The Chickahominy Report

News about Earth, Atmosphere, Water, and Life

Ocean acidification threatens shellfish in estuaries, coastal ecosystems

Whitman Miller of the Smithsonian Environmental Research Center checks an experimental aquarium used to rear juvenile oysters. (Smithsonian Environmental Research Center)

Whit­man Miller of the Smith­son­ian Envi­ron­men­tal Research Cen­ter checks an exper­i­men­tal aquar­i­um used to rear juve­nile oys­ters. (Smith­son­ian Envi­ron­men­tal Research Center)

MECHANICSVILLE, Va. — The shell­fish of the Chesa­peake Bay and oth­er estu­ar­ies in the Mid-Atlantic region face a host of threats to their sur­vival. Over­fish­ing, pol­lu­tion, sed­i­men­ta­tion, habi­tat destruc­tion, and intro­duced species have tak­en a toll on many shelled species, such as the east­ern oys­ter (Cras­sostrea vir­gini­ca), that play impor­tant roles in main­tain­ing the health of the estu­ar­ine ecosys­tems. A new study, led by Whit­man Miller (Smith­son­ian Envi­ron­men­tal Research Cen­ter, Edge­wa­ter, Mary­land, USA), finds anoth­er poten­tial­ly dis­as­trous threat com­ing from the atmos­phere — ocean acidification.

The find­ings have sig­nif­i­cant impli­ca­tions for efforts to restore the Chesa­peake Bay ecosystem.

“Our results sug­gest that the native east­ern oys­ter is quite sen­si­tive to ele­vat­ed CO2/reduced pH,” Miller said. “Future restora­tion efforts should prob­a­bly con­sid­er the chem­i­cal con­di­tions of waters when select­ing loca­tions for oys­ter restoration.”

The acid­i­fi­ca­tion of the oceans is dri­ven by car­bon diox­ide released into the atmos­phere as a result of our com­bus­tion of fos­sil fuels. While some debate whether or not humans are caus­ing cli­mate change by these car­bon diox­ide emis­sions, the effect on the oceans is clear. Car­bon diox­ide in the air read­i­ly dis­solves in water, mak­ing the water more acidic by the for­ma­tion of car­bon­ic acid.

Acid­i­ty is mea­sured by pH — a scale from 0 to 14 in which low­er num­bers mean more acidic con­di­tions and high­er num­bers mean more basic (alka­line) con­di­tions. Neu­tral pH (nei­ther acid nor alka­line) is 7. The scale is log­a­rith­mic, such that a decrease of 1 unit on the scale — from 7 to 6, for exam­ple, rep­re­sents a ten­fold increase in acid­i­ty level.

Since the begin­ning of the Indus­tri­al Rev­o­lu­tion, the pH of the oceans has dropped by 0.1 — while the num­ber seems small, it actu­al­ly rep­re­sents a 30 per­cent increase in acidity.

The pri­ma­ry con­cern is how the increased acid­i­ty affects shell­fish species that build their shells from car­bon­ate min­er­als — pri­mar­i­ly arag­o­nite and cal­cite — extract­ed from the water. Acidic waters eas­i­ly dis­solve these min­er­als, mak­ing it hard­er for the ani­mals to extract them from the water and in turn build shells. [Remem­ber what hap­pened to the bak­ing soda (sodi­um bicar­bon­ate) after the addi­tion of vine­gar (acetic acid) in the ele­men­tary school vol­cano experiment?]

So far, most of the con­cern over ocean acid­i­fi­ca­tion was focused on marine ecosys­tems such as coral reefs which are built from car­bon­ate min­er­als deposit­ed by reef-build­ing corals and algae. Giv­en the dif­fer­ences in phys­i­cal con­di­tions between ocean­ic and estu­ar­ine envi­ron­ments, the new study, pub­lished last Wednes­day in the jour­nal PLoS ONE, assessed whether shell­fish in estu­ar­ies and coastal envi­ron­ments would be more vul­ner­a­ble than those in oceans.

Projected mean summer positions of aragonite compensation points for the Chesapeake Bay under preindustrial, present, and predicted atmospheric CO2 levels. (From Miller et al., 2009)

Pro­ject­ed mean sum­mer posi­tions of arag­o­nite com­pen­sa­tion points for the Chesa­peake Bay under prein­dus­tri­al, present, and pre­dict­ed atmos­pher­ic CO2 lev­els. (From Miller et al., 2009)

Estu­ar­ies and nearshore coastal ecosys­tems are less deep, less saline, and less alka­line than the open ocean. Any one of the three con­di­tions reduces the abil­i­ty to buffer against changes in pH. The com­pen­sa­tion point — the point below which it becomes more expen­sive in terms of ener­gy required for organ­isms to extract car­bon­ate min­er­als from the water — is also a func­tion of the salin­i­ty and tem­per­a­ture of the water. Low­er pH, low­er salin­i­ty, and increas­ing tem­per­a­tures all increase the com­pen­sa­tion point, shift­ing it sea­ward, i.e., reduc­ing the area where con­di­tions are favor­able for shell production.

Giv­en those con­di­tions, Miller and his col­leagues sus­pect­ed that lar­val oys­ters would be the most severe­ly affect­ed as they relied more on arag­o­nite — the more sol­u­ble of the two min­er­als — to form their shells while mature oys­ters relied more on calcite.

To test its hypoth­e­sis, the research team grew lar­val oys­ters of two species, east­ern oys­ter and Sum­i­noe oys­ter (Cras­sostrea ari­ak­en­sis), a species native to Asia, in estu­ar­ine water under four dif­fer­ent atmos­pher­ic con­cen­tra­tions of car­bon diox­ide — 280, 380, 560, and 800 ppm. The con­cen­tra­tions cor­re­spond with prein­dus­tri­al, cur­rent, and pro­ject­ed atmos­pher­ic con­cen­tra­tions of atmos­pher­ic car­bon diox­ide 50 and 100 years from now.

Oyster larvae as seen through a microscope. (Smithsonian Environmental Research Center)

Oys­ter lar­vae as seen through a micro­scope. (Smith­son­ian Envi­ron­men­tal Research Center)

Sum­i­noe oys­ters showed no notice­able effect from the increased car­bon diox­ide, either in terms of growth (shell area) or in terms of shell cal­ci­um con­tent. East­ern oys­ters, how­ev­er, showed a 16 per­cent decrease in growth rate and a 42 per­cent decrease in cal­ci­um con­tent when com­par­ing prein­dus­tri­al to year 2100 car­bon diox­ide concentrations.

Miller and col­leagues con­clude that, while the effects of acid­i­fi­ca­tion will vary from species to species and place to place, it may lead to reduc­tions in growth or even in the geo­graph­ic dis­tri­b­u­tion of estu­ar­ine shell­fish. Giv­en the ener­gy costs of shell for­ma­tion, the increas­ing dif­fi­cul­ty in extract­ing car­bon­ate min­er­als from the water may force the ani­mals to take ener­gy away from oth­er impor­tant process­es, such as immune defense or repro­duc­tion. This may be exac­er­bat­ed by oth­er stres­sors in the envi­ron­ment, such as extreme tem­per­a­tures and pol­lu­tion. Slow­er shell for­ma­tion and growth may — by forc­ing lar­vae to spend longer peri­ods in the plank­ton­ic, or float­ing, stage — reduce the per­cent­age of oys­ter lar­vae sur­viv­ing to maturity.

The researchers are very con­cerned about the future of estu­ar­ine and coastal ecosys­tems, which because of their greater envi­ron­men­tal vari­abil­i­ty, can be much more stress­ful for their inhab­i­tants than open ocean ecosys­tems. Dis­as­trous envi­ron­men­tal and eco­nom­ic con­se­quences may result if organ­isms that form shells based on car­bon­ate min­er­als give way to those that do not.

Chris French, Vir­ginia Direc­tor of the Alliance for the Chesa­peake Bay, would like to see this research con­firmed by oth­er stud­ies as well as expand­ed to oth­er mol­lusk species, such as clams. He also won­ders what the con­se­quences may be for those who make their liv­ing on the water.

“It’s def­i­nite­ly con­cerns me that, giv­en the cur­rent state of the oys­ter pop­u­la­tion, it looks like we have anoth­er poten­tial chal­lenge ahead of us as far as the restora­tion of the native oys­ter,” French said. “It def­i­nite­ly rais­es some addi­tion­al con­cerns, not just on the ecol­o­gy of the Bay restora­tion effort, but also of the eco­nom­ic impli­ca­tions because we’ve seen so many peo­ple in the water­men’s sec­tor whose liveli­hoods depend on the shell­fish resources.”

— David M. Lawrence

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