Oceans ‘suf­fo­cat­ing as dead zones grow’

Qua­dru­pling since 1950 of oxy­gen-starved ar­eas threat­ens marine life

The Guardian Weekly - - International News - Damian Car­ring­ton

Ocean “dead zones”, where there is no oxy­gen left, have quadru­pled in size since 1950, sci­en­tists have warned, while the num­ber of sites near coasts with very low oxy­gen has mul­ti­plied ten­fold.

Most sea life can­not sur­vive in th­ese zones and the process, on cur­rent trends, will even­tu­ally lead to mass ex­tinc­tion, risk­ing dire con­se­quences for hun­dreds of mil­lions of peo­ple who rely on the sea. Cli­mate change caused by burn­ing fos­sil fu­els causes the largescale de­oxy­gena­tion, as warmer wa­ters hold less oxy­gen. Coastal dead zones are due to fer­tiliser and sewage runoff.

The anal­y­sis, pub­lished in the jour­nal Sci­ence, is the first com­pre­hen­sive anal­y­sis of the ar­eas. It states: “Ma­jor ex­tinc­tion events in Earth’s his­tory have been as­so­ci­ated with warm cli­mates and oxy­gen-de­fi­cient oceans.”

Denise Bre­it­burg, of the Smith­so­nian En­vi­ron­men­tal Re­search Cen­ter in the US, who led the study, said: “Un­der the cur­rent tra­jec­tory, that is where we would be headed. But the con­se­quences to hu­mans of stay­ing on that tra­jec­tory are so dire that it is hard to imag­ine we would go quite that far down that path.” She added: “This is a problem we can solve. Halt­ing cli­mate change re­quires a global ef­fort, but even lo­cal ac­tions can help with nu­tri­ent-driven oxy­gen de­cline.”

Bre­it­burg pointed to re­cov­er­ies in Ch­e­sa­peake Bay in the US and the Thames in Bri­tain, where bet­ter farm and sewage prac­tices led to dead zones dis­ap­pear­ing. How­ever, Pro­fes­sor Robert Diaz at the Vir­ginia In­sti­tute of Marine Sci­ence, who re­viewed the new study, said: “Right now, the in­creas­ing ex­pan­sion of coastal dead zones and de­cline in open-ocean oxy­gen are not pri­or­ity prob­lems for gov­ern­ments around the world. Un­for­tu­nately it will take se­vere and per­sis­tent mor­tal­ity of fish­eries for the se­ri­ous­ness of low oxy­gen to be re­alised.”

The oceans feed more than 500 mil­lion peo­ple, es­pe­cially in poorer na­tions, and pro­vide jobs for 350 mil­lion peo­ple. But at least 500 dead zones have now been re­ported near coasts, up from fewer than 50 in 1950. Lack of mon­i­tor­ing in many re­gions means the true num­ber may be much higher.

The open ocean has nat­u­rally oc­cur­ring low-oxy­gen ar­eas, usu­ally off the west coast of con­ti­nents, due to the way the ro­ta­tion of the Earth af- fects ocean cur­rents. But th­ese zones have ex­panded dra­mat­i­cally, in­creas­ing by mil­lions of square kilo­me­tres since 1950, roughly equiv­a­lent to the area of the Euro­pean Union.

Fur­ther­more, the level of oxy­gen in all ocean wa­ters is fall­ing, with 2% – about 77bn tonnes – hav­ing been lost since 1950. This can re­duce growth of marine life, im­pair re­pro­duc­tion and in­crease dis­ease, the sci­en­tists warn. One irony is that warmer wa­ters not only hold less oxy­gen but also mean marine or­gan­isms must breathe faster, us­ing up oxy­gen more quickly.

There are other dan­gers. Mi­crobes that pro­lif­er­ate at very low oxy­gen lev­els pro­duce lots of ni­trous ox­ide, a green­house gas 300 times more po­tent than car­bon diox­ide.

Fer­tiliser, ma­nure and sewage pol­lu­tion along coasts cause al­gal blooms, and when the al­gae de­com­pose oxy­gen is sucked out of the wa­ter. In some places, the al­gae can lead to more food for fish and in­crease catches around dead zones. But this may not be sus­tain­able, says Bre­it­burg. “There is a lot of con­cern that we are chang­ing the way th­ese sys­tems func­tion and that the over­all re­silience of th­ese sys­tems may be re­duced.”

The new anal­y­sis came from an in­ter­na­tional work­ing group set up in 2016 un­der Unesco’s In­ter­gov­ern­men­tal Oceano­graphic Com­mis­sion. Kirsten Isensee, a spe­cial­ist in car­bon sinks at the com­mis­sion, said: “Ocean de­oxy­gena­tion is tak­ing place all over the world as a re­sult of the hu­man foot­print, there­fore we also need to ad­dress it glob­ally.”

Lu­cia von Reusner, cam­paign di­rec­tor at the en­vi­ron­men­tal group Mighty Earth, which re­cently ex­posed a link be­tween the dead zone in the Gulf of Mex­ico and large-scale meat pro­duc­tion, said: “Th­ese dead zones will con­tinue to ex­pand un­less the ma­jor meat com­pa­nies that dom­i­nate our global agri­cul­tural sys­tem start clean­ing up their sup­ply chains to keep pol­lu­tion out of our wa­ters.”

Diaz said the pace of ocean suf­fo­ca­tion was breath­tak­ing: “No other vari­able of such eco­log­i­cal im­por­tance to coastal ecosys­tems has changed so dras­ti­cally in such a short pe­riod of time from hu­man ac­tiv­i­ties as dis­solved oxy­gen.”


Al­gae victims … a Chilean beach lit­tered with dead sar­dines

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