What is ocean acidification and why is it worse at night?

Ocean acidification is the chemical process that is set into motion when the Earth’s oceans, lakes, and rivers absorb atmospheric carbon dioxide produced by automobiles, airplanes, and power plants (doi:10.1146/annurev.marine.010908.163834).

Basically, when carbon dioxide (CO2) dissolves in water, the water becomes “carbonated” like a fizzy drink, and the carbon dioxide gives rise to carbonic acid (H2CO3). Like square dancers switching partners, carbonic acid reacts with water to create aqueous bicarbonate (HCO3−) and hydronium (H3O+) ions. It is these hydronium ions that lower the pH of marine waters, making them increasingly corrosive.

But the damage doesn’t end there because bicarbonate ions form a volatile chemical equilibrium with calcium (Ca2+) and carbonate (CO32-) ions, and with carbon dioxide. As pH decreases, solid calcium is dissolved, giving rise to aqueous calcium bicarbonate. In fresh water, calcium comes from limestone in the soil. (This also is the source of the white scale that you struggle to remove from your sinks.) But in seawater, the main calcium sources are shells and skeletons of marine creatures. As a result of acidification, these dissolved calcium ions are unavailable for building shells or skeletons, for producing eggs, or for other important biological functions. In high enough concentrations, carbon dioxide can even cause these shells and skeletons to dissolve entirely.

Significant changes in water chemistry can be measured directly: already, the pH of the world’s oceans has decreased by more than 0.1 pH units since preindustrial times (doi:10.1038/425365a), representing an increase of almost 30% in the concentration of hydronium ions. Additionally, scientists estimate that the process of ocean acidification will gain momentum over the next few decades. Research projections show that the pH of the world’s oceans will further decrease by between 0.07 to 0.33 pH units by 2100 (doi:10.5194/bg-10-6225-2013), thereby attaining a level of oceanic acidity was last seen on Earth 20 million years ago. But this dramatic transformation will occur within our children’s lifetimes.

As if this situation is not serious enough, ocean acidification fluctuates over a 24 hour period of time. This cycle is intensified in coastal tide pools, as a team of scientists based in California just reported (doi:10.1038/srep22984).

In their study, the researchers measured the chemistry in coastal tide pools at the UC-Davis Bodega Marine Reserve, California. Coastal tide pools are isolated from the open ocean during low tide. When low tide occurs during daylight hours, this is not a problem: photosynthesis captures the Sun’s energy and converts dissolved carbon dioxide into sugars that are used as an energy source. This effectively reduces or even reverses ocean acidification. But when low tide occurs at night, there is no sunlight to support photosynthetic activities, and therein lies the problem: plants, animals and microscopic zooxanthellae respire by taking up oxygen and releasing carbon dioxide. Thus, night time low tides aggravate the already elevated concentration of dissolved carbon dioxide in these isolated tide pools, which further magnifies ocean acidification and its effects. Not only does this increased acidity cause calcium shells and skeletons in intertidal communities to dissolve, but it increases their dissolution rate.

“Unless carbon dioxide emissions are rapidly curtailed, we expect ocean acidification to continue to lower the pH of seawater”, said lead author, Lester Kwiatkowski, a postdoctoral researcher at the Carnegie Institution for Science.

These conditions damage a wide variety of species, particularly invertebrates, such as snails, limpets and corals, although tide pool fishes are also harmed.

“This work highlights that even in today’s temperate coastal oceans, calcifying species, such as mussels and coralline algae, can dissolve during the night due to the more-acidic conditions caused by community respiration”, said Dr Kwiatkowski in a statement.

“If what we see happening along California’s coast today is indicative of what will continue in the coming decades, by the year 2050 there will likely be twice as much nighttime dissolution as there is today”, said co-author, climate scientist Ken Caldeira, also of Carnegie Institution for Science and Professor at Stanford University.

Will marine creatures be able to evolve quickly enough to meet these changing conditions?

“Nobody really knows how our coastal ecosystems will respond to these corrosive waters, but it certainly won’t be well”, said Professor Caldeira.

GrrlScientist, Forbes, 31 March 2016. Article.

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