The document summarizes water quality monitoring results from the Severn River in Maryland. It finds two "dead zones" with low oxygen levels below the surface, especially during summer months. New tests detected hydrogen sulfide gas in these low-oxygen areas, indicating bacterial activity under anoxic conditions. Strong density gradients formed pycnoclines that inhibited mixing of low-oxygen bottom waters with better-oxygenated surface waters.

1. Tracking bad water below the surface of Maryland’s Severn River
By Pierre Henkart, PhD
2013 results of the Severn Riverkeeper Water Quality Monitoring Program
Follow the ups and downs of two Severn “dead zones” independent of the Chesapeake’s
New assays for hydrogen sulfide confirm widespread bottom anoxia

2. Annapolis
N
SR0 – Our “near Chesapeake” station south of Greenbury
Point, in the channel, with a depth of 6 meters (a bit less than
20 ft). SR0 is near the NOAA “Annapolis” buoy providing
great continuous on-line water quality data at 1 meter depth.
SR1 – Our “USNA” station in
mid-channel opposite College
Creek, with a depth of 7 meters
SR2 – Our “Rte 50 bridge” station with a depth of 7
meters. We get to watch the peregrine falcons that nest
on the bridge. We also get to compare our data with the
monthly MD DNR monitoring data (their station WT7.1)
on the “Eyes on the Bay” website.
SR3 – Our “Joyce” station, south of Joyce
Point, is our deepest, with a depth of 12-13
meters (~40 feet). This is representative of
the mid-Severn’s narrow “deep trench”.
RBS – Our “Round Bay South”
station, which is interesting because the
bottom sometimes gets anoxic in the
summer. It has a typical Severn depth of ~
7 meters.
SR5 – Our mid Round Bay station with a depth of 6-7
meters. Bottom anoxia usually sets in by early July, and
in the absence of storms, persists until September.
RBN – Our “Round Bay North”
station, with a typical Severn depth
of 6-7 meters. This is the heart of
the Severn summer dead zone.
SR6 – Our Severn Narrows station with a depth of 5 meters. We
generally notice fresher water near the surface, the influence of the
fresh Severn Run entering to the northeast. Summer bottom anoxia
is pronounced here.
SR7 – Our shallow
(~1.5m) upper
station with
fresher water from
nearby Severn
Run, especially
after rains.
Turbidity is high.
Severn mainstem monitoring stations
The distance
from SR0 to SR7:
18 km = 11 miles

3. Oxygen depth profiles show habitat stress and “hypoxic squeeze”
Dissolved
oxygen,
mg/liter
We show our water quality data as depth profile bar graphs. You can
think of yourself as a scuba diver entering the water from our monitoring
boat and then heading straight for the bottom. As you go down oxygen
levels will change. We’ve plotted the depth in a downward direction, so
the longer the bar, the deeper the water. The oxygen levels are color
coded according to the oxygen concentrations needed by different
marine organisms. The Severn’s large active fish need at least 5 mg/liter
oxygen, and levels greater than that are colored green. Since most
oxygen in the water comes from the air, the top of the water column has
higher oxygen. In most cases, water near the surface has more than 5
mg/liter, so the tops of most bars are green. Smaller fish like white perch
are adapted to live with lower oxygen levels, but will avoid water with
less than three mg/liter dissolved oxygen. They will utilize both the
green and yellow portions of the water column. Benthic organisms that
live in or on the bottom (oysters, worms, etc) are adapted to yet lower
oxygen levels down to 1 mg/liter, and they will tolerate bottom water in
the orange 1-3 mg/liter oxygen range. Oxygen levels below 1 mg/liter
(red) are stressful to tough benthic organisms, even for short durations.
Truly anoxic conditions exist below 0.2 mg/liter oxygen, where only
anaerobic bacteria can live. This anoxic water will suffocate even the
toughest multicellular organisms quickly.
The lower oxygen levels in deeper water will “squeeze” fish habitat
toward the surface. The extent of squeeze will depend on the oxygen
preference of the fish involved, but few fish will be found in waters with
less than 1 mg/liter dissolved oxygen.
1
0
2
3
4
5
6
Depth,meters
>5
3-5
1-3
.2-1
<.2

4. Hydrogen Sulfide is produced by bacterial metabolism in the absence of oxygen
Multicellular organisms produce energy by metabolic pathways requiring oxygen, but In its
absence, many bacteria have the ability to utilize less efficient metabolic pathways not involving oxygen.
One such less efficient pathway uses the naturally occurring sulfate ion in seawater, reducing it to
hydrogen sulfide, H2S. Famous for its characteristic strong odor of rotten eggs, H2S is toxic to higher
organisms, but reacts chemically with oxygen, so it cannot persist for long when oxygen is present. The
presence of H2S in the water column has been used by oceanographers to characterize anoxic water
(“dead zones”), e.g., in the Black Sea and the Chesapeake’s deep channel.
A semi-quantitative assay for Hydrogen Sulfide
The Hach HS-C assay for hydrogen sulfide
utilizes a CuSO4 coated filter disc (#1,
right) inserted into the cap of a jar loaded
with 100 ml of sample water. After
dropping 2 Alka-Seltzer (!!) tablets into
the sample, the cap is screwed on, forcing
expressed gases through the filter. As
seen in disc #2 (right), H2S converts the
pale blue CuSO4 to brown/black CuS,
whose color can be evaluated according
to the calibration chart provided by Hach.
Disc 2 is the much-faded result from an assay
sample obtained from 4 meters in Asquith Creek,
Severn River, June 18, 2013.
1
2
Cap

5. During the summer, estuaries have outward flowing warm
fresher water layered over inward flowing cooler saltier water
Deg C
Temperature
Salinity
Havre
de Grace
Norfolk
Chesapeake Bay
Program
During the summer, both salinity and temperature contribute to
the density differences between top and bottom water in estuaries.
The distinct layering you see above with both salinity and
temperature reflects the existence of a pycnocline.

6. Pycnocline strength reflects the intensity of density change with depth, a
measure of the water column’s resistance to vertical mixing
= 1002.48 kg/m3
Sal = 8.0 psu; Temp = 27.1oC
= 1003.03 kg/m3
Sal = 8.2 psu; Temp = 25.8 oC
2
4
6
8
10
2
4
6
8
10
Depth,mDepth,m
Sharp density changes with depth as seen in the
lower example are pycnoclines, which often form in
estuaries as lighter, fresher, water flows seaward
over denser, saltier water. Such intense pycnoclines
resist vertical mixing more effectively than the
gradual density changes with depth shown in the
upper example.
Pycnocline
intensity
~0.1 kg/m3/m
~0.5 kg/m3/m
= 1002.48 kg/m3
Sal = 8.0 psu; Temp = 27.1oC
= 1003.03 kg/m3
Sal = 8.2 psu; Temp = 25.8 oC
Water density depends on salinity, temperature
and pressure:
http://www.es.flinders.edu.au/~mattom/Utilities/density.html
Small differences in salinity and temperature like
those shown here can make significant density
changes that resist vertical mixing mixing induced
by waves or turbulence from horizontal flow. .

7. Dissolved
Oxygen,
mg/liter
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m
6-4

8. 6-18
Dissolved
Oxygen,
mg/liter
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10

9. 6-25
Dissolved
Oxygen,
mg/liter
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10

10. 7-2
Dissolved
Oxygen,
mg/liter
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10

11. 7-9
Dissolved
Oxygen,
mg/liter
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10
Dissolved
Oxygen,
mg/liter

12. 7-16
Dissolved
Oxygen,
mg/liter
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10

13. 7-23
Dissolved
Oxygen,
mg/liter
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m

14. 7-30
Dissolved
Oxygen,
mg/liter
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m

15. 8-6
Dissolved
Oxygen,
mg/liter
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m

16. 8-13 Dissolved
Oxygen,
mg/liter
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m

17. Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m
Dissolved
Oxygen,
mg/liter
8-20

18. 8-27
Dissolved
Oxygen,
mg/liter
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10

19. 9-3
Dissolved
Oxygen,
mg/liter
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m

20. 9-10
Dissolved
Oxygen,
mg/liter
Hydrogen sulfide, mg/liter
<.1 .1-1.0 1.0-10 >10
Pycnocline intensity,
(density increase/depth increase)
.1-.2 .2-.3 .3-1 >1
Kg/m3/m