Lecture 3b: Effect of rising CO2 Aquatic Flashcards
Ocean acidification
Oceans are a major sink for atmospheric CO2 and therefore central to climate change mitigation
Oceans have absorbed ~29% of global CO2 emissions since the end of the pre-industrial era.
From 2008-2017 ~40 gigatons of emissions of heat-trapping gases have been added to the atmosphere each year from the burning of fossil fuels and land-use change.
When CO2 enters the ocean, it dissolves in saltwater. First, it forms carbonic acid. Then, this carbonic acid dissociates – producing bicarbonate ions and hydrogen ions.
Ocean acidification results from an increased concentration of hydrogen ions and a reduction in carbonate ions due to the absorption of increased amounts of CO2.
see: Results of previous meta-analyses on the effects of climate drivers on biological responses of marine animals. https://doi.org/10.1038/s41467-024-47064-3
For millions of years, the exchange of CO2 between the surface of the ocean and the atmosphere remained constant. In the past ca. 150 years, the ocean has absorbed about 29 percent of this additional carbon and become ca. 30% more acidic.
Anthropogenically sourced CO2 is gradually invading into oceanic deep waters.
Impacts of Anthropogenic CO2 on oceans
Surface ocean waters have a pH of ~7.9 to 8.3, slightly alkaline. Anthropogenic CO2 is thought to have decreased the mean pH of the ocean by 0.1 unit since 1800.
If atmospheric CO2 concentrations continue to rise unabated (i.e., projections based on SRES A2 “business as usual”, pH levels will drop a further 0.3 by 2100.
In parts of the ocean, these levels would be extremely damaging to organisms that build their skeletons out of CaCO3, which is very sensitive to CO2 addition. E.g. coral and diatoms as well as crustaceans and other shellfish – planktonic creatures are at the base of the food web
CaCO3 is the dominant material used by invertebrate organisms to build their skeletons. There are two different minerals made of CaCO3, known as polymorphs: calcite and aragonite.
Dissolution reaction: CaCO3 (solid) = Ca2+ + CO3^2-
Precipitation reaction: Ca^2+ + CO3^2- = CaCO3 (solid)
An increase in CO2 from the atmosphere damages skeletons formed from CaCO3
The H+ ions and carbonate ions (CO3^2-) that derive from the dissociation of carbonic acid combine to form bicarbonate ions (HCO3-).
This rapid reduction in available carbonate ions limits calcification by organisms with aragonite- and calcite-based skeletons.
Increase in seawater hydrogen and carbonate ions in recent years suggests current ocean acidification rates are the fastest in the last 66 million years
Increasing concentrations of seawater hydrogen ions result in a decrease of carbonate ions through their conversion to bicarbonate ions.
The concentration of carbonate ions in seawater affects states for calcium carbonate compounds, which many marine species use to build their shells and skeletons.
Evidence suggests that the current rate of ocean acidification is the fastest in the last 66 million years (the K-Pg boundary) and possibly even the last 300 million years (when the first pelagic calcifiers evolved providing proxy information and also a strong carbonate buffer, characteristic of the modern ocean).
Some of the presumed acidification events in Earth’s history have been linked to selective extinction events suggestive of how guilds of species may respond to the current acidification event.
Rising carbon dioxide concentrations will change marine habitats and fish communities
Using submerged natural CO2 seeps off the Japanese Island of Shikine, an international team of marine biologists showed that even slightly higher CO2 concentrations than those existing today may cause profound changes in marine habitats and the fish that rely upon them.
Behavior of carbonate species in the ocean with the addition of CO2 from anthropogenic sources.
see: https://www.youtube.com/watch?v=9EaLRcVdTbM
Credit: National Research Council of the National Academies
nder elevated dissolved CO2 conditions, habitats are dominated by a few ephemeral algae.
Species such as complex corals and canopy-forming macroalgae mostly disappeared.
This shift from complex reefs to habitats dominated by opportunistic low-profile algae led to a 45% decrease of fish diversity, with a loss of coral-associated species and a rearrangement of feeding behaviour.
Both marine and freshwater species are affected by acidifying water in ways that disrupt the entire food web.
Many aquatic photosynthetic organisms (phytoplankton and macroalgae) also possess a CO2-concentrating mechanism based on the activity of an enzyme, carbonic anhydrase and an active transport of CO2 and bicarbonate so that increasing concentrations are of no avail (see earlier!).
see: https://youtu.be/5cqCvcX7buo
Acidification is likely to most impact cold water areas and where warm water rises to the surface this is where the most productive fisheries in the world are located
