Global warming and acidification are damaging the phytoplankton at the basis of the oceans’ enormous food web, putting the entire biosphere in jeopardy. Dr. Mae-Wan Ho
Imagine vast expanses of oceans devoid of life as far and wide as you can project your senses, no whales, no fish, no seabirds and no corals beneath. The warm seawater is thick with floating plastic wastes and slime, and the oppressive heavy air putrid with death and decay.
That’s not a scene from a science-fiction film, but a likely future scenario unless we take appropriate action now: stop polluting and exploiting the oceans and shift comprehensively to renewable energy options to drastically reduce carbon emissions (Which Energy?).
Increase in carbon dioxide concentration in the atmosphere and global warming are threatening the oceans’ phytoplankton that supports all marine life from zooplankton to whales. Phytoplankton is also the fastest assimilator of carbon, clearing carbon dioxide from the atmosphere to prevent it building up as a greenhouse gas that warms the earth.
When phytoplankton is in jeopardy, all life is in jeopardy, on land and at sea. Marine life will literally starve to death, and the decay and decomposition that follow would release enormous amounts of carbon dioxide from the estimated 800 Gt of the ocean’s standing biomass, resulting in further global warming on a massive scale.
A little girl who was taken to the beach for the first time in her life was moved to declare she loved the ocean because it was “always open”. It is unthinkable that the final curtains may soon fall over the oceans.
Oceans take up carbon dioxide passively by dissolving it in the water of the surface layers, and as carbon dioxide increases in the atmosphere, so too does the concentration of carbon dioxide in the water. This makes the surface water more acidic and interferes with calcification in organisms that make their shells or external skeletons from calcium carbonate [1]. The ‘calicifers’ span the marine food web from phytoplankton that make their own food by means of photosynthesis, to practically all other organisms that depend directly or indirectly on phytoplankton for food. Califiers include coccolithophores among the phytoplankton; foraminifera and pteropods (tiny marine snails) among the zooplankton; and corals. Under normal conditions, calcite and aragonite (forms of calcium carbonate) are stable in surface waters where the carbonate ion is at supersaturating concentrations. However as the water becomes more acidic, the concentration of carbonate ion falls, and structures made of calcium carbonate can now dissolve.
Researchers have already found that corals, coccolithophore algae and pteropods have reduced calcification or enhanced dissolution of their shells and skeletons when exposed to elevated levels of carbon dioxide.
Acidity is measured as pH, the negative logarithm to the base 10 of the concentration of hydrogen ion, H+. The scale of pH goes from 0 to 14, pH 7 is neutral, pH greater than 7 is alkaline and less than 7 acidic. The pH of the oceans is slightly alkaline at 8.0 to 8.2; and has dropped 0.1 unit since the industrial revolution. By the end of this century, it will become another 0.3-0.4 unit lower, representing a 100 to 150 percent increase in hydrogen ion concentration.
As pH drops, so does the concentration of carbonate, making it more difficult for marine organisms to form calcium carbonate. There is substantial experimental evidence indicating that calcification rates will decrease in both low latitude corals that form reefs out of aragonite, the metastable form of calcium carbonate, and phytoplankton that form their shells out of calcite, the stable form of calcium carbonate.
An international team of 27 climate scientists from France, the United States, Japan, Switzerland Germany, Australia, and UK used 13 models of the ocean carbon cycle to assess calcium carbonate saturation under the ‘business as usual’ scenario [1]. In their projections, the surface water of the Southern Ocean (that which surrounds the Antarctica) will begin to become under-saturated with respect to aragonite by 2050. By 2100, this could extend throughout the entire Southern Ocean and into the sub-Arctic Pacific Ocean. When live pteropods were exposed to the predicted level of under-saturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution.
The researchers said their findings indicate that conditions harmful to ecosystems at high-latitude could develop within decades and not centuries as previously thought.
They computed modern-day ocean carbonate chemistry from the observed alkalinity and dissolved inorganic carbon (DIC) from data collected during the CO2 Survey of the World Ocean Circulation Experiment (WOCE), part of the World Climate Research Programme (WCRP) which used resources from nearly 30 countries to make in situ and satellite observations of the global ocean between 1990 and 1998 [2], and the Joint Global Ocean Flux Study (JGOFS), an international multi-disciplinary programme involving 20 nations to study carbon fluxes between the atmosphere and the surface and interior of the oceans and their sensitivities to climate change [3].
Modern-day surface carbonate ions concentration varies with latitude from 105 mmol/kg in the Southern Ocean (all waters south of 60 S) to 240 mmol/kg in the tropics. Low temperature and large amounts of upwelled deep water in the Southern Ocean containing high levels of carbon dioxide from organic matter decomposition is responsible for the low concentrations of carbonate there.
Carbon dioxide generated by human activities has already reduced modern surface carbonate by more than 10 percent since the industrial revolution (29 mmol/kg in the tropics and 18mmol in the Southern Ocean.).
By year 2100, as atmospheric carbon dioxide reaches 788 ppm (parts per million) under the business as usual scenario, average tropical surface carbonate will decline to 149 +14 mmol/kg, a 45 percent reduction relative to pre-industrial levels, and that agrees with previous estimates. In the Southern Ocean however, surface concentrations will dip to 55 + 5 mmol/kg, which is 18 percent below the threshold (66 mmol/kg) at which aragonite becomes under-saturated. These changes extend well below the sea surface. Throughout the Southern Ocean, the entire water column becomes under-saturated with respect to aragonite. The aragonite saturation horizon shifts from its present average depth of 730 m all the way to the surface. Simultaneously, in a portion of the sub-Arctic Pacific, the aragonite saturation horizon moves from its present depth of about 120 m to the surface. In the North Atlantic, surface waters remain saturated with respect to aragonite, but the saturation horizon shallows dramatically. North of 50 deg N, it shallows from 2 600 m to 115 m. The greater erosion in the North Atlantic is due to deeper penetration and higher concentrations of anthropogenic carbon dioxide, a tendency already evident in present-day estimates based on data and in models.
The changes in aragonite concentration could have severe consequences for calcifying organisms, particularly shelled pteropods, the major planktonic producers of aragonite. Pteropod population densities are high in polar and subpolar waters, but only 5 species typically occur in such coldwater regions, and of these only one or two species are common at the highest latitudes. High latitude pteropods have one or two generations per year and form integral components of food webs, and are typically found in the upper 300 m where they may reach densities of hundreds to thousands of individuals per cubic millilitre. In the strongly seasonal high latitudes, sedimentation pulses of pteropods frequently occur just after summer. In the Ross Sea, pteropods account for the majority of the annual export flux of both carbonate and organic carbon to the sea floor. South of the Antarctic Polar Front, pteropods also dominate the export flux of calcium carbonate to the sea floor.
Pteropods may already be unable to maintain shells in waters under-saturated with respect to aragonite. Data from sediment traps indicate that empty pteropod shells show pitting and partial dissolution as soon as they fall below the agaronite saturation horizon. In vitro measurements confirm the rapid rates of pteropod shell dissolution. New experimental findings suggest that even the shells of live pteropods dissolve rapidly once surface waters become under-saturated with aragonite. The authors show that when live subartic pteropod Clio pyramidata is exposed to a level of under-saturation similar to that predicted for Southern Ocean surface waters in the year 2100 under business as usual scenario, a marked dissolution at the growing edges of the shell occurs within 48 hours.
The failure of pteropods to thrive will affect many species, as they contribute to the diet of diverse species of other zooplankton, small fishes, the North Pacific Salmon, mackerel herring, cod and baleen whales.
Surface dwelling calcite plankton such as foraminifera and coccolithophorids may fare better in the short term. However, the beginning of high-latitude-calcite under-saturation will only lag aragonite by 50 to 100 years. The diverse bottom-dwelling calcareous organisms in high latitude regions may also be threatened, including cold-water corals, which provide essential fish habitat. Cold-water corals seem much less abundant in the North Pacific than in the North Atlantic where the aragonite saturation horizon is much deeper. Some important groups in the Arctic and Antarctic bottom-dwelling communities secrete magnesium calcite, which can be more soluble than aragonite. These include gorgonians, coralline red algae and echinoderms. At twice the normal concentration of carbon dioxide, juvenile echinoderms stopped growing and produced more brittle and fragile exoskeletons in a subtropical six-month experiment. Experimental evidence from many lower-latitude, shallow-dwelling calcifiers reveals a reduced ability to calcify with a decreasing carbonate saturation state. At twice the normal carbon dioxide concentration, calcification rates in some shallow-dwelling calcareous organisms may decline by up to 50 percent.
Article first published 26/07/06
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