The Chickahominy Report

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)

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)

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