Journal Article

Reversal of ocean acidification enhances net coral reef calcification

Ninokawa, Aaron, Maclaren, Jana K., Mason, Benjamin M., Wolfe, Kennedy, Rivlin, Tanya, Zhu, Kai, Pongratz, Julia, Nebuchina, Yana, Silverman, Jacob, Shamberger, Kathryn, Kwiatkowski, Lester, Hosfelt, Jessica, Albright, Rebecca, Sesboüé, Marine, Schneider, Kenneth, Ricke, Katharine L., Caldeira, Ken, Caldeira, Lilian

2016

Overview

A manipulative experiment in which a reef is alkalinized in situ shows that calcification rates are likely to be lower already than they were in pre-industrial times because of acidification. Ocean acidification effects on coral Ocean acidification is one of several factors projected to threaten coral reef ecosystems, but disentangling its effects from other factors such as temperature is difficult. These authors used a manipulative experiment in which sodium hydroxide was added to seawater flowing over a natural coral reef community in situ . When ocean chemistry was restored closer to pre-industrial conditions, net community calcification increased. This suggests calcification rates are already lower than they were in pre-industrial times because of acidification. Deliberate alkalinization has been proposed as a geoengineering technique to offset ocean acidification, and this work suggests that the method could be effective, but only on a small scale — in protected bays or lagoons, for example. Approximately one-quarter of the anthropogenic carbon dioxide released into the atmosphere each year is absorbed by the global oceans, causing measurable declines in surface ocean pH, carbonate ion concentration ([CO 3 2− ]), and saturation state of carbonate minerals ( Ω ) 1 . This process, referred to as ocean acidification, represents a major threat to marine ecosystems, in particular marine calcifiers such as oysters, crabs, and corals. Laboratory and field studies 2 , 3 have shown that calcification rates of many organisms decrease with declining pH, [CO 3 2− ], and Ω . Coral reefs are widely regarded as one of the most vulnerable marine ecosystems to ocean acidification, in part because the very architecture of the ecosystem is reliant on carbonate-secreting organisms 4 . Acidification-induced reductions in calcification are projected to shift coral reefs from a state of net accretion to one of net dissolution this century 5 . While retrospective studies show large-scale declines in coral, and community, calcification over recent decades 6 , 7 , 8 , 9 , 10 , 11 , 12 , determining the contribution of ocean acidification to these changes is difficult, if not impossible, owing to the confounding effects of other environmental factors such as temperature. Here we quantify the net calcification response of a coral reef flat to alkalinity enrichment, and show that, when ocean chemistry is restored closer to pre-industrial conditions, net community calcification increases. In providing results from the first seawater chemistry manipulation experiment of a natural coral reef community, we provide evidence that net community calcification is depressed compared with values expected for pre-industrial conditions, indicating that ocean acidification may already be impairing coral reef growth.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 704/106/47 / 704/106/829/826 / 704/106/829/827 / 704/172/4081 / 704/47/4113 / Accretion / Acidification / Alkalinity / Analysis / Animals / Anthozoa - chemistry / Anthozoa - metabolism / Anthropogenic factors / Calcification / Calcification, Physiologic / Calcium Carbonate - metabolism / Carbon / Carbon Cycle / Carbon dioxide / Chemical analysis / Chemistry / Coloring Agents / Coral Reefs / Coral reefs and islands / Crustaceans / Ecosystems / Environmental factors / Humanities and Social Sciences / Hydrogen-Ion Concentration / letter / Marine ecosystems / multidisciplinary / Ocean acidification / Oceans / Oceans and Seas / Oysters / Physiological aspects / Science / Seawater / Seawater - chemistry / Shellfish / Temperature / Water analysis