Bottom-Dwellers Suffer in Hypoxic Corner of Corpus Christi Bay

Bottom-Dwellers Suffer in Hypoxic Corner of Corpus Christi Bay
An area of low oxygen levels in the southeast corner
of Corpus Christi Bay is bigger and lasts longer than
first suspected, but scientists point to natural factors,
including wind and the movement of higher salinity water
into the area from the Laguna Madre and Oso Bay.
A condition known as hypoxia, with oxygen levels below
2 milligrams per liter, can be stressful and even lethal
to organisms. Some effects are felt in waters with
less than 3 milligrams of oxygen per liter.
Episodic hypoxia has been recorded during summer
months in the bottom layer of this water column since
1988, when monitoring began, researchers Dr. Paul
Montagna and Kevin Nelson, both of the Harte Research
Institute, said in a study recently presented to
Coastal Bend Bays & Estuaries Program.
Only one reported fish kill has been associated with
this hypoxic zone as most fish are able to swim out of
the problem area.
“Typically it’s the things that are stuck on the bottom
that die,” Montagna said.
Crabs, clams and shrimp often die in the hypoxic zone,
leaving less food for fish and a less productive bay system,
he said.
Studying the area helps determine what’s causing the
hypoxia and if anything can be done, Nelson
said.
“This is something right in our backyard
that can be a kind of gauge as to what’s
happening,” he said.
Corpus Christi Bay is experiencing only
bottom level hypoxia, meaning it’s found
just in the bottom levels and not the entire
water column. That makes it much less severe
than the largest known hypoxic area,
just off the mouth of the Mississippi River
in the Gulf of Mexico. That area, called the
Dead Zone, is hypoxic because of nutrient
loading. Nutrients from the river cause algae
blooms, which consume more oxygen
thus dropping the levels in the water.
While there are nutrient inputs from Oso
Bay, Corpus Christi Bay’s nutrient levels
are low, and the low oxygen levels don’t
appear related to nutrients, the research
has shown.

The strong southwest winds of summer blow
higher salinity water from the Laguna Madre
and Oso Bay into Corpus Christi Bay, and
this contributes to stratification of the water
column. This layering of the water column
has two effects: bottom-water is isolated and
oxygen is consumed by benthic organisms,
and the layering prevents mixing, which
would bring oxygen-rich water from the
surface to the lower levels. The layering occurs
in low spots in the bay between bottom
ridges.
Another factor, the water here, with higher
temperature and higher salinity, holds less
oxygen, further reducing its levels.
The next area of study will look at whether
contours along the bay bottom are interfering
with water flow and resulting in unmixed and
hypoxic water layers.
“There may be an engineering solution to this problem,” Montagna said. Adjusting the bottom elevation could
improve water circulation, resulting in increased oxygen levels in these bottom layers.
While the factors creating hypoxia are naturally occurring, research indicates it appears to be getting worse.
Nelson said the research provides a starting point to monitor incoming nutrients to assure they don’t increase
and become a factor in the future.
“It’s not nutrient-driven now,” he said. “We want to make sure it stays that way.”
Montagna compared the hypoxic area to the flu. It’s recurring problem that gets better over time. “But if we don’t
take care of it and it starts happening over and over again, it can become a serious problem,” he said. “We have
to be very diligent about other stressors in the environment. We already have fragile environments.”
Read the full report on hypoxia at www.cbbep.org.
The Coastal Bend Bays & Estuaries Program is a non-profit organization
dedicated to protecting and restoring bays and estuaries in the 12-county
region of the Texas Coastal Bend. CBBEP is partially funded by the Texas
Commission on Environmental Quality and the U.S. Environmental Protection
Agency.
For more information about the Coastal Bend Bays & Estuaries Program,
contact Beth Wilson at (361) 885-6246 or bwilson@cbbep.org.
This report published in July 2009.