http://www.brown.edu/Administration/News_Bureau/2005-06/05-108.html
PROVIDENCE, R.I. Fish kills, foul odors and closed beaches hit
Rhode Islands Narragansett Bay during the summer of 2001. The
culprit was hypoxia, or oxygen depletion, which literally
suffocates sea life. While some evidence of this dead zone could
be seen on the bays surface, Brown University ecologists went
underwater and discovered a massive mussel die-off.
In a survey of nine mussel reefs located in the central bay,
researchers found one reef completely wiped out. Of the remaining
eight, seven were severely depleted. The ecologists estimate that
the number of mussels that died was roughly 4.5 billion, or about
80 percent of the reefs population.
Just one month before hypoxia hit, researchers surveyed the same
reefs and saw acres of healthy, densely packed mussels blanketing
the estuary floor.
What we saw was a local extinction, said Andrew Altieri, a new
Ph.D. graduate from Brown Universitys Department of Ecology and
Evolutionary Biology. The mussel population was devastated. If the
magnitude of this die-off was visible from the surface, there
wouldve been public alarm.
Altieri conducted the surveys with Jon Witman, a marine ecologist
and an associate professor in the Department of Ecology and
Evolutionary Biology. In a report on their research, published in
Ecology, Altieri and Witman show that mussel die-off had a lasting
effect.
In fall 2002, one year after the die-off, the pair found that only
one of the nine reefs was recovering. Altieri and Witman wondered
how the loss of so many mussels, which filter minute algae called
phytoplankton from the water, might affect the bays
ecosystem.
So Altieri calculated the filtering capacity of mussels in the
reefs, before and after the hypoxic event. They found that the
healthy mussels could filter the equivalent of the entire volume of
Narragansett Bay in just 20 days. But within weeks of the die-off,
that filtering capacity dropped by 75 percent.
Altieri said this is an important, and troubling, finding for water
quality and sea life in the bay.
Hypoxia can start when fertilizer or sewage spills into coastal
waters, carrying nitrogen, phosphorus and other nutrients. Often
fueled by warm temperatures and a lack of circulation, this
nutrient rush can cause algae blooms. When the algae dies, it sinks
to the bottom, where it is consumed by bacteria along with
dissolved oxygen. This is what happened in Narragansett Bay in the
summer of 2001 and again in the summer of 2003.
Mussels, however, can help control nutrient overload and hypoxia by
consuming phytoplankton, which reduces bottom-dwelling bacteria.
When we lose mussels, we may be losing the ability to prevent
future dead zones from forming, Altieri said. So these sorts of
extinctions may trigger a downward spiral, with coastal zones less
able to handle environmental degradation.
According to a 2004 United Nations Environment Program report, the
number of areas hit by hypoxia worldwide has doubled since 1990.
Dead zones can be found along the east coast of the United
States, in the seas of Europe, as well off Australia, Brazil, and
Japan. One of the biggest dead zones is in the Gulf of Mexico,
where it has grown to an area as big as New Jersey.
Altieri and Witman said lessons from Narragansett Bay could be
applied to other dead zones.
When you lose a foundation species such as mussels which filter
water and provide food and habitat for other organisms you see a
large and lasting effect on the ecosystem, Witman said. Added
Altieri: Weve already seen this in Chesapeake Bay and other
coastal estuaries, where loss of filter-feeding oysters has led to
runaway effects of pollution and hypoxia and prevented restoration
of these shellfish.
