7.4 Ocean chemistry and acidification
3 min read•august 7, 2024
and acidification are crucial aspects of marine ecosystems. The ocean's chemical composition, including and dissolved gases, plays a vital role in supporting marine life. However, human activities are altering these delicate balances.
, caused by increased CO2 absorption, threatens marine organisms with structures. This process, along with oxygen depletion in some areas, poses significant challenges to ocean ecosystems and the life they support.
Ocean Chemistry
Seawater Composition and Properties
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- Seawater has a pH range of 7.5 to 8.4, making it slightly alkaline due to the presence of dissolved ions and buffer systems
- The carbonate system in the ocean consists of dissolved (CO2), carbonic acid (H2CO3), ions (HCO3-), and carbonate ions (CO32-), which play a crucial role in regulating ocean pH and
- in the ocean is essential for marine life and varies with temperature, salinity, and biological activity (photosynthesis and respiration)
- Salinity, the concentration of dissolved salts in seawater, averages around 35 parts per thousand (ppt) and influences the density and circulation patterns of ocean water
Chemical Processes and Interactions
- The dissolution of atmospheric CO2 in seawater forms carbonic acid (H2CO3), which dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-), lowering the pH of the ocean
- The carbonate system acts as a buffer, helping to maintain a relatively stable pH in the ocean by absorbing excess hydrogen ions and releasing them when needed
- Dissolved oxygen levels in the ocean are influenced by gas exchange with the atmosphere, photosynthesis by marine plants (which produces oxygen), and respiration by marine organisms (which consumes oxygen)
- Variations in salinity can affect the solubility of gases (such as oxygen and CO2) in seawater, with higher salinity generally leading to lower gas solubility
Ocean Acidification
Causes and Mechanisms
- Ocean acidification refers to the ongoing decrease in the pH of the Earth's oceans, primarily caused by the absorption of anthropogenic CO2 from the atmosphere
- Anthropogenic CO2, released through human activities such as fossil fuel combustion and deforestation, dissolves in seawater and increases the concentration of hydrogen ions (H+), lowering the ocean's pH
- The ocean acts as a carbon sink, sequestering a significant portion of anthropogenic CO2 emissions (about 30-40%), which helps mitigate global warming but leads to ocean acidification
Impacts on Marine Life
- Ocean acidification can have detrimental effects on marine organisms that build calcium carbonate (CaCO3) shells or skeletons, such as corals, mollusks, and some plankton species
- , a form of calcium carbonate, becomes less stable in more acidic conditions, making it harder for calcifying organisms to build and maintain their structures
- , the level at which seawater is saturated with respect to aragonite, decreases as ocean acidification progresses, threatening the survival of coral reefs and other calcifying ecosystems
Oxygen Depletion
Hypoxia and Dead Zones
- occurs when dissolved oxygen levels in the ocean become too low to support most marine life, typically defined as less than 2 milligrams of oxygen per liter of water
- Hypoxic conditions can lead to the formation of "," areas in the ocean where oxygen levels are so low that most marine life cannot survive
- Dissolved oxygen depletion can be caused by a combination of factors, including (excessive nutrient input), stratification, and increased water temperature
Anthropogenic Influences
- Anthropogenic CO2 emissions not only contribute to ocean acidification but also indirectly affect oxygen levels in the ocean
- Warmer ocean temperatures, resulting from global warming, can reduce the solubility of oxygen in seawater and increase stratification, limiting the mixing of oxygen-rich surface waters with deeper, oxygen-poor waters
- Eutrophication, often caused by agricultural runoff and sewage discharge, can lead to algal blooms that consume large amounts of oxygen when they decompose, contributing to hypoxia in coastal regions (Gulf of Mexico)
Key Terms to Review (24)
Aragonite: Aragonite is a crystalline form of calcium carbonate (CaCO₃) that is commonly found in marine environments, especially in the shells of mollusks and the skeletons of corals. Its formation and stability are closely linked to ocean chemistry, particularly the carbonate system, making it highly relevant in discussions about ocean acidification and its impacts on marine ecosystems.
Aragonite Saturation: Aragonite saturation refers to the measure of the availability of aragonite, a calcium carbonate mineral, in seawater for marine organisms to build their shells and skeletons. It is a critical concept in understanding ocean chemistry, especially as it relates to acidification and the health of marine ecosystems. The saturation state indicates how likely it is for calcium carbonate to precipitate or dissolve, which is vital for organisms such as corals and shellfish that rely on it for structural integrity.
Bicarbonate: Bicarbonate is a chemical compound, specifically an anion (HCO₃⁻), that plays a crucial role in maintaining pH balance in natural waters, including oceans. It acts as a buffer, helping to neutralize acids and stabilize the pH of marine environments, which is essential for the survival of aquatic organisms and overall ocean chemistry. The balance of bicarbonate in ocean water is particularly important in the context of acidification caused by increased carbon dioxide levels in the atmosphere.
Calcium carbonate: Calcium carbonate is a chemical compound with the formula CaCO₃, commonly found in nature as a mineral and is a key component of rocks such as limestone and marble. It plays a crucial role in ocean chemistry, particularly as a building block for marine organisms like corals and mollusks, which utilize it to form their shells and skeletons. The balance of calcium carbonate in ocean waters is essential for maintaining healthy marine ecosystems and is significantly impacted by ocean acidification.
Carbon dioxide: Carbon dioxide (CO2) is a colorless, odorless gas that is naturally present in Earth's atmosphere and plays a crucial role in various biogeochemical processes. It is produced by the respiration of animals and plants, combustion of fossil fuels, and decomposition of organic matter, making it a key player in ocean chemistry, atmospheric conditions, climate regulation, and the carbon cycle.
Carbon Sequestration: Carbon sequestration is the process of capturing and storing atmospheric carbon dioxide (CO2) to mitigate climate change and its associated effects. This process can occur naturally through biological systems, such as forests and oceans, or through technological methods aimed at reducing CO2 levels in the atmosphere.
Carbon trading: Carbon trading is a market-based approach to controlling pollution by providing economic incentives for reducing greenhouse gas emissions. This system allows countries or companies to buy and sell carbon credits, which represent the right to emit a certain amount of carbon dioxide or equivalent greenhouse gases. By facilitating the exchange of these credits, carbon trading aims to encourage emission reductions where it is most cost-effective, supporting efforts to mitigate climate change and manage ocean chemistry.
Climate feedback loops: Climate feedback loops are processes that can either amplify or dampen the effects of climate change by interacting with the climate system. These loops involve changes in the environment that lead to additional changes, creating a cycle of response that can either accelerate or slow down climate change impacts. Understanding these feedback loops is essential as they connect various elements like ocean chemistry, the carbon cycle, and the broader implications of climate change on Earth systems.
Coral bleaching: Coral bleaching is a phenomenon where corals lose their vibrant colors and turn white due to stress, primarily caused by elevated water temperatures, changes in salinity, or pollution. This process occurs when symbiotic algae called zooxanthellae, which provide corals with nutrients and their color, are expelled from the coral tissue. When bleached, corals are more susceptible to disease and can lead to significant ecosystem impacts.
Dead Zones: Dead zones are areas in aquatic ecosystems, typically oceans or large lakes, where oxygen levels are so low that marine life cannot survive. This phenomenon is primarily caused by nutrient pollution, particularly from agricultural runoff and wastewater, leading to excessive algae growth, which depletes oxygen when decomposed. Understanding dead zones is crucial as they significantly impact biodiversity and the health of marine ecosystems.
Dissolved oxygen: Dissolved oxygen refers to the amount of oxygen that is present in water, which is crucial for the survival of aquatic life. This oxygen enters the water primarily through atmospheric diffusion and photosynthesis from aquatic plants and phytoplankton. The levels of dissolved oxygen are influenced by various factors, including water temperature, salinity, and biological activity, making it a key indicator of water quality and ecosystem health.
Eutrophication: Eutrophication is the process by which a body of water becomes overly enriched with nutrients, often leading to excessive growth of algae and other aquatic plants. This phenomenon can result in decreased oxygen levels in the water, harming aquatic life and disrupting ecosystems. It connects to several environmental aspects, including nutrient cycling, water quality management, and ecosystem health.
Hypoxia: Hypoxia refers to a condition in which there is a deficiency of oxygen in a specific environment, often leading to detrimental effects on marine life and ecosystems. This condition can result from various factors, including nutrient pollution and changes in ocean chemistry, especially acidification, which can exacerbate the conditions leading to reduced oxygen levels. The impacts of hypoxia are significant as they affect not just aquatic life but also the quality of water and overall ecosystem health.
Marine biodiversity: Marine biodiversity refers to the variety of life forms in oceanic ecosystems, encompassing the diversity of species, genetic variations, and the different ecological roles they play. This rich tapestry of life supports ecosystem functionality and resilience, influencing food webs, nutrient cycles, and overall ocean health. It is significantly impacted by factors such as ocean chemistry and acidification, as well as ocean currents and global circulation patterns, which shape habitats and distribution of marine organisms.
Marine Protected Areas: Marine Protected Areas (MPAs) are designated regions of the ocean where human activities are regulated or restricted to protect the marine environment and conserve biodiversity. They play a crucial role in safeguarding ecosystems, enhancing resilience to climate change, and maintaining fish populations while promoting sustainable use of marine resources.
Nutrient loading: Nutrient loading refers to the introduction of excessive amounts of nutrients, particularly nitrogen and phosphorus, into aquatic ecosystems, often as a result of human activities such as agricultural runoff, wastewater discharge, and industrial pollution. This process can lead to significant environmental issues, including algal blooms and subsequent oxygen depletion, which can adversely affect marine life and overall water quality.
Ocean acidification: Ocean acidification refers to the process by which the ocean becomes more acidic due to increased carbon dioxide (CO2) levels in the atmosphere, primarily from human activities like burning fossil fuels. This change in ocean chemistry affects marine life, ecosystems, and the overall health of the planet's integrated systems, highlighting the interconnectedness of Earth's spheres.
Ocean chemistry: Ocean chemistry refers to the study of the chemical composition and properties of seawater, including its dissolved gases, nutrients, and trace elements. This area of study is crucial for understanding the ocean's role in global biogeochemical cycles and how it interacts with the atmosphere, land, and living organisms. It also includes the investigation of how various factors, such as human activities and climate change, influence these chemical processes and the overall health of marine ecosystems.
Ocean salinity: Ocean salinity refers to the concentration of dissolved salts in seawater, typically measured in parts per thousand (ppt). This key characteristic of ocean water influences various chemical processes, biological activities, and physical properties of the ocean. Salinity levels can vary due to factors such as evaporation, precipitation, river inflow, and ice melting, playing a significant role in ocean chemistry and the overall health of marine ecosystems.
Ocean warming: Ocean warming refers to the increase in sea surface temperatures due to climate change, primarily driven by human activities such as the burning of fossil fuels and deforestation. This rise in temperature impacts marine ecosystems, alters ocean chemistry, and contributes to phenomena like coral bleaching and changes in species distribution, which can have cascading effects on ocean health and global weather patterns.
Oceanic food webs: Oceanic food webs are complex networks of feeding relationships among organisms in the ocean, illustrating how energy and nutrients flow through marine ecosystems. They highlight the interdependence of various marine life forms, from phytoplankton at the base to larger predators like sharks at the top, showcasing how each level impacts the others. Oceanic food webs are significantly influenced by factors such as ocean chemistry and acidification, which can alter species composition and overall ecosystem health.
PH levels: pH levels measure the acidity or alkalinity of a solution on a scale of 0 to 14, where 7 is neutral. In the context of ocean chemistry and acidification, pH levels are crucial for understanding how changes in carbon dioxide concentrations affect seawater chemistry, influencing marine life and ecosystems.
Revelle Factor: The Revelle Factor is a dimensionless number that quantifies the buffering capacity of seawater against changes in pH due to increased carbon dioxide levels. It indicates how effectively ocean waters can absorb CO2 without experiencing significant acidification. The Revelle Factor is essential for understanding ocean chemistry, especially in the context of increasing atmospheric CO2 and its implications for marine ecosystems.
Roger Revelle: Roger Revelle was a prominent American oceanographer who played a crucial role in raising awareness about climate change and ocean acidification in the mid-20th century. He was one of the first scientists to demonstrate the connection between rising atmospheric carbon dioxide levels from human activities and the increasing acidity of the oceans, highlighting the potential dangers this posed to marine ecosystems and global climate stability.