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User:Shanell.lovelace/Ocean acidification in the Arctic Ocean

Effects of Ocean Acidification on Arctic Organisms[ edit]
Organisms in Arctic waters are already challenged with stressors of living in the arctic ocean, such as dealing with cold temperatures, and it is thought that because of this, additional stressors such as ocean acidification, will cause ocean acidification effects on marine organisms to appear first in the Arctic. There exists a significant variation in the sensitivity of marine organisms to increased ocean acidification. Calcifying organisms generally exhibit larger negative responses from ocean acidification than non‐calcifying organisms across numerous response variables, with the exception of crustaceans, which calcify but were not negatively affected. This is due, mainly, to the process of marine biogenic calcification, that calcifying organisms utilize.

Calcifying Organisms
The uptake of CO₂ by seawater increases the concentration of hydrogen ions, which lowers pH and, in changing the chemical equilibrium of the inorganic carbon system, reduces the concentration of carbonate ions (CO₃²⁻). Carbonate ions are required by marine calcifying organisms such as plankton, shellfish, and fish to produce calcium carbonate (CaCO₃) shells and skeletons.

For either aragonite or calcite, the two polymorphs of CaCO₃ produced by marine organisms, the saturation state of CaCO₃ in ocean water is expressed by the product of the concentrations of CO₂²⁻ and Ca²⁺ in seawater relative to the stoichiometric solubility product at a given temperature, salinity, and pressure. Waters which are saturated in CaCO₃ are favourable to precipitation and formation of CaCO₃ shells and skeletons, but waters which are undersaturated are corrosive to CaCO₃ shells, and in the absence of protective mechanisms, dissolution of calcium carbonate will occur. Because colder arctic water absorbs more CO₂, the concentration of CO₃²⁻ is reduced, therefore the saturation of calcium carbonate is lower in high-latitude oceans than it is in tropical or temperate oceans. In model simulations of the Arctic Ocean, it is predicted that aragonite saturation will decrease, because of an increased amount of freshwater input from melting sea ice and increased carbon uptake as a result of sea ice retreat. This simulation predicts that Arctic surface waters will become undersaturated with aragonite within a decade. The undersaturation of aragonite will cause the shells of organisms which are constructed from aragonite to dissolve. This would have a profound effect on a large variety of marine organisms and has the potential to do devastating damage to keystone species and to the marine food web in the arctic ocean. Laboratory experiments on various marine biota in an elevated CO₂ environment show that changes in aragonite saturation cause substantial changes in overall calcification rates for many species of marine organisms, including coccolithophore, foraminifera, pteropods, mussels, and clams.

Although the undersaturation of arctic water has been proven to have an effect on the ability of organisms to precipitate their shells, recent studies have shown that the calcification rate of calcifiers, such as corals, coccolithophores, foraminiferans and bivalves, decrease with increasing pCO₂, even in seawater supersaturated with respect to CaCO₃. Additionally, increased pCO₂ has been found to have complex effects on the physiology, growth and reproductive success of various marine calcifiers. CO₂ tolerance seems to differ between various marine organisms, as well as differences in CO₂ tolerance at different life cycle stages (e.g. larva and adult). The first stage in the life cycle of marine calcifiers which are at a serious risk by high CO2 content is the planktonic larval stage. The larval development of several marine species, primarily sea urchins and bivalves, are highly affected by elevations of seawater pCO₂. In laboratory tests, numerous sea urchin embryos were reared under different CO₂ concentrations until they developed to the larval stage. It was found that once reaching this stage, larval and arm sizes were significantly smaller, as well as abnormal skeleton morphology was noted with increasing pCO₂.

Similar findings have been found in CO₂ treated-mussel larvae, which showed a larval size decrease of about 20% and showed morphological abnormalities such as convex hinges, weaker and thinner shells and protrusion of mantle. The larval body size also impacts the encounter and clearance rates of food particles, and if larval shells are smaller or deformed, these larvae are more prone to starvation. In addition, CaCO₃ structures also serve vital functions for calcified larvae, such as defence against predation, as well as roles in feeding, buoyancy control and pH regulation. Another example of a species which may be seriously impacted by ocean acidification is Pteropods, which are shelled pelagic molluscs which play an important role in the food-web of various ecosystems. Since they harbour an aragonitic shell, they could be very sensitive to ocean acidification driven by the increase of anthropogenic CO₂ emissions.Laboratory tests showed that calcification exhibits a 28% decrease at the pH value of the Arctic ocean expected for the year 2100, compared to the present pH value. This 28% decline of calcification in the lower pH condition is within the range reported also for other calcifying organisms such as corals. In contrast with sea urchin and bivalve larvae, corals and marine shrimps are more severely impacted by ocean acidification after settlement, while they developed into the polyp stage. From laboratory tests, the morphology of the CO₂-treated polyp endoskeleton of corals was disturbed and malformed compared to the radial pattern of control polyps.

This variability in the impact of ocean acidification on different life cycle stages of different organisms can be partially explained by the fact that most echinoderms and mollusks start shell and skeleton synthesis at their larval stage, whereas corals start at the settlement stage. Hence, these stages are highly susceptible to the potential effects of ocean acidification. Most calcifiers, such as corals, echinoderms, bivalves and crustaceans, play important roles in coastal ecosystems as keystone species, bioturbators and ecosystem engineers. The food web in the arctic ocean is somewhat truncated, meaning it is short and simple. Any impacts to key species in the food web can cause exponentially devastating effects on the rest on the food chain as a whole, as they will no longer have a reliable food source. If these larger organisms no longer have any source of nutrients, they too will eventually die off, and the entire Arctic ocean ecosystem will be affected. This would have a huge impact on the arctic people who catch arctic fish for a living, as well as the economic repercussions which would follow such a major shortage of food and living income for these families.