperature affects these biogeochemical reactions as well as the solubility of gases, which results in lower pH (more CO₂) in colder waters in equilibrium with the atmosphere. These biochemical processes also occur in the sediment, especially in surface layers where “bioturbation” by the fauna causes mixing of interstitial water with deep waters in the water column. When anoxic water meets oxic water the reduced chemical species are oxidised in reactions that also involve hydrogen ions. For example, when iron(II) is oxidised H⁺ is produced: $Fe {(II)}^{2 +} + {2 H}_{2} O \to Fe (III) OOH + {2 H}^{+}$ (7) But when hydrogen sulphide is oxidised to elemental sulphur H⁺ is consumed: ${HS}^{-} + 0.5 O_{2} + H^{+} \to H_{2} O + S^{0}$ (8) Or if iron sulphide precipitates H⁺ is produced: $Fe {(II)}^{2 +} + {HS}^{-} \to FeS (s) + H^{+}$ (9)

Box 3: Chemical consequences of ocean acidification in the Baltic-Skagerrak System

In the surface mixed layer of the oceans, the photosynthetic capture of light energy to combine CO₂, macro-nutrients (such as nitrate and phosphate), and micro-nutrients in the form of trace metals (Me²⁺, such as iron(II)) to create organic matter and oxygen can be formulated as

$140 {CO}_{2} + {16 H}^{+} + {16 NO}_{3}^{-} + {HPO}_{4}^{2 -} + {Me}^{2 +} + {123 H}_{2} O \to {({CH}_{2} O)}_{91} {({CH}_{2})}_{16} {({NHCH}_{2} CO)}_{16} ({CHPO}_{4} Me) + {172 O}_{2}$ (1)

Decomposition of sedimenting organic matter in deeper water runs in the opposite direction, releasing CO₂ and H⁺. In the following formulation, this CO₂ release is illustrated by balancing it to the bicarbonate ion, HCO₃⁻, the dominating form of dissolved inorganic carbon at typical seawater pH (Box 1).

${({CH}_{2} O)}_{91} {({CH}_{2})}_{16} {({NHCH}_{2} CO)}_{16} ({CHPO}_{4} Me) + {172 O}_{2} + {17 H}_{2} O \to 140 {HCO}_{3}^{-} + {156 H}^{+} + {16 NO}_{3}^{-} + {HPO}_{4}^{2 -} + {Me}^{2 +}$ (2)

Thus decomposition produces hydrogen ions, and hence lowers pH. This reaction normally occurs deep in the water column or at the sediment surface. In waters with limited exchange, the decomposition process (reaction 2) can sometimes completely deplete the available oxygen, resulting in strongly hypoxic or anoxic bottom water. Under these circumstances, other “electron acceptors” are needed to replace oxygen in the decomposition process. The most energetically favourable electron acceptor after oxygen is nitrate, and hence in hypoxic and anoxic areas, decomposition leads to denitrification:

${({CH}_{2} O)}_{91} {({CH}_{2})}_{16} {({NHCH}_{2} CO)}_{16} C ({MeHPO}_{4}) + {112 NO}_{3}^{-} \to 140 {HCO}_{3}^{-} + {12 H}^{+} + {56 N}_{2} + {16 NH}_{4}^{+} + {HPO}_{4}^{2 -} + {Me}^{2 +} + {23 H}_{2} O$ (3)

When comparing reactions (2) and (3), it can be seen that denitrification generates far fewer hydrogen ions per bicarbonate ion produced. If decomposition proceeds to deplete all the nitrate, then other electron acceptors step in. In seawater, these are (in order) manganese(IV), iron(III) and sulphate. When these are used as electron acceptors the following reactions (4–6) occur (here organic matter is simplified to “carbohydrates”; CH₂O_(org)):

${CH}_{2} O_{(org)} + 2 Mn (IV) O_{2} + {3 H}^{+} \to {HCO}_{3}^{-} + {2 H}_{2} O + 2 Mn {(II)}^{2 +}$ (4)

${CH}_{2} O_{(org)} + 4 Fe (III) OOH + {7 H}^{+} \to {HCO}_{3}^{-} + {6 H}_{2} O + 4 Fe {(II)}^{2 +}$ (5)

${CH}_{2} O_{(org)} + 0.5 {SO}_{4}^{2 -} \to {HCO}_{3}^{-} + 0.5 H^{+} + 0.5 {HS}^{-}$ (6)

These reactions have very different impacts on pH as both manganese and iron reduction consume H⁺, whereas sulphate reduction produces H⁺. An important consequence of this is that the sulphide bottom-water that occurs in the Baltic Proper has close to constant pH, even if the sulphide concentration increases with depth (Fig. 5). Note that temperature affects these biogeochemical reactions as well as the solubility of gases, which results in lower pH (more CO₂) in colder waters in equilibrium with the atmosphere.

These biochemical processes also occur in the sediment, especially in surface layers where “bioturbation” by the fauna causes mixing of interstitial water with deep waters in the water column. When anoxic water meets oxic water the reduced chemical species are oxidised in reactions that also involve hydrogen ions. For example, when iron(II) is oxidised H⁺ is produced:

$Fe {(II)}^{2 +} + {2 H}_{2} O \to Fe (III) OOH + {2 H}^{+}$ (7)

But when hydrogen sulphide is oxidised to elemental sulphur H⁺ is consumed:

${HS}^{-} + 0.5 O_{2} + H^{+} \to H_{2} O + S^{0}$ (8)

Or if iron sulphide precipitates H⁺ is produced:

$Fe {(II)}^{2 +} + {HS}^{-} \to FeS (s) + H^{+}$ (9)