OCEAN ACIDIFICATION

Oceans are considered an important reservoir of CO_2, absorbing a significant quantity of one-third of it produced by anthropogenic activities & effectively buffering climate change.

Ocean acidification is the change in ocean chemistry. Lowering of ocean pH i.e. increase in concentration of hydrogen ions driven by the uptake of carbon compounds by the ocean from the atmosphere.

Effect of Carbon dioxide

• If the uptake of atmospheric carbon dioxide by the ocean increases, the concentration of hydrogen ions in the ocean increases and the concentration of carbonate ions decreases, the pH of the oceans decreases & the oceans become less alkaline. This process is known as Ocean Acidification.
• The uptake of atmospheric carbon dioxide is occurring at a rate exceeding the natural buffering capacity of the ocean.
• The pH of the ocean surface waters decreased by about 0.1 pH unit(26% increase in ocean hydrogen ion concentration) since the beginning of the Industrial Revolution.
• The ocean currently has a pH of around 8.0 & is “Basic”, which is nearly impossible chemically, for all of it to become a pH less than 7.0.
• Acidification is the direction of travel, the trend, regardless of the starting point. Acidification refers to lowering pH from any starting point to any endpoint on the pH scale.

Calcium Carbonate

• Calcite & aragonite are two different forms of calcium carbonate.
• Calcite is the mineral form found in the shells of planktonic algae, amoeboid protists, corals, echinoderms and some molluscs (oysters); it is relatively less soluble.
• Aragonite is a more soluble form of calcium carbonate; it is found in most corals, most molluscs (planktonic snails) also some species of algae.

Other Factors

Various factors can locally influence the chemical reactions of carbon dioxide with seawater & add to the effects of ocean acidification.

Acid Rain: Acid rain can have a pH between 1 & 6 which has an impact on surface ocean chemistry. It has a major effect on ocean acidification locally & regionally but is very small globally.

Eutrophication: Coastal waters are also affected by excess nutrient inputs, mostly nitrogen from agriculture, fertilizers & sewage. The resulting eutrophication leads to large plankton blooms & when blooms collapse, and sink to the sea bed, the subsequent respiration of bacteria decomposing the algae leads to a decrease in seawater oxygen & an increase in carbon dioxide i.e, the decline in pH.

Reaction: The term ocean acidification summarizes several processes that occur when CO₂ reacts with seawater. Two reactions are particularly important. Firstly, the formation of carbonic acid with subsequent release of hydrogen ions;

CO₂            +          H₂O             →   H₂CO₃ ↔   H+     ↔    HCO3^-

^{(Carbon dioxide)               (Water)                          (Carbonic acid)           (Hydrogen ions)   (Bicarbonate ions)}

• The above reaction & release of hydrogen ions increases acidity and thus pH level is reduced.
• A second reaction, between carbonate ions, CO₂ & water produces bicarbonate ions.
• The combined effect of both these reactions not only increases acidity but also lowers the availability of carbonate ions.

1. Ocean Acidification Effects

• Sea water absorbs CO₂ to produce carbonic acid (H₂CO₃), bicarbonate ions(HCO₃-), carbonate ions(CO₃^2-).
• These carbonate ions are essential to the calcification process that allows certain marine organisms to build their calcium carbonate shells, and skeletons (Hard tropical corals, cold-water corals, molluscs, crustaceans, sea urchins, lobsters).
• An increase in atmospheric carbon dioxide levels leads to a decrease in pH level and, an increase in the concentration of carbonic acid & bicarbonate ions, causing a decrease in the concentration of carbonate ions.
• Thus carbonate ions are less available & calcification is therefore harder to achieve, maybe prevented altogether.
• This impact of ocean acidification may therefore have potentially catastrophic consequences for ocean life and marine species of economic importance.

Mitigation

• Promoting government policies to cap CO₂
• Reducing CO
• Eliminate offshore drilling.
• By advocating for energy efficiency.
• Alternative energy sources like wind power, solar etc.

Saturation Horizons

• Deep, cold ocean waters are naturally under-saturated with carbonate ions causing the shells of most calcifying organisms to dissolve.
• The saturation horizon is the level below which calcium carbonate minerals undergo dissolution.
• Surface waters are over-saturated with carbonate ions & do not readily dissolve shells of calcifying organisms.
• The organisms that can survive below the saturation horizon do so due to special mechanisms to protect their calcium carbonate from dissolving.
• As ocean acidification causes this horizon to rise vertically in the water column more calcifying organisms will be exposed to under-saturated water & hence vulnerable to dissolution of their shells, and skeletons.
• The saturation horizon of calcite occurs at a greater ocean depth than that for aragonite, but both horizons have moved closer to the surface presently when compared to the 1800s.

2. Fate of Carbon in the System

• On timescales more than 100,000 years, there is a natural balance maintained between the up-take & release of carbon dioxide on earth, the carbon dioxide produced by volcanoes, the main natural source of CO₂, is taken up by the production of organic matter by plants & by rock weathering on land.
• Rock weathering takes tens of thousands of years so will not remove the current anthropogenic input of CO₂ to the atmosphere & ocean fast enough.
• On shorter time scales less than 1,000 years, the ocean has an internal stabilizing feedback linking the ocean carbon cycle to the underlying carbonate-rich sediment known as carbonate compensation.
• The upper layers of the ocean tend to be supersaturated with CaCO₃, and little dissolution takes place, whilst the deep ocean is unsaturated and carbonate readily dissolves.
• The first boundary between these two states is known as the lysocline, the depth at which dissolution strongly increases in the deep ocean.
• The CaCO₃ in the form of dead shells sink to the sea bed. If it is of shallow water depth, the majority is buried in the sediment which was trapped for a long time. But the shells sink in deep water nearly all the CaCO₃ is dissolved, thereby not locking the carbon away for millions of years.
• The increased rate of dissolution of atmospheric CO₂ into the ocean results in an imbalance in the carbonate compensation depth (CCD), the depth at which all carbonate is dissolved.
• As the pH of the ocean falls, it results in a shallowing of the lysocline and the CCD, exposing more of the shells trapped in the sediments to unsaturated conditions causing them to dissolve, which will help buffer ocean acidification but over a long time scale of a thousand years.

Upwelling

• The coastal surface regions periodically experience upwelling events where deeper ocean water circulates onto continental shelves & near-shore areas.
• This exposes the productive upper ocean ecosystems to colder water containing more nutrients & more CO
• As ocean acidification makes the upper oversaturated layer of seawater shallower each year, the natural upwelling events will more often cause unsaturated water to well up and flow to the shore.
• Coastal marine organisms that form shells are unaccustomed to such events and periodic exposures to these significantly different conditions affect these communities.

The growth and level of photosynthesis of certain marine phytoplankton & plant species may increase with higher CO₂ levels, but this is by no means a general rule. For others, higher CO₂ and rising acidity may have either negative or neutral effects on their physiology.

Therefore, particular marine plants will be winners, while others will be losers and some may show no signs of change but change is inevitable.

A reduction in atmospheric CO₂ levels is essential to halt ocean acidification before it is too late.