Freshwater acidification

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Diagram depicting the sources and cycles of acid rain precipitation. Origins of acid rain.svg
Diagram depicting the sources and cycles of acid rain precipitation.

Freshwater acidification occurs when acidic inputs enter a body of fresh water through the weathering of rocks, invasion of acidifying gas (e.g. carbon dioxide), or by the reduction of acid anions, like sulfate and nitrate within a lake. [1] Freshwater acidification is primarily caused by sulfur oxides (SOx) and nitrogen oxides (NOx) entering the water from atmospheric depositions and soil leaching. [1] Carbonic acid and dissolved carbon dioxide can also enter freshwaters, in a similar manner associated with runoff, through carbon dioxide-rich soils. [1] Runoff that contains these compounds may incorporate acidifying hydrogen ions and inorganic aluminum, which can be toxic to marine organisms. [1] Acid rain is also a contributor to freshwater acidification. It is created when SOx and NOx react with water, oxygen, and other oxidants within the clouds. [2]

Contents

What contributes to freshwater acidification?

The buffering capacity of soils and bedrocks within the freshwater ecosystem can contribute to the acidity of the water. [1] Each freshwater reservoir has a capacity to resist changes in pH, but an excess input of acids into the reservoir can cause the buffering capacity to decrease, eventually causing the water to become more acidic. [1] An increase in atmospheric CO2 affects freshwater acidity as the more of it dissolves into the water, the more acidic it becomes. [3] It is difficult to quantify the effects of anthropogenic CO2 due to the various carbon fluxes in freshwater ecosystems. [4] High levels of freshwater acidification is harmful to various aquatic organisms. Nonetheless, there are many freshwater systems, including the Great Lakes, where pH levels could be decreasing, most likely due to CO2 accumulation in the atmosphere, however, increased monitoring is necessary to determine the full effects of acidification on pH levels. [5]

Freshwater vs. ocean acidification

A basic summary of the relationship between anthropogenic CO2 and ocean acidification. Ocean Acidification Infographic.jpg
A basic summary of the relationship between anthropogenic CO2 and ocean acidification.

The ocean and the atmosphere are constantly exchanging massive amounts of CO2. [6] Over the last 800,000 years, the concentration of CO2 in the atmosphere has remained around 172-300 parts per million by volume (ppmv). [6] With increases of anthropogenic CO2 emissions, this number has risen to 387 ppmv in 2009. [6] From 2000-2008, 26% of anthropogenic CO2 was absorbed by the ocean. [6] CO2 is the primary factor affecting ocean pH, though other aspects can also play a role. [6] When dissolved in water, CO2 acts as a weak acid that primarily affects carbonate chemistry. [6] Dissolved CO2 increases the concentration of bicarbonate ions (HCO3−) and dissolved inorganic carbon (CT) as well as lowering pH levels. [6] Similar to oceans, freshwater bodies also absorb atmospheric CO2, lowering the pH of the water. [7] In addition to CO2, freshwater reservoir's pH values are altered by acid rain, nutrient runoff, and anthropogenic pollutants. [7] Freshwater takes up CO2 in the same mechanism as seawater; however, freshwater alkalinity is much more variable than seawater, due to differences in rocks present in the watershed and decreased salt concentrations. [7] Without this salt-buffer, pH changes in freshwater tend to be more pronounced than in ocean water. In freshwater systems, newly released H+ ions are not buffered by as many bicarbonate (HCO3−) ions as ocean water. Therefore, freshwater biota tends to have a higher evolutionary pH tolerance than seawater biota. [7]

Causes

CO2

Carbon dioxide reacts with water to form carbonic acid, bicarbonate, carbonate, and acidic protons via the following equilibria:

CO2(aq) + H2O ⇌ H2CO3⇌ HCO3− + H+⇌ CO32− + 2 H+ [8]

Carbon dioxide molecule Carbon-dioxide-3D-vdW.png
Carbon dioxide molecule

Dissociation of carbonic acid decreases the pH of the solution. [9] The degree of dissociation is controlled by the overall chemistry of the solution; in particular, alkalinity and temperature are primary controlling factors. [9] There has been a clear increase of pCO2 in some freshwater ecosystems in the last century due to anthropogenic influence that is contributing to freshwater acidification. [6] It is often difficult to quantify the role of increasing pCO2 (partial pressure) in freshwater due to the various sources of carbon dioxide and the many factors that affect it, such as the surrounding landscape, climate, organisms present, the water's chemistry, and biological processes (e.g. photosynthesis, respiration). [7] The dominant type of inorganic carbon present in freshwater indicates pH levels as more CO32- is present in basic water and free CO2 is in acidic water. [7] When the latter dissolves into the surface of freshwater it reacts to form carbonic acid. [7] Along with the overall trend of increasing CO2 in the atmosphere that is being absorbed by bodies of water, the levels of carbon dioxide fluctuate daily and seasonally. [10]

SOx and NOx

Two of the other main contributors to freshwater acidification are sulfur oxides and nitric oxides. [11] The accelerated burning of fossil fuels over the past two centuries has largely contributed to the acidification of freshwater ecosystems. [11] International cooperation and environmental legislation have reduced SOx and NOx in recent decades, as sulfate emissions peaked in the 1970s, with nitrogen following behind 10 years later. [11] High levels of sulfate concentration in runoff due to increased acidity inputs, coupled with both an increase in base cation run-off and a decrease of bicarbonate, creates the acidifying effects in aquatic systems. [12] Acidic rain seeps into and reacts with clay particles in the soil which leads to the leaching of aluminum into nearby bodies of water. Thus, as the pH levels decrease, aluminum levels will increase. [13] The higher levels of aluminum pose a risk of contamination to drinking water which can lead to several health diseases. [13] This creates a toxic environment for marine species and their habitat which can lead to extinction, reductions in population size, and an overall decrease in biodiversity. [13] Most of the nitrogen in its natural state found in terrestrial ecosystems will be utilized by vegetation. However, in large amounts, not all of it can be taken up by vegetation so the excess gets washed away with runoff in the form of nitrate, contributing to acidification in the same manner as sulfate. [12]

Buffering capacity

A map depicting Atlantic Canada. Atlantic Canada - Natural Earth.jpg
A map depicting Atlantic Canada.

The buffering capacity of ecosystems helps them resist changes in pH. When this is lacking, it can lead to the acidification of freshwater reservoirs. [14] For example, the Atlantic region of Canada has the lowest acid deposition rates in Eastern North America, yet it has the most acidic waters on the continent. [14] This is due to the low buffering capacity of the regional bedrock and the addition of natural organic acids produced from close by wetlands. [14] Specifically, in Southwestern and Eastern Nova Scotia, there is a combination of high organic acidity, poor buffering, and high acid deposition to produce very low surface water pH levels and acid neutralization capacity (ANC) values. [14] In most of the Atlantic region, granite and shale bedrock are found, which contain very little buffering material. [14] Soil formed from low-buffering materials and the waters that drain from them are, therefore, susceptible to acidification, even under low acid deposition. [14] Soil that undergoes acidification can, in turn, have negative repercussions on agriculture. [15] Some species are able to withstand low pH levels in their environment. For example, frogs and perches can withstand a pH level of 4. [16] This allows these species to be unaffected by the acid deposition in their aquatic environment, allowing them to survive in these conditions. [16] However, most aquatic species, such as clams and snails, are unable to withstand low pH levels which negatively impacts their growth and survival. The high acidic levels deteriorate their thick shells decreasing their protection from predators. [16]

Harmful effects on aquatic ecosystems

This pond shows an overabundance of Sphagnum. Nationaal Park Drents-Friese Wold. Locatie Fochteloerveen. Waterveenmos (Sphagnum cuspidatum), Veenpluis (Eriophorum angustifolium) 03.JPG
This pond shows an overabundance of Sphagnum.

Acidification of freshwater ecosystems may have significant negative effects. Changes in pH as a result of freshwater acidification imposes physiological challenges on individual organisms, may decrease native biodiversity, and can alter ecosystem structure and function entirely. [12] Macro-invertebrates and large vertebrates alike are particularly sensitive to acidification as they exhibit higher mortality and lower reproductive rates under acidified conditions. [12] These species are forced to expend more energy on buffering their body conditions to retain a livable pH and, therefore, must limit energy expenditure on processes such as hunting, sheltering, and reproducing. [12] Thus, embryonic development, and species' success, is also compromised in acidified freshwaters. [12]

Conversely, algae thrive in acidified environments, and may quickly dominate these habitats, outcompeting other species. [12] In most acidic freshwater reservoirs, there is an increase in the development of mosses and algae. [12] In particular, it is common to see an increase in the abundance of the sphagnum. [12] Sphagnum has a high capacity to exchange H+ for basic cations within freshwater. The thick layer of sphagnum restricts the exchange between surface water and sediment, further contributing to reduction in nutrient cycling in the ecosystem. [12]

Aquatic biomonitoring can be used to examine the health of aquatic ecosystems by assessing water quality and temperature.

Reducing acidification

Current and emerging chemical techniques

There are processes that can remediate the acidification of freshwaters. Liming is one such practice, where calcium carbonate (CaCO3) is added to these systems. [17] Liming aids freshwater chemical and biological recovery by increasing pH levels and essentially helping the habitat return to a similar condition to how it was prior to acidification. [17] Otherwise, recovery on its own would be very extensive and take a lot longer to achieve. When added to rivers, liming showed some positive effects on wildlife, increasing the abundance of fish and acid-sensitive invertebrates. [17] However, these effects are variable. In fact, other ecosystems showed a decrease in invertebrate abundance. [17] New technologies have been developed to reduce emissions of nitrogen oxide and sulfur dioxide, linked to acid rain and water acidification. These include, wet lime, gypsum denitrification, Ammonia reduction denitrification, Electron beam irradiation desulfurization and denitrification, and Pulse plasma chemical desulfurization and denitrification. [15]

Government regulations and policies

A large decrease of acid rain and acidic bodies of water in the past couple of decades has been a direct result of governmental regulations on anthropogenic emissions, specifically SOx and NOx. [18] For example, the Canada-United States Air Quality Agreement has greatly reduced acid rain and ozone levels by 78% in Canada and 92% in the United States, as of 2020. [19] Moreover, investing in scientists to monitor and collect data is essential to create a model used to establish successful policies. [20] For instance, a protocol can be implemented to mitigate the issue. [20] Also, governments could invest funds to subsidize companies to decrease their pollution and incentivize them to use innovative methods of production, to lower both greenhouse gas emissions and the amount of acidic substances created. Furthermore, government institutions across the globe can connect on the issue of acidification and work together to find a feasible solution through international agreements. [15] Some successful government implementations include the Acid Rain Program [21] established in the United States in 1995, and the most recent Gothenburg Protocol, established by the United Nations Economic Commission for Europe (UNECE) to reduce acidification. [22]

Reduce, Reuse, Recycle Post-it Reduce, Reuse, Recycle post-it.png
Reduce, Reuse, Recycle Post-it

Public education

Another important factor to consider when looking at reducing freshwater acidification are the choices people make to protect the environment everyday. Having a basic understanding of environmental problems, such as climate change and acid rain, can influence people to act differently by being more conscious of these issues. Following a circular approach to reduce, reuse and recycle can reduce resource depletion and waste minimization, including decreasing water acidity. [23] Also, establishing school programs to ensure children learn from a young age the importance of sustainability and environmental protection. Moreover, the practice of waste separation is fundamental as it allows for the breakdown of the chemicals that cause acid rain. [15] And, finally, being altogether more aware of the effect human actions have on the environment to better protect the planet. [15]

Related Research Articles

<span class="mw-page-title-main">Acid rain</span> Rain that is unusually acidic

Acid rain is rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions. Most water, including drinking water, has a neutral pH that exists between 6.5 and 8.5, but acid rain has a pH level lower than this and ranges from 4–5 on average. The more acidic the acid rain is, the lower its pH is. Acid rain can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids.

<span class="mw-page-title-main">Carbon dioxide</span> Chemical compound with formula CO₂

Carbon dioxide is a chemical compound with the chemical formula CO2. It is 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, and as the source of available carbon in the carbon cycle, atmospheric CO2 is the primary carbon source for life on Earth. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. When carbon dioxide dissolves in water, it forms carbonate and mainly bicarbonate, which causes ocean acidification as atmospheric CO2 levels increase.

<span class="mw-page-title-main">Phytoplankton</span> Autotrophic members of the plankton ecosystem

Phytoplankton are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words φυτόν, meaning 'plant', and πλαγκτός, meaning 'wanderer' or 'drifter'.

Carbonic acid is a chemical compound with the chemical formula H2CO3. The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is quite stable at room temperature. The interconversion of carbon dioxide and carbonic acid is related to the breathing cycle of animals and the acidification of natural waters.

<span class="mw-page-title-main">Nitrogen cycle</span> Biogeochemical cycle by which nitrogen is converted into various chemical forms

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmospheric nitrogen, making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems.

A hydrogen ion is created when a hydrogen atom loses an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle-free space. Due to its extremely high charge density of approximately 2×1010 times that of a sodium ion, the bare hydrogen ion cannot exist freely in solution as it readily hydrates, i.e., bonds quickly. The hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes. Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions.

<span class="mw-page-title-main">Alkalinity</span> Capacity of water to resist changes in pH that would make the water more acidic

Alkalinity (from Arabic: القلوية, romanized: 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 of a buffer solution composed of weak acids and their conjugate bases. It is measured by titrating the solution with an acid such as HCl until its pH changes abruptly, or it reaches a known endpoint where that happens. Alkalinity is expressed in units of concentration, such as meq/L (milliequivalents per liter), μeq/kg (microequivalents per kilogram), or mg/L CaCO3 (milligrams per liter of calcium carbonate). Each of these measurements corresponds to an amount of acid added as a titrant.

<span class="mw-page-title-main">Solubility pump</span> Physico-chemical process which transports carbon

In oceanic biogeochemistry, the solubility pump is a physico-chemical process that transports carbon as dissolved inorganic carbon (DIC) from the ocean's surface to its interior.

<span class="mw-page-title-main">Ocean acidification</span> Climate change-induced decline of pH levels in the ocean

Ocean acidification is the decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide levels exceeding 410 ppm. CO2 from the atmosphere is absorbed by the oceans. This produces carbonic acid which dissociates into a bicarbonate ion and a hydrogen ion. The presence of free hydrogen ions lowers the pH of the ocean, increasing acidity. Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.

Soil acidification is the buildup of hydrogen cations, which reduces the soil pH. Chemically, this happens when a proton donor gets added to the soil. The donor can be an acid, such as nitric acid, sulfuric acid, or carbonic acid. It can also be a compound such as aluminium sulfate, which reacts in the soil to release protons. Acidification also occurs when base cations such as calcium, magnesium, potassium and sodium are leached from the soil.

<span class="mw-page-title-main">Carbon dioxide in Earth's atmosphere</span> Atmospheric constituent; greenhouse gas

In Earth's atmosphere, carbon dioxide is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis and oceanic carbon cycle. It is one of several greenhouse gases in the atmosphere of Earth. The current global average concentration of CO2 in the atmosphere is 421 ppm as of May 2022 (0.04%). This is an increase of 50% since the start of the Industrial Revolution, up from 280 ppm during the 10,000 years prior to the mid-18th century. The increase is due to human activity. Burning fossil fuels is the main cause of these increased CO2 concentrations and also the main cause of climate change. Other large anthropogenic sources include cement production, deforestation, and biomass burning.

<span class="mw-page-title-main">Human impact on the nitrogen cycle</span>

Human impact on the nitrogen cycle is diverse. Agricultural and industrial nitrogen (N) inputs to the environment currently exceed inputs from natural N fixation. As a consequence of anthropogenic inputs, the global nitrogen cycle (Fig. 1) has been significantly altered over the past century. Global atmospheric nitrous oxide (N2O) mole fractions have increased from a pre-industrial value of ~270 nmol/mol to ~319 nmol/mol in 2005. Human activities account for over one-third of N2O emissions, most of which are due to the agricultural sector. This article is intended to give a brief review of the history of anthropogenic N inputs, and reported impacts of nitrogen inputs on selected terrestrial and aquatic ecosystems.

Marine chemistry, also known as ocean chemistry or chemical oceanography, is influenced by plate tectonics and seafloor spreading, turbidity currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology. The field of chemical oceanography studies the chemistry of marine environments including the influences of different variables. Marine life has adapted to the chemistries unique to Earth's oceans, and marine ecosystems are sensitive to changes in ocean chemistry.

<span class="mw-page-title-main">Environmental impact of the petroleum industry</span>

The environmental impact of the petroleum industry is extensive and expansive due to petroleum having many uses. Crude oil and natural gas are primary energy and raw material sources that enable numerous aspects of modern daily life and the world economy. Their supply has grown quickly over the last 150 years to meet the demands of the rapidly increasing human population, creativity, knowledge, and consumerism.

The Revelle factor (buffer factor) is the ratio of instantaneous change in carbon dioxide (CO2) to the change in total dissolved inorganic carbon (DIC), and is a measure of the resistance to atmospheric CO2 being absorbed by the ocean surface layer. The buffer factor is used to examine the distribution of CO2 between the atmosphere and the ocean, and measures the amount of CO2 that can be dissolved in the mixed surface layer. It is named after the oceanographer Roger Revelle. The Revelle factor describes the ocean's ability to uptake atmospheric CO2, and is typically referenced in global carbon budget analysis and anthropogenic climate change studies.

Estuarine acidification happens when the pH balance of water in coastal marine ecosystems, specifically those of estuaries, decreases. Water, generally considered neutral on the pH scale, normally perfectly balanced between alkalinity and acidity. While ocean acidification occurs due to the ongoing decrease in the pH of the Earth's oceans, caused by the absorption of carbon dioxide (CO2) 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 (CO2) to mixes with water (H2O) forming carbonic acid (H2CO3). Through wave motion this chemical bond is mixed up, allowing for the further break of the bond, eventually becoming carbonate (CO3) which is basic and helps form shells for ocean creatures, and two hydron molecules. This creates the potential for acidic threat since hydron ions readily bond with any Lewis Structure to form an acidic bond. This is referred to as an oxidation-reduction reaction.

<span class="mw-page-title-main">Ocean acidification in the Great Barrier Reef</span> Threat to the reef which reduces the viability and strength of reef-building corals

Ocean acidification threatens the Great Barrier Reef by reducing the viability and strength of coral reefs. The Great Barrier Reef, considered one of the seven natural wonders of the world and a biodiversity hotspot, is located in Australia. Similar to other coral reefs, it is experiencing degradation due to ocean acidification. Ocean acidification results from a rise in atmospheric carbon dioxide, which is taken up by the ocean. This process can increase sea surface temperature, decrease aragonite, and lower the pH of the ocean. The more humanity consumes fossil fuels, the more the ocean absorbs released CO₂, furthering ocean acidification.

<span class="mw-page-title-main">Ocean acidification in the Arctic Ocean</span>

The Arctic ocean covers an area of 14,056,000 square kilometers, and supports a diverse and important socioeconomic food web of organisms, despite its average water temperature being 32 degrees Fahrenheit. Over the last three decades, the Arctic Ocean has experienced drastic changes due to climate change. One of the changes is in the acidity levels of the ocean, which have been consistently increasing at twice the rate of the Pacific and Atlantic oceans. Arctic Ocean acidification is a result of feedback from climate system mechanisms, and is having negative impacts on Arctic Ocean ecosystems and the organisms that live within them.

<span class="mw-page-title-main">Human impact on marine life</span>

Human activities affect marine life and marine habitats through overfishing, habitat loss, the introduction of invasive species, ocean pollution, ocean acidification and ocean warming. These impact marine ecosystems and food webs and may result in consequences as yet unrecognised for the biodiversity and continuation of marine life forms.

<span class="mw-page-title-main">Total inorganic carbon</span> Sum of the inorganic carbon species

Total inorganic carbon is the sum of the inorganic carbon species.

References

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