Warm­ing cli­mate af­fects oxy­gen lev­els, wa­ter tem­per­a­ture, al­ter­ing ma­rine life


The her­ring here — both a sym­bol of the sea­side re­gion and a sta­ple food that lo­cals eat salted, pick­led or fried — have plum­meted to about a third of their pop­u­la­tion in the 1990s. Cod have de­clined dras­ti­cally, too. And they are get­ting smaller and thin­ner. Sci­en­tists have ob­served fish in their sam­ples whose white fatty un­der­bel­lies had all but dis­ap­peared. “They just looked like they were starv­ing,” ma­rine ecol­o­gist Jan Dierk­ing said.

Then came this past sum­mer’s heat wave, which in­creased Baltic Sea tem­per­a­tures to an un­prece­dented 27 C, killing starfish and other frag­ile crea­tures.

It all could be a sign of things to come in a warm­ing world. The Baltic Sea is a sort of ex­per­i­men­tal pres­sure cooker for ma­rine life, a test for how species fare — and whether they can sur­vive at all — in con­di­tions the world’s oceans may soon ex­pe­ri­ence. “Many of the pres­sures have hit here much ear­lier and more in­tensely than in other world re­gions,” said Thorsten Reusch, a ma­rine ecol­o­gist based here at Ger­many’s largest ocean re­search in­sti­tute.

That is in part due to the Baltic’s small size: It is roughly the size of Cal­i­for­nia, or 1/250th of the At­lantic. It is also tucked be­tween nine coun­tries — in­clud­ing Swe­den to the north, Rus­sia to the east and Ger­many to the south — whose res­i­dents pol­lute and dump waste into, travel over and swim through its del­i­cate ma­rine ecosys­tems. As a re­sult, con­di­tions have been chang­ing rapidly. The tem­per­a­ture of the Baltic has risen at roughly three times the av­er­age rate of global oceans over the past decade. It has ex­pe­ri­enced a ten­fold ex­pan­sion of no-oxy­gen “dead zones” that wipe out fish and their habi­tats in the past 115 years. And it is see­ing in­creas­ing lev­els of acid­i­fi­ca­tion.

By study­ing the Baltic, and com­pre­hen­sive data sets that span five decades, sci­en­tists are learn­ing valu­able lessons from na­ture. At times, the ecosys­tem is seem­ingly un­af­fected by a huge flux in con­di­tions. At other times, species and their habi­tats are dev­as­tated when a tiny nudge in tem­per­a­ture or salin­ity dis­rupts the bal­ance. The Baltic of­fers an omi­nous, but also, in some ways, hope­ful vi­sion of the ca­pac­ity for species to adapt and what peo­ple work­ing in col­lab­o­ra­tion might be able to do to mit­i­gate en­vi­ron­men­tal dam­age. One prob­lem Baltic re­searchers have ob­served is an­i­mals and their food sources can fall out of sync. That seems to have hap­pened with baby her­ring and the small crus­taceans they feed on. Re­searcher Ca­tri­ona Clemme­sen-Bock­el­mann said higher tem­per­a­tures have prompted her­ring to hatch early, be­fore their main food source has time to grow.

Cod, mean­while, have been sep­a­rated ge­o­graph­i­cally from their main fish prey. To es­cape oxy­gen dead zones, cod have con­cen­trated them­selves in the south­ern Baltic, while the sprat they like to eat have shifted to the rel­a­tively cooler north­ern wa­ters. Cod still have ac­cess to tiny or­gan­isms on the sea floor as a food source. But the com­pe­ti­tion is fierce, and weaker cod may not get as much as they need. Some species are evolv­ing in ways that help them cope with the Baltic’s chang­ing con­di­tions — at least to a point. For ex­am­ple, cer­tain lo­cal pop­u­la­tions of mus­sels, jel­ly­fish and stick­le­back have man­aged to adapt to a lower salt level in the wa­ter. And some­times, as if to make up for a loss, a new species will ap­pear in the Baltic for the first time, said Bjorn Fis­cher, a 53-year old fish­er­man who is as proud of his apro­pos name as he is of the seven gen­er­a­tions of fish­er­men be­fore him. Fis­cher noted species such as sar­dines, sea bass and mack­erel have mi­grated from south­ern wa­ters that have be­come too warm for them. “Some­where an­other door al­ways opens,” he said. “As a fish­er­man, you al­ways have to be ready to adapt,” he said.

Most sci­en­tists who study the Baltic Sea are less san­guine about the ef­fects of cli­mate change. But they see po­ten­tial in the way the sur­round­ing coun­tries have co­op­er­ated in re­sponse to en­vi­ron­men­tal problems such as over­fish­ing and pol­lu­tion.

In 1974, lead­ers from the Baltic­fac­ing coun­tries over­came their Cold War dif­fer­ences to es­tab­lish an en­vi­ron­men­tal pact and com­mis­sion to man­age their shared re­source. The sci­ence-backed fish­ing quo­tas that fol­lowed, to­gether with other reg­u­la­tions that are now be­ing en­forced by the Euro­pean Union and Rus­sia, have helped to re­verse some harm­ful trends. They have curbed the over­fish­ing of some fish species and re­duced toxic pol­lu­tants to re­vive the pop­u­la­tions of seals, cor­morants and ea­gles liv­ing near the Baltic’s coasts. The en­vi­ron­men­tal pro­tec­tion regime has its short­com­ings. There has been no crack­down on coun­tries that al­low mas­sive amounts of agri­cul­tural run-off into the Baltic. The coun­tries sur­round­ing the Baltic Sea also have been slow to turn their at­ten­tion to cli­mate change.

Reusch noted the suc­cesses of the fish­ing quo­tas aimed at help­ing stocks re­cover are be­ing erased by the ef­fects of higher tem­per­a­tures. The solutions to the most vi­cious problems, he said, start many miles away from the wa­ter. They re­quire peo­ple to change how they farm, how much money they are will­ing to spend on food, how they travel.


A re­search ves­sel heads out of the Baltic Sea’s Kiel fiord close to the Geo­mar Helmholtz Cen­tre for Ocean Re­search in the north­ern Ger­man city of Kiel. The Baltic Sea, sur­rounded by nine coun­tries, in­clud­ing Swe­den, Rus­sia and Ger­many, has seen her­ring pop­u­la­tions plum­met since the 1990s and cod lose much of their fat stores.

Ma­rine bi­ol­o­gist Ca­tri­ona Clemme­sen-Bock­el­mann feeds lit­tle her­rings in Kiel, Ger­many, as part of a re­search pro­ject in­ves­ti­gat­ing fall­ing oxy­gen lev­els in the Baltic Sea. Oxy­gen lev­els are at an un­prece­dented low.

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