Harald Franzen Scientific American
Hurricanes
destroy homes and flood neighborhoods but they can also tip entire
ecosystems out of balance, according to a new study published
in today's Proceedings of the National Academy of Sciences. Several
major hurricanes that hit the North Carolina coast in 1999 flooded
Pamlico Sound, the nation's largest lagoonal estuary, flushing
nutrients and sediment into the ecosystem and diluting the salty
water. "Within six weeks, the entire water content of Pamlico
Sound was replaced by the flood," lead author Hans Paerl
from the University of North Carolina at Chapel Hill says. "Normally
that would take a year because the sound has only four small inlets,
restricting exchange with the sea."
As a result,
the salinity of the water in the sound dropped by two-thirds and
the nitrogen levels soared by over 50 percent. Such a boost of
fertilizer lets algae and other microorganisms flourish, creating
a low-oxygen environment that threatens other wildlife like crab,
shrimp and fish. In Pamlico Sound, it led many species to migrate
out of the area. Last spring and summer, winds in the lagoon kept
the water well-mixed and oxygen-levels within limits. Because
water stays in the sound for a long time before running off into
the ocean, though, most of the nitrogen is still in the lagoon
estuary. Scientists fear that if the winds subside this year,
the algae will choke the finfish and shellfish that have remained.
There are
indications that the ecosystem is slowly recovering for now, but
even if the summer winds return, it may take several years for
the estuary to return to normal. Paerl is also concerned about
the effects of future storms: "The real question is: if we're
going to have 20 or more years of more frequent hurricanes as
our meteorologist friends predict," he says, "how many
times can the system bounce back?"
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