From the Chickens to the Crabs - The Ripple Effect: The Fate of Freshwater in the Twenty-First Century - page 73

“Welcome to the dead zone,” someone said.
A dead zone is an area where oxygen levels are low, or hypoxic , a condition caused by
excess nutrients in the water. Nitrogen and phosphorus are the two primary nutrients
for microscopic organisms called phytoplankton. In small quantities phytoplankton are
invisible to the eye, but in large numbers they cause algae blooms that cloud the water
red, green, yellow, or brown and block sunlight from other underwater plants. When
the phytoplankton die, they are eaten by bacteria, which uses up much of the oxygen in
the water. Fish and crabs either leave the area or suffocate. Worms and clams emerge
from their sedimentary hideaways in an attempt to reach oxygen. Other animals lower
their metabolism and simply shut down. Those that can't move die.
Thus, the excess nutrients mount a double attack: they promote phytoplankton
growth, which blocks sunlight and destroys plants and grasses that provide food and
habitat for marine creatures; and the bacteria that feed on dying phytoplankton suck
away oxygen normally used by fish and crabs. Once every bit of oxygen has been used
up, a new suite of bacteria bloom, often producing a strong sulfurous odor.
To Hirsch, the Chesapeake Bay is something like a giant petri dish in which multiple
diseases are festering. “Most people don't understand what's happening here,” he said.
“But they do like [to eat] their rockfish and oysters and crabs. And those creatures are
all being affected by the water quality of the bay. It's in big trouble.”
Rockfish, also known as striped bass, a black-and-white-striped sport fish, are fun to
catch and delicious to eat. But in the mid-1980s, the Chesapeake's rockish were being
overfished, and a moratorium was imposed. By 1995, the population had revived—in
what wildlife experts hailed as “a rare triumph”—and the fishing ban was lifted. In 1997,
though, pods of dead and diseased rockfish appeared in the bay, forcing beach closings
and threatening a fishing industry worth $300 million a year. The rockfish suffered from
painful-looking lesions—similar to those on the freshwater bass in local rivers—and
from mycobacteriosis, a wasting disease that causes fish to lose weight and is the result
of bacteria in the water. (It can cause skin infections in humans.) What sorts of bacteria
are causing mycobacteriosis, and how they ended up in the Chesapeake, is the subject
of ongoing investigations.
Even more distressing, the Chesapeake oyster, a quintessential part of local culture,
is facing catastrophic population declines. Since the Civil War, a large percentage of the
oysters eaten in America were the teardrop-shaped Atlantic oysters, Crassostreavir-
ginica,from the Chesapeake. Oysters spawn in the warm season (hence the injunction
against eating the bivalves in months that don't feature an rin their name), by releasing
gametes into the water, which fertilize when opposite gametes meet. A female Atlant-
ic oyster can exude ten thousand to 60 million eggs , only a fraction of which will find