An annual
model-based report on
"dead-zone" conditions in
the Chesapeake Bay during
2018 indicates that the
total volume of low-oxygen,
"hypoxic" waters
was very similar to the
previous year, but a
sharp drop in hypoxia
during late July shows
the critical role of wind
mixing in short-term
variations in the oxygen
content of Bay waters.
The duration of hypoxia
in 2018 was greater than
in recent years.
Dead zones are
one of the major water quality
concerns facing
the Bay. They form when
rivers carry in excess
nitrogen from fertilizers,
wastewater, and other
sources, fueling short-lived
blooms of algae.
Bacteria then eat the
dead, sinking algae, consuming
from bottom
waters the dissolved oxygen
that fish, shellfish,
crabs, and other animals
need to survive. Bay dead
zones peak during summer,
when hot weather
encourages algal growth
and drives gases from the
water, while calm winds
typically preclude the
mixing of relatively oxygen-
rich surface waters
into the depths.
The Annual
Chesapeake Bay Hypoxia
Report Card is the brainchild
of Dr. Marjy
Friedrichs of the Virginia
Institute of Marine
Science and Dr. Aaron
Bever of Anchor QEA, an
environmental and engineering
consulting firm.
Bever earned his Ph.D.
from William and Mary's
School of Marine Science
at VIMS in 2010.
The team's report
card summarizes oxygen
conditions in the Bay
each year as estimated by
their 3-D, real-time
hypoxia forecast model,
originally developed
with funding from
NOAA. The model is
based on thirty years of
water quality data collected
by the Chesapeake
Bay Program, and is
forced daily by wind data
provided by NOAA and
river-input data provided
by the U.S. Geological
Survey. The modeling
team, which includes Dr.
Raleigh Hood of the
University of Maryland
Center for Environmental
Science, also generates
dissolved oxygen statistics
for previous years for
comparative purposes.
Because springtime
inflows from the
Susquehanna River were
high in 2018, scientists
predicted that summer
2018 would have an
above average amount of
hypoxia, a forecast that
held true through mid-
July, when unusually
strong winds reduced the
Bay's hypoxic volume to
near zero. Hypoxia
increased rapidly again
in early August and
peaked at a higher value
in early September than
in previous years. Strong
winds in September
again mixed Bay waters,
resulting in a large reduction
in hypoxic volume.
Says Bever, "The
lack of hypoxia in late
July was very atypical of
historical dissolved oxygen
conditions for midsummer."
Friedrichs
adds, "It's fascinating
how the hypoxic volume
was so low in late July
and so high in early
September, but overall
the hypoxic volume was
basically the same as last
year."
"Overall," she
says, "our model indicates
that the total
amount of hypoxia in
2018 was similar to 2017,
but that the seasonal patterns
in hypoxia were
very different. In addition
to late-July's major
hypoxic dip, low-oxygen
conditions started earlier
and lasted longer in 2018
than in recent years."
Friedrichs notes that
related research—by
recent VIMS Ph.D. graduate
Ike Irby—suggests
that an earlier onset of
spring hypoxia may be
growing more common,
as May temperatures
increase due to climate
change.
To put this year's
late-July "hypoxia hiatus"
in perspective, Friedrichs
notes that low-oxygen
waters occupy on average
about 7% of the Bay's
entire volume during
summer, and about 20%
of the volume of
Maryland's Chesapeake
waters—where Bay
hypoxia is most pronounced.
This year,
hypoxia occupied 14.4%
of the Bay's mainstem
waters on July 5 (a volume
of 9.9 cubic kilometers),
but by July 26—just
three weeks later—had
fallen to only 0.14% of
mainstem water, with a
volume of only 0.1 km3.
The findings of
the VIMS hypoxia model
and report card generally
match the monitoring-based
report provided by
the Maryland
Department of Natural
Resources for the
Maryland portion of the
Bay. Variations in results
are partially due to different
reporting periods, as
the Virginia report
includes results from the
onset of hypoxia in
spring to its cessation in
the autumn, while
Maryland's DNR reports
on Maryland conditions
from June through
September, the period
that specifically relates to
the EPA's water-quality criteria
assessment for
summer dissolved oxygen.
