United States
Environmental Protection
Agency
Chesapeake Bay
Program
Annapolis MD 21401
Research and Development
EPA-600/S3-83-080 May 1984
Project Summary
Intensive Watershed Study:
The Patuxent River Estuary
Charles Bostater, Diane McCraney, Stephanie Berlett, and David Pushkar
This study was one of five intensive
watershed studies designed by the
Chesapeake Bay Program's Eutrophica-
tion Work Group to provide detailed
non-point source export rates and
ambient water quality data within the
Chesapeake Bay drainage area.
The study was conducted within the
Patuxent Estuary Watershed and con-
sisted of estuarine slack tide surveys.
intensive 24-hour water quality surveys,
primary productivity measurements,
sediment oxygen demand and sediment
nutrient flux measurements, phyto-
plankton and nitrifying bacterial longi-
tudinal surveys, non-point source
monitoring at five subwatersheds,
current speed and direction measure-
ments, as well as rainfall quality and
quantity measurements.
This Project Summary was developed
by EPA's Chesapeake Bay Program,
Annapolis, MD, to announce key
findings of the research project that is
fully documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
This study was one of five watershed
studies funded by the U.S. Environmental
Protection Agency, Chesapeake Bay
Program. The study was designed to
provide non-point source chemical export
data for typical subwatersheds within the
Chesapeake Bay Basin, as well as to
provide information concerning the
ambient concentrations of nutrients
within the tidal river and estuary of the
Patuxent River Basin. This estuarine
system was chosen for study by the State
of Maryland, Department of Natural
Resources and the Chesapeake Bay
Program because of concern for main-
taining an economically important fishery
and shellfish industry. Recent reports
have indicated that land use activities
have increased nutrient enrichment. The
Patuxent Basin has experienced greater
population increases and land use
changes than most other watersheds
within the Chesapeake Bay system.
Therefore, a research monitoring study
was selected for this basin in order to
provide additional information for man-
agement of its water and biological
resources.
The results of the study have been used
by the Chesapeake Bay Program for
developing baywide non-point source
(NPS) data used for calibrating the
Chesapeake Bay Basin Non-Point Source
Model and for characterization of the
nutrient enrichment of the Patuxent
estuary, relative to other subestuaries
within the Chesapeake Bay Basin. Data
from this study are currently being used
for water quality modeling purposes by
other programs.
This report constitutes an initial interpre-
tation to date of what is the most
intensive study of this estuarine system.
Careful documentation of the data
collection efforts has been included in this
report for future nutrient enrichment and
habitat assessment evaluations.
Monitoring Program
Estuary and tidal river sampling
stations were monitored between June
1980 and August 1981, This program
included 17 slack water surveys and two
24-hour surveys. Figure 1 indicates the
locations of the sample sites. The
intensive water quality surveys (IWQS)
were designed to provide information
concerning lateral and vertical homogene-
ity of water column variables, as well as
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South River
WXTOO45
Western
BRWW
Nottingham tS PXT0402
XED949oli:°wer Marlboro
Patuxent River
0123 Miles
RT4
Figure 1. Mainstem of Patuxent River and
Estuary showing locations of
major sampling stations.
temporal changes within several tidal
cycles. All data were stored in STORET.
Variables measured included stage
height, current velocity, wind, light
penetration, pH, dissolved oxygen, tem-
perature, relative humidity, secchi disc,
chlorophyll-a, pheophytin, BOD5, BOD 20,
total organic carbon, dissolved organic
carbon, chemical oxygen demand, total
persulphate phosphorus, orthophospho-
rus,nitrate, nitrite, total suspended
solids, dissolved reactive silicate, ammonia,
alkalinity, and chlorides. Separate moni-
toring included additional measurements
of sediment oxygen demand, total partic-
ulate nitrogen, total paniculate carbon,
and sediment nutrient fluxes. Several
longitudinal surveys were conducted in
order to determine the genera and to
some degree, the species of algae along
the estuary.
Five subwatersheds in the basin were
selected for measurement of chemical
export representing non-point source
contributions from the land surface
during rainfall events. Chemical export
measurements were determined for
ammonia, nitrate plus nitrite, total
nitrogen, total phosphorus, orthophos-
phorus, BOD5, BOD30, total suspended
solids, total organic carbon, chemical
oxygen demand and alkalinity. Four
subwatersheds were predominantly
agricultural and one was an undisturbed
forested site. The subwatersheds ranged
in size from 34 to 144 acres, with three
sites in the coastal plain province and two
sites in the piedmont province. Samples
were collected using automated flow-
compositing techniques and equipment.
Flow control devices, (H-S flumes or V-
notch weirs) were installed at each site.
At each site, a recording tipping bucket
raingauge was installed. All equipment
was run by installation of alternating
current electricity. Agricultural sites
were dominated by corn and tobacco or
field corn and pasture land uses.
Results and Summary
Data collected during this study indi-
cate that the Patuxent estuary sediment
oxygen demand measurements are
among the highest values measured and
reported in the literature (Figure 2)*. High
sediment oxygen demand is a classical
•D-Elia, C.F., et al., 1981. Benthic Nutrient Studies
on the Lower Patuxent River, Final Report, DNR
Contract #39753.
Patuxent River
Selby Landing 80-8 J
Jones Pt 79-80
Potts Pt 78-79
Potts Pt 79-80
example of secondary impacts from
nutrient enrichment.
Nutrient budgets were calculated from
the data collected during this intensive
study in order to indicate to managers and
researchers the potential major sources
of nutrient enrichment. The results of
these budgets shown in Figures 3 thru 7
indicate the major sources of nutrient to
the estuary during this study. Data
indicate the major sources of total
nitrogen are from fluvial sources at Rt.
50, followed by the lower estuary
sediment flux and NFS loads. The data
also indicate that there is a net exchange
of total nitrogen put of the estuary. Figure
4 shows the dissolved NO2 + N03 budget.
This budget indicates that the largest
source to the estuary is also from fluvial
sources above Rt. 50, followed by the
source from Chesapeake Bay.
The ammonia budget (Figure 5) indicates
that the sediment flux in the upper and
lower river may be the greatest source
followed by the source from Chesapeake
Bay. The total phosphorus budget (Figure
6) indicates that the lower estuary
SOD Rates in a Variety of Estuarine Ecosystems
Buena Vista 78-79
Buena Vista 79-80
Marsh Pt 73-80
Marsh Pt. 80-81
Jacks Bay 80-81
Broomes Island 80-81
Softerly Point 80-81
St Leonard Cr. 80-81
Other Ecosystems
Sublittoral Area
Georgia
Brackish Lake
Louisiana
New York Bight
Chesapeake Bay
Maryland
Narragansett Bay
Rhode Island
La Jolla California
Carstle Harbor
Bermuda
Long Island
New York
Sea Loch Scotland
Puget Sount
Washington
-Q-
-B-
-Q-
-Q-
Q V9.7
-B-
Sediment Oxygen Demand fgO2m~1d''lJ
Figure 2. Sediment oxygen demand rates in estuarine systems compiled by D'Elia, et al. for
this study.
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sediment flux and non-point sources in
the lower estuary may be the same order
of magnitude and represents the largest
sources. The dissolved orthophosphorus
budget indicates that the lower estuary
sediment flux and fluvial sources above
Rt. 50 are the major sources followed by
the potential source from Chesapeake
Bay.
Theoretical conservative mixing dia-
grams also indicate potential sources of
nutrients in the estuary. These diagrams
indicated high variability from month to
month but on a seasonal and annual
basis trends were apparent. A mid to
lower estuary peak was observed from
station averages for orthophosphorus,
total phosphorus, and silica conservative
mixing diagrams. These diagrams support
the view, as well as the phosphorus
budgets that a source of phosphorus to
the estuary water column exists in the
area of the turbidity maximum.
The estimated flushing time of the tidal
river and estuary is around 315 days. This
information, in conjunction with the
dissolved nitrogen conservative mixing
diagram, indicates that the estuary
serves as a sink for most of the fluvial
nitrogen. Conservative mixing diagrams
for nitrate, nitrite, and dissolved nitrogen
also indicated a sinkof nitrogen in the mid
to upper estuary and a source near the
mouth of the estuary. If this is the case,
one would expect high sediment-nutrient
fluxes of inorganic nitrogen, especially
ammonia, due to remineralization of
organic matter, and associated high
sediment oxygen demand due to settling
out and resulting decay of organic
material. The budget for ammonia (Figure
5) indicates a major source of ammonia in
the estuary is from the sediments.
Data also indicate that ammonia
behaves as a conservative substance in
the water column, except in the lower
estuary where a source is indicated (due
to higher bottom concentrations). Thus,
the fate of nitrogen introduced into the
estuary appears to be that it remains in
the system for a relatively long period and
is probably remineralized by sedimentation
and biological processes, yielding high
ammonia concentrations in bottom
waters.
Thus, the role that sediments play in
regulating water column concentrations
in the Patuxent estuary appears to be
important, as reported by other studies.
As part of this study, D'Elia et al.
calculated that the sediments may supply
approximately 30% of the daily phyto-
plankton demand for water column
ammonium. This fact also indicates that
Net Exchange is
16.05 X 1(f Ibs (20.9%) T
from C. Bay to the Estuary
1
>
i
Figure 3.
Figure 4.
Western Branch WWTP^
3.83 XI0s Ibs. (5.0%) \
Atmosphere 1.13X105 Ibs. (1.5%)
Fluvial Sources Above Rt. 50
Avg. Freshwater Inflow
21.23XI0*Ibs. (27.7%)
Storm Events 3.42 X 1O* Ibs.
(4.5%)
* Fluvial Sources Below Rt. 50
Avg. Freshwater Inflow
4.89 XI0s Ibs. (6.4%)
Storm Events 5.43 X 10s Ibs.
(7.1%)
Sediment Flux
Upper River 2.02 X 10s Ibs (2.7%)
Lower River 18.56X 105lbs. (24.2%)
Patuxent Estuary estimated total nitrogen budget for April thru October.
Western Branch
1.41 XI0s Ibs. (4.8%)
Atmosphere 0.89 X 10s Ibs. (3.1%)
Fluvial Sources Above Rt. SO
Avg. Freshwater Inflow
13.97X1'O5 Ibs. (47.8%)
Storm Events 1.06 X 10s Ibs.
(3.6%)
, Fluvial Sources Below Rt. 50
Avg. Freshwater Inflow
1.62 X 10sIbs. (5.6%)
Storm Events 1.68 X 70s Ibs.
(5.7%)
Sediment Flux
Upper River -1.98X 10s Ibs (-6.8%)
Lower River 0.46 X 10s Ibs. (1.6%)
Patuxent Estuary dissolved NOi + NOa budget for April thru October.
Net Exchange is
10.12 X 10s Ibs (34.6%)
from C. Bay to the Estuary
I
*•
Western Branch WWTP ^
0.68 XI0s Ibs. (1.7%)
Net Exchange is
6.60X10s Ibs (16.0%)
from C. Bay to the Estuary
Atmosphere 0.12 X 10s Ibs. (0.3%)
Fluvial Sources Above Rt. 50
Avg. Freshwater Inflow
> 5.67 XI0s Ibs. (13.8%)
Storm Events 0.33 X W* Ibs.
(0.8%)
* Fluvial Sources Below Rt. 50
Avg. Freshwater Inflow
0.38 XI0* Ibs. (0.9%)
Storm Events 0.53 X /0s Ibs.
(1.3%)
Figure 5.
Sediment Flux
Upper River 8.74 X 10s Ibs (21.2%)
Lower River 18.1 X 10s Ibs. (44.O%)
Patuxent Estuary dissolved NH3 budget for April thru October.
3
-------
the sediments may be responsible for
controlling water column concentrations
occasionally.
A typical concern in estuarine manage-
ment is determination of the limiting
factors to plant or algal production.
Although temperature is one of the most
dominant controlling factors, knowledge
of the controlling nutrient has been used
to help focus management scenarios for
point and non-point source nutrient
controls.
Unfortunately, the limiting nutrient
affecting plant production is not consis-
tent spatially and temporally which
makes management strategies difficult to
develop as well as to determine their
effectiveness. An evaluation of the
redfield ratio (N/P ratio) was conducted
using data collected during this study.
This analysis showed that the apparent
limiting nutrient varies longitudinally as
well as monthly. A multiple regression
analysis indicated that approximately 75%
of the variability of the redfield ratio could
be explained by location (i.e. nautical
mile), salinity, timing of the survey to the
previous storm event and characteristics
which describe the size of the previous
storm event, i.e. peak daily CFS, average
daily CFS during storm event, and sum of
daily CFS from beginning to the end of the
storm. From this analysis, as well as
examination of the data, it can be inferred
that storm events and their associated
export of nutrients cause pulses of
nutrients to enter the estuary, which in
turn determine the limiting nutrient.
From a management perspective, this
clearly shows the need for considering
the effect of storm events and associated
chemical export from land use activities
on the management of the estuarine
resources. It also supports the fact that
nutrient limitation would be poorly
estimated from steady state eutrophication
models.
One measure of the suitability of an
aquatic habitat for fisheries and shellfish
production is the level of dissolved
oxygen needed for growth and reproduc-
tion. The effects of nutrient enrichment
can be expressed in the dissolved oxygen
found in the water column. One effect of
increasing nutrient enrichment should be
observed through low dissolved oxygen in
bottom waters. Historical dissolved
oxygen measurements (approximately
5,000) were collected from historical
reports and studies conducted in the
mainstem Patuxent River. Statistical
analyses were performed in order to
determine trends. A mean dissolved
oxygen deficit occurred throughout the
mainstem estuary. Monthly average
Western Branch WWTP,
0.65 XI (fibs. (3.9%)
Net Exchange is
0.45X 10s Ibs (2.7%) "^
from C. Bay to the Estuary
I1
b
*
i
Atmosphere 0.25 X J 0s Ibs. (1.5%)
Fluvial Sources Above Ftt. 50
Avg. Freshwater Inflow
3.46X1'0sIbs. (20.7%)
Storm Events 2.62 X 103 Ibs.
(15.7%)
• Fluvial Sources Below fit. 50
Avg. Freshwater Inflow
0.81 X 10s Ibs. (4.9%)
Storm Events 4.16 X 10s Ibs.
(24.9%)
Sediment Flux
Upper River - 0.56 X W* Ibs (3.3%)
Lower River 3.76 X 10s Ibs. (22.5%)
Figure 6. Patuxent Estuary total phosphorus budget for April thru October.
Western Branch
0.31 X 10* Ibs. (4.4%)
Net Exchange is,
-0.03 XI 0s Ibs (-0.4%)
from C. Bay to the Estuary
Atmosphere 2400 Ibs. (0.3%)
Figure 7.
Fluvial Sources Above Rt. 50
Avg. Freshwater Inflow
2.23 X 10s Ibs. (31.3%)
Storm Events 7,200 Ibs. (1.0%)
' Fluvial Sources Below Ftt. 50
Avg. Freshwater Inflow
10,300 Ibs. (1.4%)
^ Storm Events 11.500 Ibs. (1.6%)
Sediment Flux
Upper River -0.56X10s Ibs (7.7%)
Lower River 3.76X10s Ibs. (52.7%)
Patuxent Estuary dissolved orthophosphorus budget for April thru October.
summer deficits are around 2 mg/l with
a maximum average deficit around 2.3
mg/l in September. The mean dissolved
oxygen deficit is around 3 mg/l in the
lower river (10-24 ppt salinity) at depths
greater than 30 feet. Yearly mean
dissolved oxygen plots show no apparent
trends for the entire river. However, a
strong trend towards lower dissolved
oxygen is observed for a river segment
above the Western Branch tributary
based upon seasonal mean dissolved
oxygen. A low dissolved oxygen trend is
observed when historical dissolved
oxygen deficits are regressed for the
upper estuary. A similar trend is indicated
in the lower estuary when all values are
regressed versus time, although it is not
as apparent as in the upper estuary. A
detailed analysis of dissolved oxygen
deficits categorized by months, depths
and time of day clearly showed the lack of
consistent monitoring in the estuary,
especially in deeper waters in the lower
estuary where dissolved oxygen reaches
anoxia level. Lack of consistent historical
measurements indicates ambiguous
results when more detailed statistical
analyses of dissolved oxygen deficits are
performed.
The relative importance of freshwater
flow effects on nutrient concentrations
was examined by conducting regression
analysis between station nutrient con-
centrations, surface to bottom salinity
differences, and transformations of
average freshwater inflow before the
estuarine water quality surveys. This
analysis was performed to indicate the
effect of advective processes on water
-------
column stratification. The results of this
analysis indicated that stratification at
each station may be uniquely controlled.
In addition, water column stratification
(indicated by surface to bottom salinity
differences) and freshwater inflow
explained approximately 50% of the
variability of nutrient concentrations.
Further analyses should be performed to
more fully explain dominant physical
processes at different longitudinal sta-
tions.
The relative importance of NFS loads to
the basin is indicated by the nutrient
budgets. The NFS chemical export data
indicated that the predominant NFS loads
of total phosphorus and total nitrogen
may come from agricultural lands (Table
1). These data are consistent with other
estimates. Table 2 shows the ratio of
agricultural to forested chemical export
during storm events. Data from this study
indicate that total phosphorus is six to
seven times higher in agricultural runoff
than from forested land runoff. Total
suspended solids (representative of
suspended sediment) was five times
higher in agricultural runoff than in the
forested site monitored in this study.
Fluvial sources above Rt. 50 are the
major source of NOi + NOi with Chesa-
peake Bay being the major source. Non-
point source export of ammonia on a
basin-wide basis appears insignificant.
Total phosphorus NFS export is indicated
as the major source to the estuary as
shown in Figure 6. NFS chemical export
of orthophosphorus is insignificant
compared to the sediment flux in the
estuary.
The duncan multiple range test was
applied to the data for characterizing zones
of chemical similarity. This test clearly
shows river segments which have
relatively unique concentrations. In many
cases, this analysis identified the surface
water concentrations as a uhique class in
the turbidity maximum region of the
estuary. Unique zones were identified in
the turbidity maximum region for silica,
salinity, BOD5, total suspended solids,
and total organic carbon. Unique zones
were identified in the lower estuary for
surface water pH; bottom water alkalinity
and chlorophyll-a near the mouth of the
estuary; and silica in bottom water and
surface water at the mouth of the
estuary. It is interesting to note a unique
zone in the lower estuary was identified
for dissolved oxygen and dissolved
nitrate. This zone showed the lowest
average concentrations for both dissolved
oxygen and dissolved nitrite. This zone or
region of the estuary occurs downstream
'of the mid-estuary sill. This zone is also
Table 1.
Estimated A verage Potential Watershed Chemical Export for Total Phosphorus During
Storm Events to the Patuxent River Basin
Land Use Activity
Agricultural
Forested
Other (i.e. residential,
commercial, industrial,
and idle)
All Activities
Estimated
Acres In
Basin*
205,743
272,738
58,843
537,324
Potential
Load
Ib/yr.
532.813
104,131
37,574
674,518
Loading
Rate
Ib/acre/vr
2.5897
.3818
1.180
Source
this study
this study
*
"Provided by Maryland Department of State Planning.
Table 2.
Variable
BODS
BOD30
TSS
/VO2
N03
NO2NO3
NH3
TKN
TKND
TPHOS
TPHOSD
DPO,
TOO
COD
ALKLIN
Relative Comparison of Estimated
Average Agricultural to Forested
Wastershed Chemical Export
Ratio of average
Agricultural to Average
Forested Export f/b/acre/
inch of rain)
2.7
2.3
5.2
0.3"
1.6
4.2
3.2
3.0
1.8
6.8
2.4
2.5
2.5
2.8
3.6
"Suspect data due to holding time and analy-
tical procedure used.
the region where lower estuary dissolved
oxygen values have historically been
close to zero. Observations from the slack
water survey data indicate that this may
also be a region of upwelling of anoxic
bottom water due to a combination of
advective and tidal mixing. The existence
midriver of the sill (near nautical mile 25)
as well as the fact that the rate of change
of water depth increases in this area,
makes the region susceptible to upwelling
phenomena. Plots of temperature and
salinity profiles in this region of the
estuary indicated inversions, i.e. higher
salinity and lower temperature in surface
waters. High chlorophyll-a values in this
region of the lower estuary may also be
due to upwelling of nutrient rich bottom
waters. Dinoflagellates (cells/ml) increase
in this region where upwelling may
predominate under certain mixing condi-
tions. The predominance of marine
dinoflagellates in the region just below
the apex of the midriver sill and upstream
of the bottom water D.O. minimum, as
well as vertical instability of the water
column, is one of the most interesting
findings of the study. Figure 8 (a and b)
shows that low dissolved oxygen has
historically occurred downstream of the
region where upwelling processes are
indicated.
Conclusions
Data collected and initially analyzed
under this grant indicate that a trend
exists for lower dissolved oxygen above
Western Branch, based on a seasonal
analysis. Fluvial inputs (including point
source loads) represent the greatest
sources of NOa + NOs, orthophosphorus,
total nitrogen, and total phosphorus.
Simple nutrient budgets indicate Chesa-
peake Bay may be a major source for N0z+
NO3(34.6%), NH3(16%)andtotal nitrogen
(20.9%). A significant source of total
phosphorus from NFS chemical export
(40% of the total) to the estuary is
indicated. The data suggest that upwelling
of lower estuary water, rich in nutrients
from sediment-nutrient fluxes may be
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8.
!§•
c *•
1
5
18.
0 <°
"8,
Q
8
0
Year 1936(aJ
x
x
X „ X
X K X
x xx *xs,2 ,,*« xx
*;*„ *
X * » X
X X
X
X
.00 6.00 12'00 18.00
Year 1980(b)
o.oo
Salinity (ppt)
6.00 12.00
Salinity(ppt)
raoo
Figure 8. Historical dissolved oxygen versus salinity for years 1936(a) and 1980(bl in the
Patuxent Estuary.
responsible for episodes of high chloro-
phyll-s in the lower estuary.
Recommendations
• Investigations should continue to
determine the future trends of lower
dissolved oxygen in the upper
estuary.
• A statistically based monitoring
program should be established
around Sheridan point and above, in
order to determine if historically low
dissolved oxygen is increasing in
extent and frequency of occurrence.
• The above monitoring should be
coupled with dynamic monitoring in
order to determine to what extent
lower estuary high chlorophyll-a
may reflect periods and processes
related to upwelling of bottom, and
nutrient rich waters, characteristic
of phytoplankton derived from Ches-
apeake Bay.
• Continued monitoring of sediments
and sedimentation is necessary for
developing a baseline of SOD and
Sediment Nutrient Fluxes.
• Once completed, the variability over
time with respect to SOD and
Sediment Nutrient Fluxes should be
monitored to determine the dominant
processes involved, as well as the
effectiveness of point and non-point
source controls.
• Monitoring of dissolved oxygen in
oyster bed areas will be needed to
determine if these living resources
are being impacted by point or non-
point source land use activities and
low dissolved oxygen.
• Data collected during this study
should be used to calibrate and
validate a real-time model of water
quality and hydrodynamics.
Real-time hydrological simulation
modeling will be needed in order to
prioritize subwatersheds in the
basin for applying best management
practices. Data from this study
should be used for this purpose.
Charles Bostater, Diane McCraney, Stephanie Berlett, and David Pushkar are
with the Maryland Department of Natural Resources Coastal Zone fC-2),
Annapolis, MD 21401.
James Smullen is the EPA Project Officer (see below).
The complete report, entitled "Intensive Watershed Study: The Patuxent River
Estuary." (Order No. PB 83-251 470; Cost: $64.00. subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Chesapeake Bay Program
U.S. Environmental Protection Agency
2083 West Street. Suite 5G
Annapolis, MD 21401
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
U.S. GOVERNMENT PRINTING OFFICE: 1984-759-102/985
4
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