Estuarine acidification happens when the
pH balance of water in coastal marine ecosystems, specifically those of
estuaries
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environmen ...
, decreases. Water, generally considered neutral on the
pH scale, normally perfectly balanced between
alkalinity
Alkalinity (from ar, القلوي, al-qaly, lit=ashes of the saltwort) is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale.
Alkalinity is the strength ...
and
acidity
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...
. While
ocean acidification
Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
occurs due to the ongoing decrease in the pH of the Earth's oceans, caused by the absorption of
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
(CO
2) from the atmosphere, pH change in estuaries is more complicated than in the open ocean due to direct impacts from land run-off, human impact, and coastal current dynamics. In the ocean, wave and wind movement allows carbon dioxide (CO
2) to mixes with water (H
2O) forming carbonic acid (H
2CO
3). Through wave motion this chemical bond is mixed up, allowing for the further break of the bond, eventually becoming
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
(CO
3) which is
basic and helps form shells for ocean creatures, and two
hydron Hydron has the following meanings:
* Hydron (chemistry)
In chemistry, the hydron, informally called proton, is the Ion, cationic form of atomic hydrogen, represented with the symbol . The general term "hydron", endorsed by the International Unio ...
molecules. This creates the potential for acidic threat since hydron ions readily bond with any
Lewis Structure
Lewis structures, also known as Lewis dot formulas, Lewis dot structures, electron dot structures, or Lewis electron dot structures (LEDS), are diagrams that show the bonding between atoms of a molecule, as well as the lone pairs of electrons t ...
to form an acidic bond. This is referred to as an
oxidation-reduction reaction.
The basic chemical equation is as follows:
:CO
2 + H
2O H
2CO
3 HCO
3 + H
+ CO
3 + 2 H
+
When this pattern of absorption is transferred into an estuary, however, acidity increases simply due to relative volume. Ocean water counts for the absorption of 30-40 percent of all CO
2 emitted into the atmosphere and yet, due to its immense volume, it remains relatively resilient.
Estuaries - being smaller by volume, sheltered from wave motion, and victim to human impact when in an urban setting - do not readily support the mixing of water, and thereby prevents basic breakdown. When this is combined with CO
2 from human impact such as car emissions or fertilizers, oxidation more readily occurs due to the overabundance of hydron ions and additional
cation, increasing the rate of occurrence and duration of acidification taking place. As the acidity of estuarine water levels continues to fluctuate, several species who use estuaries as spawning nurseries have seen decreases in reproduction levels.
Causes of variable pH
Freshwater flow
An
estuary
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...
is defined as "a water passage where the tide meets a river current". The pH of estuaries is highly variable because of freshwater flow from rivers and groundwater, as well as primary productivity (exacerbated by nutrient loading) and coastal upwelling. Fresh water from rivers typically has a lower pH than ocean water (~7 compared to ~8). Seasonal and annual changes in river flow entering an estuary can change the pH by whole units.
Photosynthesis and respiration
Primary production
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...
(plant growth) changes pH on a daily, seasonal, and annual basis. During photosynthesis, carbon dioxide is removed from the water, increasing pH. Organisms release carbon dioxide during respiration. This leads to a daily cycle of increased pH during daylight hours and a decrease in pH during the night, when respiration is dominant. Similarly, pH is higher during the winter when grazing is low compared to productivity.
Effluent
Many estuaries experience nutrient loading from runoff containing wastewater effluent or fertilizers, natural or artificial. Increased nutrients can stimulate primary productivity and alter the balance between primary productivity and respiration. This process can change pH by whole units within the estuary. Both these processes make it difficult to measure the overall change in pH associated with increased atmospheric carbon dioxide levels. This causes a change in pH by whole units in the estuary. This makes it hard to measure the overall change in pH, as well as the increased atmospheric carbon dioxide levels.
Currents
Areas with coastal upwelling such as the west coast of North America have experienced increases in acidification due to more acidic deep water upwelling into the estuary. This may have a detrimental effect on the survival of calcifying organisms
because the organisms have a much more difficult time forming and maintaining their calcium carbonate shells.
[
]
Impact on marine life
As the pH of marine systems decreases, it causes calcium carbonate (CaCO3) to dissociate
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an acid ...
[ to keep in ]chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which both the Reagent, reactants and Product (chemistry), products are present in concentrations which have no further tendency to change with time, so that there is no observable chan ...
. Calcium carbonate is vital to calcifying organisms such as shellfish, corals, and coccolithophores
Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdo ...
(a type of phytoplankton). Acidification also harms micro-organisms in the environment. These organisms either directly provide humans with a food source or supports an ecosystem important to humans.
Research
Estuarine acidification is being studied to understand the biological, chemical, and physical factors that affect pH in estuaries.
References
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Aquatic ecology
Effects of climate change