Sexual HealthShellfish Face An Uncertain Future In A High CO2 World
Overfishing and disease have decimated shellfish populations in many of
the world"s temperate estuarine and coastal ecosystems. Smithsonian
scientists, led by Whitman Miller, ecologist at the Smithsonian
Environmental Research Center in Edgewater, Md., have discovered another
serious
threat to these valuable filter feeders-rising levels of atmospheric
carbon dioxide that contribute to the acidification of open ocean, coastal
and
estuarine waters. Their findings are being published in the open-access,
peer-reviewed journal PLoS ONE, May 27.
For shellfish and other organisms that have calcium carbonate shells and
structures, the problem begins when atmospheric CO2 dissolves in seawater
and
creates carbonic acid that is then rapidly transformed into carbonate and
bicarbonate ions in the water. Increased acidity tips the balance toward
bicarbonate formation and away from carbonate. Less carbonate in the water
means that shellfish have fewer building blocks to generate their shells.
If the water is acidic enough, shells can even begin to dissolve.
"Estuarine and coastal ecosystems may be especially vulnerable to changes
in water chemistry caused by elevated CO2 because their relative
shallowness, reduced salinity and lower alkalinity makes them inherently
less buffered to changes in pH than in the open ocean," said Miller. For
many calcifying organisms, CO2-induced acidification poses a serious
challenge because these organisms may experience reduced rates of growth
and
calcification that "when combined with other environmental stresses, could
spell disaster."
Larval oysters are thought to be particularly susceptible to acidification
since larvae produce shells made of aragonite, a crystalline form of
calcium carbonate that is prone to erosion at low pH. Adult oysters
continue to build shell but generate calcite, a more durable form of
calcium
carbonate. In Miller"s study, the larvae of Eastern oysters (Crassostrea
virginica) and Suminoe oysters (Crassostrea ariakensis) were cultured in
estuarine water that was held at four separate CO2 concentrations,
reflecting atmospheric conditions from the pre-industrial era, the
present, and
those predicted in the coming 50 and 100 years. To test the effects of
acidification, Miller monitored their growth and measured the amounts of
calcium carbonate deposited in larval shells over the course of one month.
Miller and his team found that Eastern oysters experienced a 16 percent
decrease in shell area and a 42 percent reduction in calcium content when
specimens in the pre-industrial CO2 treatment were compared with those
exposed to the levels predicted for the year 2100. Surprisingly, the
closely
related Suminoe oysters from Asia showed no change to either growth or
calcification.
The results reported suggest that the impacts of acidification may be tied
to a species" unique evolutionary history and environmental setting,
implying that predictions may be more complex than previously thought. "In
the Chesapeake Bay, oysters are barely holding on, where disease and
overfishing have nearly wiped them out. Whether acidification will push
Eastern oysters, and the many species that depend on them, beyond a
critical
tipping point remains to be seen" said Miller.
With numbers so critically low-the oyster population in the Chesapeake Bay
today stands at just 2 percent of what it was in colonial times-future
losses could have dire consequences, both environmentally and
economically. Indeed, the recently enacted Federal Ocean Acidification
Research and
Monitoring Act of 2009 recognizes the urgent need to begin addressing
impacts of acidification on estuaries and their biota.
With the continued burning of fossil fuels, further acidification is
unavoidable. Miller"s team is keenly interested in what the biological and
ecological responses will be in order to better inform current and future
environmental restoration efforts. "In a high CO2 world, calcifying
organisms may well begin to lose out to competition with non-calcifiers, a
situation that could fundamentally change benthic communities.
Understanding how such changes may play out in estuaries and coastal
waters, which teem with calcifying biota, and which are also the centers
of many
commercial fisheries and human activities, seems especially urgent" said
Miller.
Funding for this research was provided by the Seward Johnson Endowment
provided through the Smithsonian Marine Science Network as well as federal
appropriations made to the Smithsonian Institution.
Citation:
"Shellfish Face Uncertain Future in High CO2 World: Influence of Acidification on Oyster Larvae Calcification and Growth in Estuaries."
Miller AW, Reynolds AC, Sobrino C, Riedel GF (2009)
PLoS ONE 4(5): e5661. doi:10.1371/journal.pone.0005661
PLoS One