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Ocean Acidification: Biological Effects

Marine organisms use ions obtained from surrounding seawater for key physiological processes as well as the formation of mineral structures such as shells or skeletons. Organisms have adapted mechanisms that can buffer changes in pH over short time scales in order to prevent internal damage. Changes in seawater chemistry caused by ocean acidification threaten to disrupt these mechanisms. It is unknown how long these compensation mechanisms can function, however experiments done at carbon dioxide levels predicted for the next 100-300 years have significantly affected the survival and development of many marine organisms. These organisms are key to the ecosystems and food webs they are within so that changes to their diversity or efficiency could affect them significantly. These observations highlight the serious threat that ocean acidification has for many marine organisms, however the complex ramifications are difficult to predict as studies on potential adaptation have not been carried out. Ocean acidification events of the past may enable us to interpret ecosystem response and adaptation potential to ocean acidification today.

CALCIFYING ORGANISMS

The resulting decrease in pH due to ocean carbon dioxide absorption will adversely affect marine organisms, in particular for oceanic calcifying organisms. Calcifying organisms span the food web and they include such organisms as coccolithophores (planktonic algae), corals, foraminifera (planktonic algae), echinoderms (starfish and sea urchins), crustaceans (crabs, lobsters, barnacles) and molluscs. Calcifying organisms use the carbonate ions in the ocean to produce calcium carbonate structures such as skeletons, liths (calcium carbonate plates) and protective shells. The three main mineral forms of calcium carbonate produced by organisms are calcite, aragonite and magnesium-calcite.

Currently surface waters are supersaturated with respect to all forms of calcium carbonate. However with increasing ocean acidification the ocean pH falls and reduces the carbonate ion concentration, making the calcium carbonate structures of organisms vulnerable to dissolution.

The mineralogy is species specific, and hence their susceptibility to dissolution will depend on their mineral form. However calcifying organisms are not only exposed to impacts of dissolution, but are also vulnerable through impacts to their physiology.

Studies on sea urchins and brittlestars (an echinoderm species) showed they were unable to compensate for longer-term changes in ocean acidification. For example, sea urchins were unable to maintain their internal pH balance for longer than seven days at carbon dioxide levels just 200 ppm higher than today. This resulted in loss of normal body functions and reduced survival rates over several months. Experiments on brittlestars showed muscle wastage in their arms, reduced juvenile success, fertilisation and larval growth was also documented at higher pH levels.

Mussels show a decrease of shell and body growth by 55% at 2 times the pre-industrial carbon dioxide levels as predicted for the year 2050. Crustaceans have shown to be more able to compensate for changes in pH, however they are still subject to dissolution impacts. Other organisms appear to be less susceptible such as sediment-dwelling organisms (e.g. worms), which may already be periodically subjected to low pH in their current environment and therefore already have compensations mechanisms in place.

Some of the organisms mentioned are not only comercially important, but are also important in cycling of nutrients between sediment and the water column. For example, sea urchins are bioturbators and efficiently mix sediments. Lowered fitness of these animals could change nutrient and carbon cycling within the water column, potentially lowering the availability of nutrients for other key organisms.

BENTHIC ORGANISMS

Benthic organisms are those that live on, in, or near the seabed. Deep sea ecosystems are extremely stable, favouring specialist organisms with their narrow niche and specific adaptation constraints affecting their ability to adapt. As a result, these organisms will be more susceptible to extinction compared with more opportunistic organisms which are adapted to more variable environments as found in the ocean surface waters. The extinction associated with the PETM (Paleocene-Eocene Thermal Maximum) was particularly strong amongst deep-sea calcifiers, whilst non-calcifying deep-sea and shallow-water species shelf showed significantly lower levels of extinction. It is likely, therefore, that many benthic marine organisms will be susceptible to extinction due to the effects of ocean acidification in the future; especially in the face of declining carbonate saturation influencing their ability to produce shells and skeletons from calcium carbonate.

Benthic foraminifers live several years, in contrast to planktonic organisms, which have bi-weekly to monthly reproduction cycles. Therefore benthic foraminifers are more vulnerable to environmental changes imposed by ocean acidification. It has also been found that these deep-sea benthic foraminifers experienced a major extinction during the PETM. For the benthic environment initial future and Palaeocene-Eocene deep-sea responses to carbon dioxide release are consistent: progressive decreases in pH and lowered calcium carbonate concentrations whilst temperature increases. Therefore future environmental change raises the possibility of similar extinctions in the future.

PELAGIC ORGANISMS

Pelagic organisms, including phytoplankton, zooplankton and other organisms living in the upper water column may also be vulnerable to ocean acidification. Coccolithophores, generating blooms so large they are visible from space, produce calcitic liths. Experiments indicate that some coccolithophore species (e.g.Emiliania huxleyi) may experience decreased rates of calcification by up to 30% at 3 times the pre-industrial carbon dioxide levels. However other species have shown no response to this increase. Copepods (small crustaceans which make up a large proportion of zooplankton), have been found to have reduced survival in their early life stages. Planktonic pteropods (sea snails and slugs) are important grazers in areas of the polar oceans and have been found to be highly susceptible to dissolution at these levels.

In addition to direct impacts on marine organisms, the lower pH expected over the next 100 years could theoretically impact the spread and production of biologically important nutrients (e.g. nitrogen, phosphate and silica) and micronutrients (e.g. iron, cobalt and manganese). Nitrification has also been shown to be pH sensitive, which may result in a reduction of ammonia oxidation rates and an accumulation of ammonia may result instead of nitrate. Over a sea shelf ecosystem model this has been predicted to result in a 20% decrease in pelagic nitrification by year 2100.

Additionally, the nutritional quality of plankton may also change with increasing acidification. Changes to calcification, nutrients or to different carbon dioxide uptake rates may result in a change in the reduction of planktonic community structure. A loss of organic carbon lowers the nutritional value of primary-produced organic matter and this in turn affects the quality of food available for zooplankton, lowering growth and reproduction. The repercussion of these changes in nutrient availability and primary production quality will affect the entire food web as there will be less energy becomes available for larger organisms.

Therefore aside from calcification, ocean acidification may cause organisms to suffer indirectly through negative impacts on food resources or directly as reproductive or physiological effects (such as acidification of body fluids). Ocean acidification may also force some organisms to reallocate resources away from feeding and reproduction in order to maintain internal cell pH. It has also been found that ocean acidification could alter acoustic properties of seawater, allowing sound to propagate further and increasing ocean noise. This could adversely impact marine animals that use sound for echolocation or communication.

CORALS

Cold-water or deep-water corals are found throughout the world's oceans. They form large frameworks that are biodiversity hotspots and are important refuges, feeding grounds and nurseries for deep-sea organisms, including commercial fish. Tropical coral reefs , often referred to as rainforests of the sea, cover only 2% of the globe but contain up to 40% of the World's overall productivity. Though they inhabit less than 1% of the surface of the ocean floor, they provide habitats for up to 25% of all marine species, forming one of the most diverse ecosystems on the planet.

Coral reefs are highly complex and fragile ecological units made up of many intricate, interdependent relationships. They are built from the accumulated exoskeletons of calcium carbonate-secreting animals, known as colonial polyps.

Problems of ocean acidification coupled with anthropogenic factors such as overfishing and organic and inorganic pollution have lead to the decline of coral reefs. This is important as coral reefs provide economic values towards coastline protection, tourism, and fisheries.

Carbonate ions are used by corals to form their calcium carbonate skeletons therefore the effects of reduced carbonate ions due to ocean acidification makes it more difficult for corals to grow.

Decreasing aragonite saturation is reducing rates of coral calcification so much so, that if this continues their rate of erosion will outpace calcification resulting in the breakdown of reef structure and loss of habitat for other organisms. Habitat disappearance might even lead to extinction in some species.


Dicynodon Illustration courtesy of John Sibbick.
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