CBP/TRS 115/94
August 1994
903R94050
Trends in Phosphorus, Nitrogen, Secchi Depth, and
Dissolved Oxygen in Chesapeake Bay, 1984 to 1992
TD
225
.C54
T74
1994
Chesapeake Bay Program
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Trends in phosphorus, nitrogen, Secchi depth, and dissolved oxygen in
Chesapeake Bay, 1984 to 1992.
Environmental Protection Agency, Annapolis, MD. Chesapeake Bay Program.
Chesapeake Bay Program,
1994
CBP/TRS 115/94
PB95-136230
32995308
Nitrogen; Phosphorus; Environmental impacts; Water pollution abatement;
Chesapeake Bay; Reductions; Nutrients; Dissolved oxygen; Monitoring;
Water quality; Trends; Runoff; Seasonal variations; Chemical analysis;
Mathematical models; Improvement; Point sources; Nonpoint sources;
Tables(Data); Graphs(Charts); Organic loading; Maryland; Pennsylvania;
District of Columbia; Virginia
Water —Chesapeake Bay (Md. and Va.)— Phosphorus content— Statistics ;
Water—Nitrogen content— Chesapeake Bay (Md. and Va.)~ Statistics ; Water-
-Dissolved oxygen— Chesapeake Bay (Md. and Va.)~Statistics ; Chesapeake
Bay (Md. and Va.)
x, 63 p. : maps, charts ; 28 cm.
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The Chesapeake Bay Program (CBP) is a Federal-State partnership working
to restore Chesapeake Bay. One of its main goals is to improve water quality
conditions for living resources. The CBP started ambient water quality
monitoring programs for Chesapeake Bay in 1984 to characterize current
water quality, to assess trends in water quality over time, and to increase
understanding of linkages between water quality and living resources.
Nutrient enrichment is a major water quality problem in Chesapeake Bay.
Spring and summer phytoplankton blooms, fueled by high nutrient levels,
cause low dissolved oxygen (DO) levels in the summer when the plankton die
and decompose. Low concentrations of DO can be lethal to Chesapeake Bay's
aquatic animals. Both point source and nonpoint source reductions of
nitrogen and phosphorus loads to Chesapeake Bay have been achieved since
1985.
"CBP/TRS 115/94". Cooperative agreement no. TCRD-93-o8-oi-ooo".
Includes bibliographical references (p. 49).
United States. Environmental Protection Agency. Chesapeake Bay Program.
{Annapolis, Md.} :
1994-
PCA04/MFA01
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EJA 19970815
EJD 19950818
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Trends in Phosphorus, Nitrogen, Secchi Depth, and
Dissolved Oxygen in Chesapeake Bay, 1984 to 1992
Cooperative Agreement No. TCRD-93-08-01-000
CBP/TRS 115/94
August 1994
Printed by the U.S. Environmental Protection Agency for the Chesapeake Bay Program
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. ENDORSEMENT
The Chesapeake Bay Monitoring Subcommittee has reviewed the assumptions and
methods of data analysis used in this report and finds them appropriate for the analysis
conducted. The findings of this report are consistent with and supported by the analytical
techniques employed.
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992 iii
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iv Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
ABSTRACT
The Chesapeake Bay Program (CBP) is a Federal-State partnership working to re-
store Chesapeake Bay. One of its primary goals is to improve water quality and habitat
conditions for living resources. The CBP began monitoring water and habitat quality in
1984 and continues to sample the main stem and tributaries for their physical and chemi-
cal makeup.
Nutrient enrichment is a major water quality problem in Chesapeake Bay. Nutrients
fuel phytoplankton growth, which has an adverse (reduction) effect on dissolved oxygen
(DO) levels. Low DO levels threaten the existence of Chesapeake Bay's aquatic animals.
DO levels should increase if nutrient levels are reduced. A computer model pre-
dicted that a 40-percent reduction in nitrogen and phosphorus would reduce nutrient
levels and cause an increase in DO levels in the main stem to Chesapeake Bay. Nitrogen
and phosphorus control programs have been initiated. Trend analyses, involving various
criteria, were performed over an 8-year period (from October 1984 through September
1992) to see how these programs affected water and habitat quality conditions in Chesa-
peake, Bay.
Results of seasonal Kendall test analysis indicate that phosphorus levels decreased
significantly baywide, especially in one upper Chesapeake Bay segment and two lower
Chesapeake Bay segments. There were also marginally significant improvements in phos-
phorus levels in two upper Chesapeake Bay segments and in one lower Chesapeake Bay
segment. Nitrogen levels were somewhat increased (marginally significant) in one seg-
ment of Chesapeake Bay. Secchi depths showed no significant trends baywide; however,
there were marginally significant trends (improvements) in upper Chesapeake Bay. DO
trends were not statistically significant baywide; however, segments at the mouth of Ches-
apeake Bay showed marginally significant degradation.
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
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Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
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CONTENTS
Page
Endorsement iii
Abstract v
Acknowledgments ix
Abbreviations ix
Executive Summary 1
Introduction 5
Methods 5
Parameters Analyzed and Data Preparation 5
Trend Analysis Methods 6
Adjustments for Changing Detection Limits 10
Susquehanna River Flow 10
Phosphorus 11
Nitrogen 11
Secchi Depth 12
Dissolved Oxygen 12
Results and Discussion 13
Susquehanna River Flow 13
Phosphorus 16
Total Phosphorus 16
Dissolved Inorganic Phosphorus (Orthophosphate) 17
Nitrogen 26
Total Nitrogen 26
Dissolved Inorganic Nitrogen 27
Secchi Depth 31
Dissolved Oxygen 39
Plans for Future Trend Analyses 46
Interpolating Above and Below Pycnocline Layers and Surface and Bottom
Layers Separately 46
Accounting for Interannual Changes in Flow 47
Adding Parametric Trend Tests 47
Adding Trend Tests on Interpolated Tributary Data 47
Summary 47
References 49
Appendix—Frequencies of Below Detection Limit Values for Dissolved
Inorganic Phosphorus and Dissolved Inorganic Nitrogen 51
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992 vii
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FIGURES
1. CBP main stem monitoring stations and segments 7
2. Total annual Susquehanna River flow (bars) and number of months above
median flow line (water years 1985 to 1992) 14
3. Total phosphorus trends in Chesapeake Bay main stem segments (October 1984
through September 1992) 18
4. Average monthly concentrations of total phosphorus and dissolved inorganic
phosphorus (1934 to 1992) 19
5. Dissolved inorganic phosphorus trends in Chesapeake Bay main stem segments
(October 1984 through September 1992) 22
6. Average monthly concentrations of dissolved inorganic phosphorus (1984
to 1992) 23
7. Total nitrogen trends in Chesapeake Bay main stem segments (October 1984
through September 1992) 28
8. Average monthly concentrations of total nitrogen and dissolved inorganic
nitrogen (1984 to 1992) 29
9. Dissolved inorganic nitrogen trends in Chesapeake Bay main stem segments
(October 1984 through September 1992) 33
10. Average monthly concentrations of dissolved inorganic nitrogen (1984 to
1992) 34
11. Secchi depth trends in Chesapeake Bay main stem segments (October 1984
through September 1992) 36
12. Average monthly Secchi depths (1984 to 1992) 37
13. Dissolved oxygen delta and dissolved oxygen deficit trends in Chesapeake
Bay main stem segments (October 1984 through September 1992) 40
14. Average monthly concentrations of dissolved oxygen and dissolved oxygen
delta (1984 to 1992) 42
15. Total volumes of water with dissolved oxygen concentrations below 0.2, 1, 3,
and 5 mg/L (June through September, 1985 to 1992) 44
TABLES
Executive Summary Table. Summary of trend results (October 1984 through
September 1992) 3
1. Correlations between log mean monthly Susquehanna River flow and mean
monthly concentrations of water quality parameters (with P Values in
parentheses) 14
2. Trend results for interpolated monthly mean total phosphorus by segment
(12 months) 17
3. Trend results for interpolated monthly mean levels of dissolved inorganic
phosphorus by segment, using four different method detection limit
treatments 21
viii Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
4. Trend results for interpolated monthly mean dissolved inorganic phosphorus
by segment (12 months) 25
5. Trend results for interpolated monthly mean dissolved inorganic phosphorus
by segment (7 months, April through October) 26
6. Trend results for interpolated monthly mean total nitrogen by segment 27
7. Trend results for interpolated monthly mean levels of dissolved inorganic
nitrogen by segment using four different method detection limit treatments 32
8. Trend results for interpolated monthly mean Secchi depth by segment
(7 months, April through October) 39
9. Trend results for interpolated monthly mean dissolved oxygen delta by
segment (4 warm weather months, June through September) 41
10. Trend results for interpolated monthly mean dissolved oxygen deficit by
segment (4 warm weather months, June through September) 46
11. Summary of trend results (October 1984 through September 1992) 48
A.I. Percent of observations with below detection limit values for dissolved
inorganic phosphorus by segment, laboratory, and water year 52
A.2. Percent of observations with below detection limit values for dissolved
inorganic nitrogen by segment, laboratory, constituent parameter, and
water year 55
A CKNO WLEDGMENTS
The Monitoring Subcommittee would especially like to express its gratitude to Peter
Bergstrom and Marcia Olson for writing this report. The Monitoring Subcommittee
would also like to express their gratitude to the field and lab crews that carefully and ex-
pertly collected and analyzed the water quality samples discussed in the report. A great
many other people provided both information insights and support that contributed to the
preparation of this report, and to all of them our heartfelt thanks.
ABBREVIATIONS
BDL Below detection limit(s)
CBL Chesapeake Biological Laboratory
CBP Chesapeake Bay Program
CRL (The U.S. Environmental Protection Agency's) Central Regional
Laboratory (in Annapolis, Md.)
DIN Dissolved inorganic nitrogen
DIP Dissolved inorganic phosphorus
DO Dissolved oxygen
KD Light attenuation
MDE Maryland Department of the Environment
MDL Method detection limit(s)
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992 ix
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ODU Old Dominion University
P Probability that an observed trend, correlation, or difference was due to
chance
SAV Submerged aquatic vegetation
TN Total nitrogen
TP Total phosphorus
VIMS Virginia Institute of Marine Science
WY Water year(s)
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
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EXECUTIVE SUMMARY
The Chesapeake Bay Program (CBP) is a Federal-State partnership working to re-
store Chesapeake Bay. One of its main goals is to improve water quality conditions for
living resources. The CBP started ambient water quality monitoring programs for Chesa-
peake Bay in 1984 to characterize current water quality, to assess trends in water quality
over time, and to increase understanding of linkages between waster quality and living
resources. Currently, over 150 stations in the tidal tributaries and main stem to Chesa-
peake Bay are sampled once or twice a month and analyzed for more than 20 physical
and chemical parameters. The main stem to Chesapeake Bay is divided into 10 segments
based on similar salinity circulation and geomorphology.
Nutrient enrichment is a major water quality problem in Chesapeake Bay. Spring
and summer phytoplankton blooms, fueled by high nutrient levels, cause low dissolved
oxygen (DO) levels in the summer when the plankton die and decompose. Low con-
centrations of DO can be lethal to Chesapeake Bay's aquatic animals. Thus, summer DO
levels should improve if nutrient levels are reduced.
A computer model of Chesapeake Bay water quality predicted that 40 percent nitro-
gen and phosphorus load reductions would reduce ambient nutrient levels sufficiently to
cause' an increase in DO levels in the deeper areas of the main stem to Chesapeake Bay.
The 1987 Chesapeake Bay Agreement and its 1992 amendments committed Pennsylva-
nia, Maryland, Virginia, and the District of Columbia to achieve a 40-percent reduction
of the 1985 nitrogen and phosphorus loads entering the main stem to Chesapeake Bay by
the year 2000.
Both point source and nonpoint source reductions of nitrogen and phosphorus loads
to Chesapeake Bay have been achieved since 1985. Trend analyses of ambient levels of
nitrogen, phosphorus, DO, and related water quality parameters were performed to deter-
mine how these source reductions are affecting water quality conditions in Chesapeake
Bay.
Trends in Chesapeake Bay main stem levels of total phosphorus (TP), dissolved in-
organic phosphorus (DIP), total nitrogen (TN), dissolved inorganic nitrogen (DIN),
Secchi depth, and DO were analyzed over 8 years (October 1984 through September
1992). Phosphorus trends were analyzed with the nonparametric seasonal Kendall test*
and were classified as marginally sufficient improvements (P<0.05) or significant im-
provements (P<0.01). Data from 49 main stem monitoring stations were spatially
averaged over 10 main stem segments using a three-dimensional interpolator. There were
no adjustments for river flow, although correlations of Secchi depth with flow and trends
in flow were examined. Monthly median values were analyzed because sampling fre-
quency varied seasonally. Method detection limits declined over time for some
parameters, but any trends that could have been caused by declining detection limits were
eliminated.
Parameters important to submerged aquatic vegetation (SAV) growth were analyzed
for trends over the whole year and also over the 7-month SAV growing season (April
through October). DO trends were only analyzed for the four warm weather months (June
through September) when most low DO conditions occur. Percent change estimates were
*The Kendall slope is a measurement of trend expressed as mg/L/yr.
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
based on the mean concentration for the first water year (October 1984 through Septem-
ber 1985) and a projection for the last water year based on the seasonal Kendall slope.
Interannual changes in Susquehanna River flow could produce the appearance of
trends in water quality if there was a trend in flow, and water quality was correlated with
flow; however, there were no significant trends in mean monthly Susquehanna River
flow over any time period. In contrast, there were significant positive and negative cor-
relations between the log mean monthly Susquehanna River flow and upper Chesapeake
Bay water quality, but they did not follow a simple pattern.
TP concentrations showed a statistically significant downward trend (P<0.01) bay-
wide, especially in one upper Chesapeake Bay segment and two lower Chesapeake Bay
segments. There were also marginally significant improvements (/><0.05) in two upper
Chesapeake Bay segments and in one lower Chesapeake Bay segment. The median bay-
wide percent change (decline) in TP over 8 years (1984 to 1992) was 16 percent, plus or
minus 8 percent (90 percent confidence interval). DIP showed significant downward
trends over 12 months at the mouth of Chesapeake Bay and over 7 months (April through
October) in central Chesapeake Bay, but the trend was not significant baywide.
TN concentrations showed a marginally significant increasing trend (degradation,
P=0.027) in Mobjack Bay, including the mouth of the York River; however, there was no
significant trend baywide or in an. other segments. The possible increase in TN in Mob-
jack Bay was probably related to similar upward trends in TN in the York River. DIN
showed no significant trends in any segments, although high detection limits made it im-
possible to assess trends in nitrogen concentrations in several lower Chesapeake Bay
segments.
Secchi depths showed no significant trends baywide over a period of 7 or 12 months
or for any segment over 12 months. There were marginally significant upward trends
(improvements) in upper Chesapeake Bay over the 7-month SAV growing season (April
through October). These trends may be related to statistically significant inverse correla-
tions between the April-through-September Secchi depth and the mean monthly
Susquehanna River flow, although there were no significant trends in flow. Secchi depth
is not measured in the Susquehanna River, so it is not known whether there were trends
in Secchi depth there.
DO concentration trends as well as trends in several metrics calculated from the
concentration were examined. These trends included oxygen delta (the difference be-
tween DO at saturation and the actual DO concentration) and DO deficit (converting the
delta concentration to the mass of DO that would have to be added to bring all the water
in that segment to saturation). The volumes of water in each segment that were below
four benchmark DO concentrations (5, 3, 1, and 0.2 mg/L) were also analyzed for trends.
DO concentration and the four metrics for volumes below specific concentrations
had no statistically significant trends (P>0.05) in any segments in the June-to-September
period. The mouth of Chesapeake Bay showed marginally significant degradation in both
DO delta and DO deficit. However, DO concentrations are generally high in the mouth of
Chesapeake Bay and DO delta is quite low, so these trends are unlikely to have any nega-
tive impact on aquatic animals living near the mouth of Chesapeake Bay.
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
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Executive Summary Table. Summary of trend results (October 1984 through September 1992).
Main Stem CBP Segments
Parameter
No. of
Months All CB1 CB2 CB3 CB4 CBS CB6 CB7 CB8 WE4 EE3
TP
DIP
DIP
TN
DIN
DIN
Secchi Depth
Secchi Depth
DO Concentration
DO Delta
DO Deficit
DO<0.2
DO<1.0
DO<3.0
DO<5.0
12 I IM I IM - - I IM I -
12-------- | --
7 ____ | ______
12 --------- DM -
12------ + + - + +
7 _-_-__ + + _ + +
12 -----------
7 -IMIM --------
4 ___________
4 ________ DM --
4 ________ DM --
4 _. _________
4 ___________
4 ___________
4 ___________
Legend and note:
I—Significant improvement (P<0.01).
IM—Marginally significant improvement (P<0.05).
DM—Marginally significant degradation.
Dash—No significant trend (P>0.05).
+—DIN trends could not be assessed in these segments because detection limits did not stop declin-
ing until July 1990.
7 months—April through October only, same as SAV growing season in lower salinity zones; 4
months—June through September only, used as period of anoxia in three-dimensional model analy-
ses.
DO data were not analyzed in EE3.
See text for explanation of DO delta, DO deficit, and DO volumes below the four concentrations.
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
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Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
INTRODUCTION
The Chesapeake Bay Program (CBP) is a Federal-State partnership working to re-
store Chesapeake Bay. One of the main goals of the CBP is to improve water quality
conditions for Chesapeake Bay living resources. CBP's water quality monitoring pro-
grams were started in 1984 to characterize current water quality and to assess trends in
water quality over time. Presently, over 150 stations are sampled once or twice a month
and analyzed for more than 20 physical and chemical parameters.
A major water quality threat in Chesapeake Bay is low summertime concentrations
of DO, a condition which is potentially lethal to Chesapeake Bay aquatic animals. Spring
and summer phytoplankton blooms, fueled by high nutrient levels, cause low DO levels
during the summer when the plankton die and decompose. Thus, summertime DO levels
should improve if nutrient levels are reduced.
A computer model of Chesapeake Bay water quality predicted that 40 percent nitro-
gen and phosphorus load reductions would reduce ambient nutrient levels sufficiently to
cause an increase in DO levels in the deeper areas of the main stem of Chesapeake Bay.
The 1987 Chesapeake Bay Agreement and its 1992 amendments committed Pennsylva-
nia, Maryland, Virginia, and the District of Columbia to achieve a 40-percent reduction
of the 1985 nitrogen and phosphorus loads entering the main stem of Chesapeake Bay by
the year 2000.
Both point and nonpoint source reductions of nitrogen and phosphorus loads to
Chesapeake Bay have been achieved since 1985.' Trend analyses of ambient levels of
nitrogen, phosphorus, DO, and related water quality parameters were performed to deter-
mine how these source reductions are affecting water quality conditions in Chesapeake
Bay.
Trends in Chesapeake Bay main stem levels of total phosphorus (TP), total nitrogen
(TN), and DO were analyzed over 6 years.2'4 This report updates the previous trend anal-
yses using 8 years of main stem monitoring data, spanning October 1984 through
September 1992.
TP levels declined significantly between 1984 and 19903; however, TN and DO lev-
els showed little or no change over the same period.2'4 This update, with 2 additional
years of data, was conducted to see if these trends continued. This update also added sta-
tistical analyses of trends in DO and trend analyses of three additional parameters:
dissolved inorganic phosphorus (DIP), dissolved inorganic nitrogen (DIN), and Secchi
depth (a measure of water clarity). These three parameters were added because their lev-
els affect submerged aquatic vegetation (SAV) growth5 and are also closely related to
phytoplankton growth.
METHODS
PARAMETERS ANALYZED AND DATA PREPARATION
The CBP monitoring program and details of sample collection and analytical chem-
istry methods used are described in three previous reports.2"4 The following water quality
parameters were analyzed in this report:
• TP concentration
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
• DIP concentration
• TN concentration
• DIN concentration
• Secchi depth
• DO concentration.
The following calculated metrics derived from the water quality parameters were
also analyzed in this report:
• DO delta: the difference between DO saturation concentration and observed
DO concentration
• DO deficit: the difference between DO mass at saturation and the observed
mass of DO
• Volume of water with DO concentrations below 5, 3, 1, and 0.2 mg/L.
A three-dimensional interpolator6 was used to estimate baywide main stem mean
concentrations and mean concentrations in each CBP main stem segment (CB1 to CBS,
EE3, and WE4). (See Figure 1.) Data from all sampling depths were used, except for
Secchi depth, which has only one measurement per station. The annual periods used were
water years (WY), from October through September (which include a complete hydro-
logical cycle). The monitoring data from October 1984 through September 1992 were
used; the previous reports included data through September 1990. The possible outliers
that were removed from nitrogen and phosphorus data in the two previous reports3-4
were checked by the data submitters and were either verified or corrected. The Maryland
monitoring data used in this report were resubmitted in 1992, incorporating numerous
data corrections; there were also corrections made to Virginia monitoring data in 1992.
Because the data had been verified or corrected, the analyses in this report used data as
currently stored in the CBP data base without deleting any possible outliers.
Data were not adjusted for river flow. In the Chesapeake Bay monitoring program,
flow is only measured at the fall line stations, and only the Susquehanna River fall line
station at Conowingo, Md. is close enough to the main stem to have a direct impact on
it.7 Trend tests were performed on mean monthly Susquehanna River flow, and correla-
tions between log mean monthly flow and all parameters were calculated to estimate the
degree of association. However, a simple flow adjustment in the main stem of Chesa-
peake Bay is not possible because it would assume that flow has either an immediate
effect on concentrations or an effect after a fixed time lag. The effects of Susquehanna
River flow on main stem water quality must be highly variable because "Chesapeake
Bay's response to a freshet is a function of Chesapeake Bay's recent history and cannot
be linearized or easily predicted."8
TREND ANALYSIS METHODS
Trend analyses of nitrogen, phosphorus, and DO were performed on monthly mean
concentrations, spatially interpolated in three dimensions. Trend analyses of Secchi depth
were performed on monthly mean depths interpolated in two dimensions. Flow data used
were monthly means of daily Susquehanna River flows measured at Conowingo, Md.
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Relative Volumes of Main Stem Segments
CB8 WE4
CB7
CB6
CB2
WE4
Figure 1. CBP main stem monitoring stations and segments.
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Nutrient and Secchi monthly means for segment EE3 were estimated without using data
from the Maryland tributary monitoring stations in this segment because the tributary
data were not available for the whole time period. DO data were not estimated for seg-
ment EE3 because it has higher spatial variability than the other parameters. Some of the
metrics used for DO were slightly different from those used before. Trend analyses were
performed only on interpolator output, not on concentrations observed at individual sta-
tions (except for flow). Trend analyses were performed on DO, which was not done
previously.2 Trend analyses for some parameters were performed on data from either
April through October or June through September to correspond with the period of maxi-
mum effects on living resources. A nonparametric trend test was substituted for the
parametric test used in the previous analyses.3'4
The trend test used was the seasonal Kendall nonparametric test, which tests for
monotonic trends. Monotonic trends need not be linear, but they are assumed to have a
slope that is consistently positive or negative.9 Trends that change from positive to nega-
tive slope (or vice versa) may not be detected. The previous nitrogen and phosphorus
reports used linear, quadratic, and cubic parametric regression to assess both monotonic
and nonmonotonic trends.3'4 The seasonal Kendall test was performed with a custom
SAS program10 using the method described by Gilbert.9
The seasonal Kendall test assumes the successive monthly values are independent
or have no serial correlation. This is not often true of Chesapeake Bay water quality
data3-4; serial correlation tends to inflate the significance of the test.9 There are modifica-
tions to the seasonal Kendall test that account for serial correlation9; however, they
assume that the correlation has a fixed structure, while the actual correlations are quite
variable.3'4 Thus, the test was used without correcting for serial correlation. To account
for the possible inflation of significance levels that results from serial correlation, P val-
ues falling between 0.05 and 0.01 were termed "marginally significant" because their
actual P value might be more than 0.05. P values less than 0.01 were termed "significant"
because their actual P value was probably less than 0.05. A similar approach to determin-
ing significant trends with the seasonal Kendall test was > jd and reported as part of the
1991 re-evaluation of the Chesapeake Bay nutrient reduction strategy,* except that mar-
ginally significant trends were not identified.
The tables of seasonal Kendall test results list the sample size in months, the median
trend slope in mg/L per year, and the Z** score for significant trend. A large Z score indi-
cates a statistically significant monotonic trend: Z between 1.96 and 2.58 was considered
"marginally significant," with P between 0.05 and 0.01; Z>2.58 was considered "signifi-
cant," with P<0.01. The %2 value for seasonality tests whether the trend is homogeneous
over different months. A small x2 value and a large P value (>0.05) indicates there were
no significant seasonal differences in the trend. (Trends were not upward in some seasons
and downward in others.) Almost all parameters in which significant trends were found
had homogeneous trends over different months. The results were corrected for ties or
concentrations from the same month that are the same in 2 or more successive years. The
results were also checked to see if ties affected more than 50 percent of the results for any
""Water Quality Characterization Report for the 1991 Re-Evaluation of the Chesapeake Bay Nutrient Re-
duction Strategy," (draft), CBP, Annapolis, Md. (1991).
**Test statistic used to determine P value.
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
month. There were no cases when ties made up more than 50 percent of the observations
for any month.
Percent change over 8 years (1984 to 1992) is shown for parameters and segments
with statistically significant and marginally significant trends. This was calculated using
the 1985 WY mean value (October 1984 through September 1985) and the median slope
from the seasonal Kendall test:
t _. Slope (Per Year) X 8 Years
Percent Change = — ino_ ..... . . x 100 .
1985 WY Mean
Multiplying the slope by the number of years gives the total estimated change over
that period. The previous phosphorus and nitrogen trend reports3'4 used percent change
values calculated using the means of the first and last years of data (1985 and 1990 WY).
The advantage of using the seasonal Kendall slope to estimate the overall percent change
is that it uses all the data. Percent change calculated with the median slope will not be
affected as much if the last year of data had unusually high or low results. However, it
will still be affected if the first year of data was unusually high or low. Using the seasonal
Kendall slope also makes it possible to put confidence limits on the percent change esti-
mate9; 90 percent confidence limits were used for the one parameter with a significant
bay wide change (for TP).
The trend line shown in the graphs of the data also came from the mean of the first
year and the seasonal Kendall slope: The start point of the line was the 1985 WY mean,
and the end point was calculated from the following equation:
End Point = 1985 WY Mean + [Slope (Per Year) x 8 Years] .
Thus, the trend line represents the same data as the percent change estimate. The
same percent change estimate could be calculated from the following equation:
Percent Change = End Point-Start Point m
r Start Point
Trend lines and percent change estimates are only shown for parameters and seg-
ments with statistically significant or marginally significant trends.
Statistically significant and marginally significant trends were called either "im-
provement" or "degradation." Declining levels are improvements for nutrients, DO delta,
DO deficit, and DO volumes below specific concentrations. Increasing levels are im-
provements for Secchi depth and DO concentration.
What improvement and degradation mean in terms of the CBP efforts to preserve
and restore Chesapeake Bay can be evaluated using the habitat requirements and CBP
goals set for Chesapeake Bay's living resources. These have been established for SAV
and for species sensitive to low DO levels. If there is an improving trend in an area that
does not currently meet one of the SAV habitat requirements or DO goals, that trend will
aid in living resource restoration efforts.
SAV habitat requirements have been established for three of the parameters ana-
lyzed: DIP, DIN, and Secchi depth.5 The habitat requirements represent the maximum
concentrations or the minimum Secchi depth that will permit SAV growth. They are
based on growing season median values; the SAV growing season is April through Octo-
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
her in most of Chesapeake Bay.5 For this reason, trends in these three parameters were
evaluated over the whole year (12 months) and also over the April through October peri-
od (7 months). There are also SAV habitat requirements for two parameters that were not
analyzed in this report5: total suspended solids and'chlorophyll a.
The benchmarks for improvements in DO are based on four target concentrations:
0.2, 1, 3, and 5 mg/L. The 0.2 mg/L benchmark is based on the anticipated effect of the
40 percent nutrient reduction strategy1: a reduction in the volume of anoxic waters (de-
fined here as water with DO concentrations less than 0.2 mg/L). The last three
benchmarks (1,3, and 5 mg/L) were established by the Habitat Restoration Goal for
DO.11
ADJUSTMENTS FOR CHANGING-DETECTION LIMITS
CBP monitoring data have method detection limits (MDL), which represent the
lowest detectable concentration of that parameter. Analytical results that are less than the
MDL are censored by setting them to the MDL and are identified with a separate vari-
able. Parameters with observations censored at the MDL pose two problems for trend
analysis: They may bias the slope, since the detection limit values are greater than the
true values, and they may produce a statistically significant trend when none existed if
the MDL changed consistent!} er time. The first problem is avoided by the use of me-
dians in the seasonal Kendall tt . Censored data have no effect on the slope as long as
the censored values make up less than half of the observations.9 Resolving the second
problem is more complex. Several of the parameters analyzed had reductions in MDL,
and the seasonal Kendall test results were apparently affected by these reductions.
Reductions in MDL occurred in all four of the nitrogen and phosphorus parameters,
but detection limits did not change for DO or Secchi depth. For nitrogen and phosphorus
parameters, below detection limit (BDL) values were set to one-half the MDL before in-
terpolation, as in the previous analyses. For one parameter (TN) the MDL reductions
were small enough to be negligible.4 The effects of moderate reductions in MDL, which
occurred in TP data, were checked by raising any lower values to the highest MDL dur-
ing the time period. When the detection limits went down substantially over time, as they
did for DIN and DIP, the possible effects were checked with four separate analyses, using
two MDL adjustments and two time periods. These included setting BDL values to zero
and analyzing only data collected after October 1988, after the largest reductions in
detection limits had occurred. The appendix provides a listing of the frequencies of BDL
values for DIP and DIN.
SUSQUEHANNA RIVER FLOW
Correlations between upper Chesapeake Bay water quality and Susquehanna River
flow were analyzed to examine the strength of any relationships. The flow data used were
log-transformed monthly means of the daily flow at Conowingo, Md. Log transforma-
tions of flow data made their distribution closer to a normal distribution. Because flow
data and water quality data tend to show serial correlation, the P values listed are approx-
imate. The Pearson (parametric) correlation was performed with the correlation
procedure in SAS.10
10 Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
PHOSPHORUS.
The two phosphorus parameters analyzed were TP and DIP, which is the same as
orthophosphate (PO4 filtered). TP was chosen because it shows total enrichment for
phosphorus, while DIP is the form most readily utilized by phytoplankton. The three
main stem laboratories changed methods for TP several times,3 but their methods were
consistent after October 1988. TP had moderate reductions in MDL levels. Although DIP
had no method changes, it had large reductions in MDL, which can complicate trend
analysis.3 Since BDL values are censored at the MDL in the CBP data base, this could
produce a significant down trend that was caused by the lower MDL. To eliminate trends
that were due to MDL changes, different approaches were used for TP and DIP.
TP, with moderate MDL reductions, was interpolated two ways: with BDL values
set to one-half the MDL and also with all values below the highest MDL raised to that
value (0.01 mg/L). The results of the analyses were very similar, so the MDL changes did
not appear to affect the TP trends.
DIP had larger MDL reductions, so four different analyses were performed: trends
over all 8 years and over 4 years, starting in October 1988, when most detection limits
had been lowered; and with BDL values set to either one-half the MDL or set to zero to
assess the effects of BDL data on trends. Thus, the four BDL treatments for DIP con-
sisted of the following:
1. Eight years of data (1984 to 1992), with BDL data set to one-half the MDL;
2. Four years of data (1988 to 1992), with BDL data set to one-half the MDL;
3. Eight years of data (1984 to 1992), with BDL data set to zero;
4. Four years of data (1988 to 1992), with BDL data set to zero.
Because DIP is one of the SAV habitat requirements, it was analyzed over 7 months
(April through October) as well as over 12 months, so there were eight sets of analyses
for DIP. The results of these four BDL treatments were compared to eliminate any signif-
icant trends that were caused by MDL changes. Statistically significant trends were
eliminated if they met one or more of the following criteria:
• If there was a significant reduction with MDL set to one-half and no trend
or a significant increase with MDL set to zero, the reduction was probably
caused by declining MDL.
• If a trend was significant over 4 years but not over 8 years, there may be a
nonmonotonic trend that is unrelated to MDL changes.
• If there was a significant increase with BDL set to zero over 8 years but not
over 4 years, the increasing trend may be caused by lowering the early BDL
values to zero.
This approach to identifying real trends was conservative because any trends that
appeared to be caused b\ declining MDL were eliminated. However, it is still possibl
that some of the DIP trends identified were affected by MDL changes.
e
NITROGEN
The two nitrogen parameters analyzed were TN and DIN. TN was chosen because it
shows total enrichment for nitrogen, while DIN includes the forms most readily taken by
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
phytoplankton. TN is calculated from total Kjeldahl nitrogen whole plus nitrite/nitrate
(NOis) in early main stem data and from total dissolved nitrogen plus paniculate nitro-
gen in later data. DIN is calculated from nitrite/nitrate plus ammonium (NH4). As with
phosphorus, the total parameter had method changes, although it had minimal MDL re-
ductions, and the dissolved inorganic parameter had no method changes and large
reductions in MDL.12
The changes in TN MDL were small enough to have no effect on trends.12 The de-
clining MDL for DIN were dealt with using the same four treatments used for DIP,
except that trends could not be estimated for segments sampled by the Virginia Institute
of Marine Science (VIMS). This affected the four segments with a majority of VIMS sta-
tions: Tangier Sound (EE3), Mobjack Bay (WE4), and lower Chesapeake Bay segments
CB6 and CB7. VIMS detection limits for DIN had a series of large reductions that con-
tinued until July 1990, which left only 2 years of data after the reductions stopped. (See
Table A.2 in the appendix.) This was not enough time to evaluate whether any trends
were affected by declining MDL.
SECCHI DEPTH
Although Secchi depth has a lower MDL (0.1 meter), it did not change and it was
almost never encountered. Thus, there were no MDL problems for Secchi depth. There
were more ties in Secchi depth than in other parameters, but they still did not exceed 50
percent for any month.
DISSOLVED OXYGEN
Eight DO metrics were calculated and interpolated, and seven were analyzed statis-
tically for trend over the whole main stem and for each of nine main stem segments. DO
saturation trends were not analyzed, and data from EE3 were not analyzed for DO trends.
All DO metrics were analyzed for trend over the 4 warm weather months (June through
September) when low DO is most frequent. This time period is also used in the assess-
ment of low DO levels in CBP time-variable model output. Other months were excluded
because they would tend to obscure any trends that occurred during the warm weather
months. DO has no detection limits, since values of zero can occur. The following eight
metrics were calculated:
1. Monthly mean DO concentration: An upward trend shows improvement.
2. Monthly mean DO saturation concentration: This is calculated from water
temperature and salinity12 and expresses the potential DO concentration at
that temperature and salinity if the water was saturated with DO. Trends are
neither improvements nor degradation but represent changes in the amount
of oxygen that can be held in solution due to changes in water temperature
and/or salinity. For this reason, trends were not calculated for saturation; it
was used as an intermediate step in calculating the next two parameters.
3. Monthly mean DO delta concentration: This is calculated from DO satura-
tion minus DO concentration. To eliminate the effects of any supersaturated
conditions, DO delta was set to zero if less than zero. A downward trend
shows improvement. (The DO concentration is getting closer to saturation.)
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
4. Monthly mean DO deficit: This is calculated from DO delta, converting it
from a concentration to the mass of DO that would need to be added to that
segment to bring all areas up to DO saturation. DO deficit is the mass of
oxygen at saturation minus mass of oxygen present, omitting any supersatu-
ration. A downward trend shows improvement. (Less DO mass would need
to be added to achieve saturation.)
5 to 8. Monthly mean volume of water below four DO concentrations: 5, 3, 1, and
0.2 mg/L. These were calculated using DO data from all depths, but since
water in the surface layers rarely has low DO, almost all the volume with
low DO was from below the pycnocline. A downward trend shows im-
provement. (A smaller volume of water was below the cutoff
concentration.)
RESULTS AND DISCUSSION
SUSQUEHANNA RIVER FLOW
Susquehanna River flow data were analyzed because some water quality parameters
may have positive or negative correlations with flow. If trends in flow were similar in
magnitude and direction to the trends in one of the parameters analyzed and levels of that
parameter were correlated with flow, that would indicate that interannual changes in flow
might be responsible for the apparent trend in the water quality parameter.
Total annual Susquehanna River flow and the number of months with mean flow
above the 1950 to 1992 median for that month are shown in Figure 2. In an average year
there should be 6 months with flow above the median and 6 months with flow below the
median. Annual Susquehanna River flow was relatively high in 1986, 1989, 1990, and
1991 and relatively low in 1985, 1988, and 1992 (see Figure 2). The WY with the highest
total flow (1991) had relatively few months (5) with mean flows above the 1950 to 1992
median. This apparent discrepancy resulted from high flows in the first 5 months of that
WY (October 1990 through February 1991), which produced the high total flow, fol-
lowed by 7 months of below average flows.
Although there was a bimodal pattern in total flows (see Figure 2), there were no
statistically significant trends (F>0.05) in either mean or total monthly Susquehanna Riv-
er flows over either 12 or 7 months, over 8 years, or the first and last 4 years, using the
seasonal Kendall tests. This means that interannual changes in flow were probably not
responsible for any of the significant water quality trends observed. The levels of some of
the water quality parameters probably were related to interannual changes in flow, but not
in a simple fashion.
The complexity of the relationships between Susquehanna River flow and upper
Chesapeake Bay water quality is shown by the correlations in Table 1. If there was a sim-
ple relationship between nutrients and flow, nutrients would show positive correlations
with flow, with the strongest correlations in segment CB1 (Susquehanna Flats), especially
in surface samples. Correlations should generally be stronger for nitrogen than for phos-
phorus because nitrogen, especially nitrate, is more soluble in water than phosphorus, and
the lagged correlations should be stronger in segments farther from the fall line because it
takes water from the fall line longer to reach these segments.
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
\ \ I I 1 1 I
1985 1986 1987 1988 1989 1990 1991 1992
Total Flow
Year
# Months > median
Figure 2. Total annual Susquehanna River flow (bars) and number of months above median flow line
(water years 1985 to 1992).
Table 1. Correlations between log mean monthly Susquehanna River flow and mean monthly con-
centrations of water quality parameters (with P values in parentheses).
Parameter Months With
CB1
CB2
CBS
CB4
CBS
TP
TP
DIP
DIP
DIP
DIP
TN
TN
DIN
12
12
12
12
7
7
12
12
12
Flow
Lag
Flow
Lag
Flow
Lag
Flow
Lag
Flow
NS
NS
NS
NS
NS
NS
0.394
(0.0001)
0.285
(0.0051)
0.432
(0.0001)
NS
NS
-0.290
(0.0042)
-0.310
(0.0022)
NS
-0.277
(0.0404)
0.651
(0.0001)
0.545
(0.0001)
0.660
(0.0001)
-0.294
(0.0037)
-0.260
(0.0109)
-0.361
(0.0003)
-0.484
(0.0001)
-0.372
(0.0048)
-0.463
(0.0004)
0.737
(0.0001)
0.672
(0.0001)
0.775
(0.0001)
-0.426
(0.0001)
-0.314
(0.0019)
-0.485
(0.0001)
-0.359
(0.0004)
-0.468
(0.0003)
NS
0.618
(0.0001)
0.715
(0.0001)
0.737
(0.0001)
-0.538
(0.0001)
-0.394
(0.0001)
-0.554
(0.0001)
. -0.360
(0.0003)
-0.540
(0.0001)
NS
0.547
(0.0001)
0.673
(0.0001)
0.678
(0.0001)
14
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Table 1. Correlations between log mean monthly Susquehanna River flow and mean monthly con-
centrations of water quality parameters (with P values in parentheses) (Continued).
Parameter Months With
CB1
CB2
CBS
CB4
CBS
DIN
DIN
DIN
Secchi Depth
Secchi Depth
Secchi Depth
Secchi Depth
DO
DO
DO Delta
DO Delta
DO Deficit
DO Deficit
12
7
7
12
12
7
7
4
4
4
4
4
4
Lag
Flow
Lag
Flow
Lag
Flow
Lag
Flow
Lag
Flow
Lag
Flow
Lag
0.310
(0.0022)
0.568
(0.0001)
0.333
(0.0130)
-0.418
(0.0001)
-0.315
(0.0019)
-0.736
(0.0001)
-0.274
(0.0426)
0.398
(0.0242)
0.361
(0.0461)
NS
0.368
(0.0418)
NS
0.368
(0.0418)
0.561
(0.0001)
0.791
(0.0001)
0.476
(0.0002)
-0.382
(0.0001)
-0.404
(0.0001)
-0.578
(0.0001)
-0.340
(0.0112)
0.448
(0.0102)
NS
NS
NS
NS
NS
0.663
(0.0001)
0.813
(0.0001)
0.360
(0.0070)
-0.222
(0.0300)
-0.319
(0.0017)
-0.334
(0.0119)
NS
NS
NS
NS
NS
NS
NS
0.753
(0.0001)
0.772
(0.0001)
0.386
(0.0036)
NS
NS
NS
NS
NS
NS
NS
-0.361
(0.0461)
NS
-0.361
(0.0461)
0.690
(0.0001)
0.697
(0.0001)
0.384
(0.0038)
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
Legend and note:
NS—Not statistically significant (R>0.05).
Months: Number of months data used: 12—all year (No. of months—96);
7—April through October (No. of months—56); 4—June through September
(No. of months—32). With: Flow—Log mean monthly flow (log of mean of daily flows);
Lag—Log mean monthly flow of previous month.
The results (see Table 1) do show stronger positive correlations for nitrogen than for
phosphorus, but the nitrogen correlations with flow were always strongest in CB3 (un-
lagged) or CB4 (lagged) and weakest in CB1. Both forms of phosphorus had negative
correlations with flow in all of the segments with significant correlations, even though a
positive correlation is expected for TP. Paniculate phosphorus, part of TP, is often at-
tached to sediment, and higher flow usually increases sediment loads. Thus, reasons for
these negative correlations of phosphorus with flow are not clear.
Only Secchi depth showed the expected pattern of the strongest correlations in CB1
(or CB2 for lagged flow). This may be because Secchi depth has only one measurement
per station, while the values for the other water quality parameters were averaged over
results from two or four depths. The correlations of flow with Secchi depth were nega-
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
15
-------
live, presumably because higher flow brings more sediment, which reduces water clarity.
Correlations with unlagged flow were stronger during April through October than during
the whole year.
Some of the DO metrics showed positive correlations with flow or lagged flow in
CB1 and CB2, but the conflicting nature of the correlations means they were probably
not meaningful. The correlations suggest that higher flow is associated both with higher
DO concentrations in the current and following months (improvement), possibly due to
increased aeration, but also with higher DO delta and deficit a month later (degradation).
PHOSPHORUS
Total Phosphorus
Results show a statistically significant downward trend (improvement, P<0.01) bay-
wide, in upper Chesapeake Bay segment CB2, and in lower Chesapeake Bay segments
CB6 and CBS (see Table 2 and Figure 3). There were also marginally significant im-
provements (P<0.05) in upper Chesapeake Bay segments CB1 and CB3 and lower
Chesapeake Bay segment CB7 (see Table 2 and Figure 3). One segment with a significant
trend (CB2) had a barely significant seasonal heterogeneity: the %2 value was 20.6, slight-
ly more than the critical value of 19.7. However, since only 3 months had increasing
trends (November, January, and T ruary), the overall decline in CB2 appeared to be val-
id.
Changes in TP detection limits had little effect on the trend results. An interpolator
run with any values below 0.01 mg/L raised to that value had significant trends in the
same segments, with very similar slopes.
Figure 4 shows average monthly TP and DIP concentrations for each segment. DIP
(thin line) is shown for comparison to TP levels; it was also graphed separately. Segments
with statistically significant or marginally significant TP trends have a trend line connect-
ing the 1984 to 1985 mean and the 1991 to 1992 projection based on the seasonal
Kendall slope.
The median bay wide percent change in TP over 8 years (1984 to 1992), based on
the seasonal Kendall slope, was 16 percent plus or minus 8 percent (90 percent confi-
dence interval). This is slightly less than the previous baywide percent change estimate
for TP, which was 19 percent.3 One reason for the lower percent change is that March
1985 TP data, which were included in the previous analysis, were subsequently deleted
from the data base due to quality assurance problems and were not used in this analysis.
Percent change values for individual segments are listed in Table 2.
The declines (improvements) in TP in upper Chesapeake Bay segments CB1, CB2,
and CB3 were probably related to declines in Susquehanna River fall line concentrations.
There were statistically significant (P<0.1) declines in flow-adjusted TP concentrations at
the Susquehanna River fall line between 1984 and 1990, using both parametric regres-
sions and seasonal Kendall trend tests.*
*B. Dobler. Maryland Department of the Environment (MDE), unpublished analyses.
16 Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Table 2. Trend results for interpolated monthly mean total phosphorus by segment (12 months).
Segment
(CBP)
All
CB1
CB2
CBS
CB4
CBS
CB6
CB7
CBS
WE4
EE3
Slope
mg/L/yr
-0.00067
-0.001
-0.002
-0.001
-
-
-0.00092
-0.0008
-0.0024
-
-
Z Trend
-2.72
-2.15
-2.87
-2.48
-
-
-3.33
-2.53
-4.86
-
-
P
0.0066
0.032
0.0042
0.013
NS
NS
<0.001
0.011
<0.0001
NS
NS
X2
Seasonal
3.28
15.64
20.63
13.59
-
-
8.67
8.5
7.88
-
-
P
>0.95
>0.1
<0.05
>0.2
-
-
>0.5
>0.5
>0.7
-
-
%
Change
16
16
29
17
-
-
21
19
36
-
-
Legend and note:
The total number of months (A/) for all segments was 96 (October 1984 through
September 1992); all depths; results shown for segments with significant
(P<0.01, underlined) trends and marginally significant (P<0.05) trends only. A
negative (down) trend shows improvement (less phosphorus); NS—Not signifi-
cant (P>0.05). X2 seasonal and its P value (last two columns) are a test for ho-
mogerieity of the trend over different months. A P value of more than 0.05 indi-
cates the trends were homogeneous; the trend in CB2 appeared to be valid
even though P<0.05.
Possible causes of the declines (improvements) in lower Chesapeake Bay segments
CB6, CB7, and CBS are less clear. None of the tributaries draining into these main stem
segments (Rappahannock, York, and James Rivers) had declining trends in TP over the
period 1984 to 1991.13 In fact, all three rivers had some .segments and seasons with sig-
nificant increases (degradation) in TP, including the segment at the mouths of the
Rappahannock and James Rivers.13 The mouth of Chesapeake Bay (segment CBS) had
both the highest concentrations (see Figure 4) and the largest percent change (see Table
2) of these three segments, suggesting that the TP declines in that segment might be re-
lated to changes in oceanic concentrations.
The downward trends in TP concentrations are consistent with TP load reductions
over the whole watershed. Point source loads of TP were reduced by 40 percent between
1985 and 1990, while controllable nonpoint source loads fell 8 percent between 1985 and
1991, based on watershed model load estimates.1 Point sources of phosphorus comprise
34 percent of the watershed total loads and 42 percent of the controllable loads (exclud-
ing atmospheric deposition). Nonpoint sources of phosphorus comprise 60 percent of the
watershed total loads and 58 percent of the controllable loads.1
Dissolved Inorganic Phosphorus (Orthophosphate)
Reductions in MDL appeared to have a major impact on trend results for DIP. For
this reason, DIP trends were analyzed with four different MDL treatments to eliminate
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Upper Chesapeake Bay
Lower Chesapeake Bay
CB7
-CBS
no significant
change
marginally
significant
improvement
significant
improvement
Trend over all
main stem segments
marginally
significant
degradation
significant
degradation
CSC.MN1D.793
Figure 3. Total phosphorus trends in Chesapeake Bay main stem segments (October 1984 through
September 1992).
18
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
0.12-
j °'1~
£ 0.08-
QL
Q
6
CL
0.04-
0.02-
—^—^ TP
DIP
Note: Segments with trend lines had significant trends.
Total Bay
TP
DIP
Oct84 Oct85 OcU" Oct87 Oct88 Oct89 OctSO Ocl91 Oct92
CB1
CB2
Oct84 Oc!85 Oct66 Oct67 OcIB8 Oc!89 Ocl 90 OctSl Ocl 92
Oct84 Oct85 OCI66 Oct87 Ocl 88 Oct69 0«90 CW91 Ocl 92
CB3
CB4
OC164 0:185 0:'65 Ocl 87 Ocl 88 Oct89 Ocl 93 0:191 0:192
Oct84 OC185 OC186 Ocl 87 Ocl 86 Ocl 69 Ocl 90 OctSl Ocl 92
Figure 4. Average monthly concentrations of total phosphorus and dissolved inorganic phosphorus
(1984 to 1992).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
19
-------
__ IP
DIP
Note: Segments with trend lines had significant trends.
0<4-r-
012-
_ 01-
- J
1. 008-
g.
O
006-
004-
002-
CB5
TP
DIP
OC184 Oc165 Oct86 Oc! 87 Ocl88 Oct 89 Oct 93 Ocl91 Oct92
CB6
OCI84 del 85 Oct86 0087 Oc'66 Oct89 Oct9C Oct91 OC192
CB7
014-
012-
01-
008-
g_
Q
00<--
DIP
OC184 Oct85 Oct86 Oct67 Oct 88 OC189 Oct 90 OCI91 Oct92
CB8
OC184 OC185 OC186 Oct 87 Oct 88 Oct 89 Od90 Oct 91 Oct 92
EE3
014-
WE4
Oc1 E9 Oct 90 Oct91 OCI92
Oct84 ones oaee octs? oase octsg Oct9o octgi Oct92
Figure 4. Average monthly concentrations of total phosphorus and dissolved inorganic phosphorus
(1984 to 1992) (continued).
20
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
trends that were caused by MDL changes. Table 3 lists the results of these treatments
over 12 months and over the 7-month SAV growing season, for a total of eight sets of
anuhses Two trends that appeared to be real occurred o\er 12 months in CBS and 7
months in CB4 (both improvements) and are shown in Figure 5. Baywide, the trend was
not significant (see inset, Figure 5).
Table 3. Trend results for interpolated monthly mean levels of dissolved inorganic phosphorus by seg-
ment, using four different method detection limit treatments.
Table 3a. BDL data set to one-half.
Main Stem CBP Segments
No. of No. of
Months Years All CB1 CB2 CBS CB4 CBS CB6 CB7 CBS WE4 EE3
12
12
7
7
8 I - - -
4
8 I
4 - - IM I
I I I
_ _ _
I* IM I
_ _ _
I I*
I*
I IM
IM
I
DM
-
_
I
-
I
_
Table 3b. BDL data set to zero.
Main Stem CBP Segments
No. of
Months
12
12
7
7
No. of
Years All CB1
8
4 - -
8
4
CB2
-
-
-
IM
CBS CB4
- -
- -
IM*
' I
CBS
DM
-
-
-
CB6 CB7 CBS
IM*
- - I*
_
- - IM
WE4
D
-
D
-
EE3
D
DM
D
-
Legend and note:
I—Significant improvement (P<0.01).
IM—Marginally significant improvement (P<0.05).
D—Significant degradation (P<0.01).
DM—Marginally significant degradation (P<0.05).
Dash—No significant trend (P>0.05).
*Trends that appear to be real, i.e., not caused by declining detection limits.
7 months—April through October only, SAV growing season in lower salinity zones.
Figure 6 shows average monthly DIP concentrations for each segment, with BDL
value^ set to one-half the MDL (thick line) and to zero (thin line). Segments with statisti-
cally significant trends have two trend lines connecting the 1984 to 1985 mean and the
1991 to 1992 projection based on the seasonal Kendall slope, one for BDL set to one-half
(thick line) and one for BDL set to zero (thin line). In one segment these lines overlap
(CB4).
Details of the trend results from Table 3 that appeared to be real are summarized in
Tables 4 and 5. Percent change was near 30 percent in both CB4 and CBS. Results over 4
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
21
-------
Mouth of Chesapeake
L:/(CE~ -12 months
Middle Chesapeake Bay
(CB4)-7 months
no significant
change
marginally
significant
improvement
significant
improvement
Trend over all
main stem segments
marginally
significant
degradation
significant
degradation
CSC.MN1D7/93
Figure 5. Dissolved inorganic phosphorus trends in Chesapeake Bay main stem segments (October
1984 through September 1992).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
0.03-
D)
i, 0.02-
Q.
Q
0.01-
Censored data set to half of detection limit.
Censored data set to zero.
Note: Segments with trend lines had significant trends.
Total Bay
Oct84 Oct85 Oct86 Oct87 Oct88 Oct89 Oct90 Oct91 Oct92
CB1
004
CB2
Oc'86 Oc!87 Oc!88 OCI89 Oct90 OC191 Oct92
OMB< Oct85 Oct86 Oct87 Oct88 Oct89 OctM Oct91 Oc1S2
CBS
CB4
Oc- 6; Oc: 85 or
Oc'.86 Oct 89 Oc!90 Oct91 Oct92
OctBI Oct85 OctSS Oct87 OctSS Ocl89 CW90 Oct91 Oct92
Figure 6. Average monthly concentrations of dissolved inorganic phosphorus (1984 to 1992).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
23
-------
Censored data set to half of detection limit.
Censored data set to zero.
Note: Segments with trend lines had significant trends.
CB5
CB6
Od84 OC185 0086 Oc!87 Oc! 88 Oc! 69 Ocl90 CM 91 Oct92
Oct84 Ocl85 Oct86 Oct87 OctSB Oct89 OctSO Ocl91 Oct92
CB7
D)
E. 002-
CB8
OctBS Ofl86 Oct87 Oct88 Oct69 0«90 Oct91 Oct92
Oct84 Oct85 Oct86 Oct87 Oct88 Oct89 Oct90 Ocl91 Ocl92
EE3
0-*
Oc!84 Ocl85 Ocl86 Ocl87 Oct88 CW89 Oc!90 Oct91 Ocl92
WE4
CW84 OcI85 Oct86 Ocl87 Oct88 Ocl89 Oct90 OCI91 Oct92
Figure 6. Average monthly concentrations of dissolved inorganic phosphorus (1984 to 1992)
(continued).
24
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
years are not shown since those analyses were only performed to assess the effects of
changing detection limits.
Table 4. Trend results for interpolated monthly mean dissolved inorganic phosphorus by segment (12
months).
Segment
(CBP)
All
CB1
CB2
CBS
CB4
CBS
CB6
CB7
CBS
CB8
WE4
EE3
BDL Slope
Treatment mg/L/yr
-
-
-
-
-
-
-
-
1/2 -0.00069
0 -0.00059
-
-
X2
Z Trend P Seasonal
- NS -
- NS -
NS
NS
NS
NS
NS
NS
-3.75 0.00018 7.62
-2.36 0.018 11.77
NS
NS
%
P Change
-
-
-
-
-
-
-
- -
>0.7 31
>0.3 29
- -
-
Legend and note:
The total number of months (N) for all segments was 96 (October 1984 through September
1992); all depths; results shown for segments with significant (P<0.01, underlined) trends
and marginally significant (P<0.05) trends only. A negative (down) trend shows improvement
(less phosphorus); NS—Not significant (P>0.05). x2 seasonal and its P value (last two col-
umns) are a test for homogeneity of the trend over different months. A P value of more than
0.05 indicates the trends were homogeneous. Where there are two slopes for the same seg-
ment, they "bracket" the true slope.
Possible reasons for the declines (improvements) in DIP in middle Chesapeake Bay
(segment CB4) and the mouth of Chesapeake Bay (segment CBS) are not clear. DIP
trends have not been analyzed in tributary segments, mainly due to high MDL. The trend
in the mouth of Chesapeake Bay (segment CBS) may be related to changes in oceanic
concentrations; as was found for TP, CBS had higher concentrations than nearby seg-
ments CB6 and CB7 (see Figure 6).
The ecological significance of these trends in terms of SAV restoration appears to
be be minimal because both of the segments that had significant down trends had already
achieved the SAV habitat requirement for DIP (0.01 rng/L in me-sohaline regions and 0.02
mg/L in other salinity regimes5). The areas where DIP levels need to be reduced to per-
mit SAV growth are in tributary segments.*
*"\\ater Quality Restoration Priorities for Living Resources Report." (draft), CBP. Annapolis, Md (1993
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
25
-------
Table 5. Trend results for interpolated monthly mean dissolved inorganic phosphorus by segment
(7 months, April through October).
Segment
(GBP)
All
CB1
CB2
CB3
CB4
CB4
CBS
CB6
CB7
CB8
WE4
EE3
BDL Slope
Treatment mg/L/yr Z Trend
- -
_
_
_
1/2 -0.0003 -2.81
0 -0.000267 -2.21
_ _
- -
- -
- -
_
- -
P
NS
NS
NS
NS
0.005
0.027
NS
NS
NS
NS
NS
NS
I2 %
Seasonal P Change
_ _ _
- -
- -
- - -
1.37 >0.95 31
2.33 >0.8 28
- -
- -
- - -
- - -
_ _
_ _
Legend and note:
The total number of months (N) for all segments was 56 (April 1985 through September
1992); all depths; results shown for segments with significant (P<0.01, underlined) trends
and marginally significant (P<0.05) trends only. A negative (down) trend shows improvement
(less phosphorus); NS—Not significant (P>0.05). x2 seasonal and its P value (last two col-
umns) are a test for homogeneity of the trend over different months. A P value of more than
0.05 indicates the trends were homogeneous. Where there are two slopes for the same seg-
ment, they "bracket" the true slope.
NITROGEN
Tola! Nitrogen
There was no significant trend baywide for TN (see Table 6 and inset, Figure 7).
There was a marginally significant increase (degradation, P=0.027) in segment WE4,
which includes the mouth of the York River (see Figure 7).
Figure 8 shows average monthly TN and DIN concentrations for each segment. DIN
(thin line) is shown for comparison to TN levels, it was also graphed separately. Seg-
ments with marginally significant or statistically significant TN trends have a trend line
connecting the 1984 to 1985 mean and the 1991 to 1992 projection based on the seasonal
Kendall slope.
The lack of any TN trend in upper Chesapeake Bay is consistent with the flow-ad-
justed TN loads at the Susquehanna River fall line. Data from this station showed no
significant changes in loads (P>0.1) between 1984 and 1990.*
L'i \iDL. unpuhli'.nco anai\-
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Table 6. Trend results for interpolated monthly mean total nitrogen by segment.
Segment
(CBP)
All
CB1
CB2
CBS
CB4
CBS
CB6
CB7
CBS
WE4
EE3
Slope
mg/L/yr Z Trend P
NS
NS
NS
- NS
- NS
NS
NS
- NS
NS
0.006 2.21 0.027
- NS
X2 %
Seasonal P Change
_ _ _
_ _ _
_
_ _ _
_
_
_
_
_
5.74 >0.8 10
_
Legend and note:
The total number of months (A/) for all segments was 96 (October 1984 through
September 1992); all depths; results shown for marginally significant (P<0.05)
trends only. A negative (down) trend shows improvement (less nitrogen);
NS—Not significant (P>0.05). x2 seasonal and its P value (last two columns)
are a test for homogeneity of the trend over different months. A P value of
more than 0.05 indicates the trends were homogeneous.
The increase (degradation) in Mobjack Bay (segment WE4) is probably related to
increases in nearby tributary segments. York River tributary segments LE4, RET4, and
TF4 also showed significant TN increases (degradation).* Trend analyses by the Virginia
Department of Environmental Quality found increasing trends in both TN and chloro-
phyll a in all of the tidal sections of the York River.13
The general lack of significant trends in TN concentrations is consistent with TN
point source load reductions over the whole watershed, which have been smaller than
point source load reductions for TP. Point source loads of TN were reduced by only 6 to 7
percent between 1985 and 1990, while controllable nonpoint source nitrogen loads fell 12
percent between 1985 and 1991, based on watershed model load estimates.1 Point
sources of nitrogen comprise 23 percent of the watershed total loads and 46 percent of the
controllable loads (excluding atmospheric deposition). Nonpoint sources of nitrogen
comprise 68 percent of the watershed total loads and 54 percent of the controllable
loads.1
Dissolved Inorganic Niirogen
As was found for DIP, reductions in MDL appeared to have a major impact on trend
results for DIN. For this reason, DIN trends were also analyzed four different ways:
1. Over all 8 years;
""Water Quality Characterization Report for the 1991 Re-Evaluation of the Chesapeake Bay Nutrient Re-
duction Strategy" (draft). CBP. Annapolis. Md (1991)
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992 27
-------
Mobjack Bay
(WE-4)
Trend overall
main stem segments
no significant
change
marginally
significant
improvement
significant
improvement
marginally
significant
degradation
significant
degradation
Figure 7. Total nitrogen trends in Chesapeake Bay main stem segments (October 1984 through
September 1992).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
——— IN
DIN
Note: Segments with trend lines had significant trends.
Total Bay
Oct84 Oct85 Oct86 Oct87 Oct88 Oct89 Ocl90 Oct91 Oct92
CB1
CB2
25-r
OC184 Oct 85 CW86 Oct67 Oct 68 Oct89 Oct90 Oct91 Oct 92
Oct64 OC185 Oct86 OC187 OC188 Oct89 Oct90 Oct91 Oct92
CBS
CB4
25-q
Oc'Si Oc!E5 Oct86 Oct 87 0:'65 Oc! 89 Oct9D OctSl Oct92
Oct85 Oct86 Ocl87 Oct88 Ocl89 Oct90 Oct91 Oct
Figure 8. Average monthly concentrations of total nitrogen and dissolved inorganic nitrogen (1984 to
1992).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
29
-------
^—^—— IN
DIN
Note: Segments with trend lines had significant trends.
CB5
CB6
Oct 84 Oct85 Oc;B6 Oct87 Ocl 88 Oct89 Oc! 90 0:191 Ocl 92
Oc:65 Oct86 0:187 Oc!88 Oct 89 0:190 Oe 91 Oc! 92
CB7
CBS
0=164 0:185 0:166 Ocl 67 Oct 88 Oct 69 Ocl 90 Ocl 91 Oct 92
0:184 Ocl 85 0:186 Oc! 87 Oct88 OCI89 Ocl 90 Oct 91 Oct92
EE3
WE4
Oc-,84 OciBS Ocl 86 Oct 87 Oct 88 Oct 89 Oct 90 0:191 0:192 Oct 84 Oct 85 Oct 86 Oct 87 Od«8 Oct 89 Oct 90 Oct 91 0:192
Figure 8. Average monthly concentrations of total nitrogen and dissolved inorganic nitrogen (1984 to
1992) (continued).
30
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
2. Over the last 4 years;
3. Starting in October 1988, when most detection limits were lowered;
4. With BDL set to either one-half the MDL or set to zero to assess the effects
of BDL data on trends.
There was no significant trend bay wide over the 8-year period (see Table 7 and in-
set, Figure 9). None of the significant trend results in Table 7 appeared to be real; the
significant trends in lower Chesapeake Bay segments CB6, CB7, WE4, and EE3 could be
caused by declining detection limits. The significant improvements in total (baywide) and
other segments only appeared over 4 years (see Table 7), so they were eliminated as
probable results of nonmonotonic trends.
Figure 10 shows average monthly DIN concentrations for each segment, with BDL
values set to one-half the MDL (thick line) and to zero (thin line). These lines usually
overlap, and the thin line is only visible in a few segments. There is a pattern of increas-
ing concentrations followed by decreasing concentrations in the three upper Chesapeake
Bay segments (CB1, CB2, and CB3) that are closest to the Susquehanna River and, thus,
most affected by its flow. This pattern was not apparent in any other segments.
SECCHI DEPTH
Results show no significant trend baywide (see inset, Figure 11) or for any segment
over 12 months (see Figure 11). There were marginally significant upward trends (im-
provements) in upper Chesapeake Bay segments CB1 and CB2 over the 7-month SAV
growing season (April through October) (see Table 8 and Figure 11). CB2 includes the
turbidity maximum in Chesapeake Bay.
Figure 12 shows average monthly Secchi depths for each segment. Segments with
significant or marginally significant trends have a trend line connecting the 1984 to 1985
mean and the 1991 to 1992 projection based on the seasonal Kendall slope. The SAV
habitat requirement for light attenuation (KD), which is related to Secchi depth, was
often not met in segment CB2. The requirement was usually met in years with low flow
and not met in years with high flow. The KD requirement for this area (2.0 m"1), is
equivalent to a Secchi depth of 0.73 m (using Secchi=1.45/KD5). In CB2, the KD/Secchi
requirement was met in 3 of the last 8 years (38 percent attainment). It was met in 1988
and 1992, both low flow years, also in 1991, a year with high total flow but a below aver-
age number of months (5) above median flow (see Figure 2). The KD/Secchi requirement
was not met in 5 years (1985 through 1987 and 1989 and 1990). Four of these years had
high Susquehanna River flow, but 1985 had relatively low flow (see Figure 2).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992 31
-------
Table 7. Trend results for interpolated monthly mean levels of dissolved inorganic nitrogen by segment
using four different method detection limit treatments.
Table 7a. Below detection limit data set to one-half.
Main Stem CBP Segments
No. of No. of
Months Years All CB1 CB2 CBS CB4 CBS CB6 CB7 CBS WE4 EE3
12
12
7
7
8
4
8
4
_
IM IM I
_
I IM I
- - -
I I IM
- - -
I I IM
I+
IM+
IM
IM
l+
IM+
I
I
IM l+ IM+
IM+
IM
- - IM
Table 7b. Below detection limit data set to zero.
Main Stem CBP Segments
No. of No. of
Months Years All CB1 CB2 CB3 CB4 CBS CB6 CB7 CBS WE4 EE3
12
12
7
7
8
4
8
4
-
IM
-
I
I I I
- -
IM I !
- -
I IM
-
I IM
l+
IM+
-
IM
l+
IM+
-
I _
IM+ IM+
-
-
IM
Legend and note:
I—Significant improvement (P<0.01).
IM—Marginally s'^:':~a-it improvement (P<0.05).
Dash—No significant trend (P>0.05).
+These trends were not identified as real because detection limits in these segments continued to de-
cline until July 1990; therefore, the marginally significant 4-year trends could be caused by declining
detection limits.
7 months—April through October only, SAV growing season in lower salinity zones.
The annual pattern of attainment of the KD requirement suggests an inverse correla-
tion between Susquehanna River flow and Secchi depth in these segments, which was
found (>ec lable 1). However, the lack of a significant monotonic trend in flow over this
period means that the marginally significant trends in Secchi depth in upper Chesapeake
Bay were IT~>' a simple consequence of a trend in flow.
32
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
no significant
change
marginally
significant
improvement
significant
improvement
Trend over all
main stem segments
marginally
significant
degradation
significant
degradation
CSC.MN1D.7/93
Figure 9. Dissolved inorganic nitrogen trends in Chesapeake Bay main stem segments (October 1984
through September 1992).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
33
-------
2.5 -q
Censored data set to half of detection limit.
Censored data set to zero.
Note: Segments with trend lines had significant trends.
Total Bay
0
Oct84 Oct85 0 -i Oct87 Oct88 Oct89 Oct90 Oct91 Oct92
CB1
CB2
Oct64 Oc!65 OC186 Oc'.87 Oc',88 Ocl 89 Ocl 90 OcI91 Ocl S
Oct84 OC185 Ocl 86 Oct87 Ocl 88 Ocl 69 OctSO Ocl 91 Ocl 92
CBS
"64 Oc;65 Oc. £5 Oc' £7 Oc188 Ocl 89 0:1 K Ocl 91 Ocl 92
25-q
1.5-
05H
CB4
0:184 Ocl 85 OC186 Ocl 87 Oct88 Oct89 Oct90 Oct91 Ocl 92
Figure 10. Average monthly concentrations of dissolved inorganic nitrogen (1984 to 1992).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Censored data set to half of detection limit.
Censored data set to zero.
Note: Segments with trend lines had significant trends.
CB5
CB6
Oct 84 Octes Oct ee Ocie? Octes Octeg 0=190 Oci9t 00192
Oct84 Octss Octee Octe? Octee oass 0090 Oct9i Oct92
CB7
CBS
25
Oct64 Oci65 Oct86 Oc: 87 Oc!S6 OC189 Oc! 90 Oct 91 Oc! 92
OCI84 Oct 85 0:186 Oct 67 Oct 68 Oct 69 Oct 90 Oct 91 Oct 92
EE3
Oct 62 Oc'85 Oc;85 0:'87 Oc! 88 Oct 69 Oc: 90 C::S' 0:192
WE4
"i:
04-j
03H
I 02-
Oct6< Oct 65 Oct 86 Oct 67 Oa88 Oct 89 Oct 90 Oct 91 Oct 92
Figure 10. Average monthly concentrations of dissolved inorganic nitrogen (1984 to 1992) (continued).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
35
-------
CB1
Northern and Upper
Chesapeake Bay -7
C02 months
no significant
change
marginally
significant
improvement
significant
improvement
Trend over all
main stem segments
marginally
significant
degradation '
significant
degradation
CSC.MN1D.7/93
Figure 11. Secchi depth trends in Chesapeake Bay main stem segments (October 1984 through
September 1992).
36
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Secchi depth
Note: Segments with trend lines had significant trends.
Total Bay
i i i r
Oct84 Oct85 Oct86 Oct87 Oct88 Oct89 Oct90 Oct91 Oct
92
E 3-
CB1
0:!64 Oct85 OclBS Oct 67 Oct 85 Oct 89 Oct 90 Oct 91 Oct 92
CB2
Oct 84 Oct 85 OC186 Oct 87 Oct 88 Oct 89 Oct 90 Oct91 Oct 92
CB3
CB4
Ocl6< OctES Oc!66 Oct 67 Oct 88 Oct 69 Oc'. K 0:'. 91 0::S2
Oct 84 Oct 85 Oct 86 Oct 87 Oct 88 Oct 89 Oct 90 Oct 91 Oct 92
Figure 12. Average monthly Secchi depths (1984 to 1992).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
37
-------
Secchi depth
Note: Segments with trend lines had significant trends.
CB5
CB6
e 3-
§25^
05-
0:!64 OCI85 Oct 65 OC187 Oct 88 Oct 63 Oct 90 Oc!91 0:; 52
-1 1-
Oct64 Oct 85 Oct 66 Oct 87 Oct 88 Oct 89 Oct 90 OctS1 Oct 92
CB7
CBS
Oc',87 Oct 68 OS 89 Oct 90 Oct 91 Oc: 92
Oct 84 Oct 85 0086 Oct 87 Oct 68 Oct 89 Oc! 90 Oct 91 Oct 92
EE3
WE4
45-
4-
•g ^
,S 15-
Oct64 Oc!85 Oct 86 Oct 67 Oc! 68 Oct 89 Oct 90 Oct 3: Oc! 92
Oct 84 Oct 85 Oct 86 Oct 87 Oct 88 Oct 89 Oc!90 Oct 91 Oct 92
Figure 12. Average monthly Secchi depths (1984 to 1992) (continued).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Table 8. Trend results for interpolated monthly mean Secchi depth by segment (7 months, April through
October).
Segment
(CBP)
All
CB1
CB2
CBS
CB4
CB5
CB6
CB7
CBS
WE4
EE3
Slope
m/yr
-
0.017
0.025
-
-
-
-
-
-
-
-
Z Trend P
NS
1.97 <0.05
2.38 <0.017
NS
NS
NS
NS
NS
NS
NS
NS
X2
Seasonal
-
4.92
8.46
-
-
-
-
-
-
-
-
%
P Change
- -
<0.7 18
>0.2 34
- -
-
-
-
- -
- -
- -
-
Legend and note:
The total number of months (A/) for all segments was 56 (April 1985 through
September 1992); all depths; results shown for marginally significant (P<0.05)
trends only. No segments had significant trends over 12 months. A positive
(up) trend shows improvement (clearer water); NS—Not significant (P>0.05).
X2 seasonal and its P value (last two columns) are a test for homogeneity of
the trend over different months. A P value of more than 0.05 indicates the
trends were homogeneous.
Secchi depth is not measured in the Susquehanna Rixer, so it is not known whether
there were trends in Secchi depth there. Total suspended solids and turbidity data col-
lected at the Susquehanna River fall line station (CB1.0) showed no significant trends
overthe 1984-to-1991 period (P>0.1*).
DISSOLVED OXYGEN
Trend results for the two DO metrics that had significant trends are shown in Tables
9 and 10. Table 9 shows results for DO delta, and Table 10 shows results for DO deficit,
both over the warm weather period (June through September). Trend results for DO con-
centration and the four metrics for volumes below specific concentrations are not shown
because no trends were statistically significant for those parameters. Segment CBS
showed marginally significant degradation in both DO delta and DO deficit (see Figure
13).
*B Dobler. MDE. unpublished analyses
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992 39
-------
Mouth of Chesapeake
Bay (CB8)-4 months,
both parameters
DO delta is DO at saturation minus DO.
DO deficit is mass at saturation minus
mass present.
no significant
change
marginally
significant
improvement
significant
improvement
Trend over all
main stem segments
marginally
significant
degradation
significant
degradation
CSC.MN1D.5TO
Figure 13. Dissolved oxygen delta and dissolved oxygen deficit trends in Chesapeake Bay main stem
segments (October 1984 through September 1992).
40
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Table 9. Trend results for interpolated monthly mean dissolved oxygen delta by segment (4 warm
weather months, June through September).
Segment
(CBP)
All
CB1
CB2
CBS
CB4
CBS
CB6
CB7
CBS
WE4
Slope
mg/L/yr
-
-
-
-
-
-
-
-
0.038
-
Z Trend P
NS
NS
NS
NS
NS
NS
NS
NS
2.50 0.012
NS
X2 %
Seasonal P Change
_
_
-
_ _ _
_
- -
- -
- - -
0.57 0.9 77
_ _
Legend and note:
The total number of months (N) for all segments was 32 (June 1985 through
September 1992); all depths; results shown for marginally significant (P<0.05)
trends only. A positive (up) trend shows less desirable conditions (degrada-
tion); NS—Not significant (P>0.05). x2 seasonal and its P value (last two col-
umns) are a test for homogeneity of the trend over different months. A P value
of more than 0.05 indicates the trends were homogeneous. DO data were not
analyzed in EE3.
Figure 14 shows average monthly DO and DO delta concentrations for each seg-
ment. .V DO (upper line) goes down in the summer, DO delta (lower line) goes up.
especially in segment CB4. Segments with marginally significant trends (DO delta in
CBS) have a trend line connecting-the 1984 to 1985 mean and the 1991 to 1992 projec-
tion based on the seasonal Kendall slope. Although some segments appear to have
declining trends in DO concentration, e.g., total segments and segment CBS, these were
not statistically significant when tested over the warm weather period only (June through
September).
Figure 15 shows the total volumes of water in each segment with DO below four
concentrations: 0.2, 1, 3, and 5 mg/L, summed over all 4 months of the warm weather
period (June through September). Hypoxia/anoxia is a problem during late spring and
summer in the deeper waters of middle Chesapeake Bay (primarily segments CB4 and
CBS). Reducing the volume of anoxic water in Chesapeake Bay is a major goal of nutri-
ent reduction strategies. The trend results for anoxic volume and the volumes of water at
the other target concentrations were not significant in any segment; in segments CB4 and
CBS (segments with relatively large total volumes), the volumes at the higher concentra-
tion categories were fairly consistent from year to year, with some shifting between the
less than 0.2 and 0.2-to-l mg/L categories. Lower Chesapeake Bay segments CB6 and
CB7, also relatively large segments, had almost no anoxic water and had more variability
in the volumes of water in the other categories. A low flow year, 1988 (see Figure 2),
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992 41
-------
DO concentration
DO delta
Note: Segments with trend lines had significant trends.
Total Bay
CONC.
DELTA
Oct84 Oct85 Oct86 Oc!87 Oc!88 Oct89 Oct90 Oct91 Oct92
CB1
CB2
- I _\_- I--1—J \_l \—l \—I—I—I—I 1—I CONC
DELTA
Oci84 oaes Octee Octe: Ocies oct89 Oct9o
Oct84 OCI85 Oct86 Oc!87 OctBB Oct89 Oct90 OcS1 Oct92
CBS
CB4
CONC
DELTA
CONC
DELTA
Ocl84 OC185
Oc'87 Oc!88 Ocl89 Oc!90 Oct91 Oct92
Ocl84 OCI85 OCI86 0«87 OcISS Oct89 Oct90 OCI91 Oct92
Figure 14. Average monthly concentrations of dissolved oxygen and dissolved oxygen delta (1984 to
1992).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
DO concentration
DO delta
Note: Segments with trend lines had significant trends.
^ 9-
2 6-
8 34
o
D
CBS
CONC
DELTA
CBS
CONC
DELTA
OC184 OC185 Oc!86 Oc!87 Oc!88 Oc!B9 OctSD Oct9; Oct92
Oc!84 Oct85 Oct86 OC187 Oct88 OctBS CW90 0«91 Oct92
15-
CB7
^ \—/-\- CONC
DELTA
CBS
CONC
DELTA
OCI84 Ocl35 OC186 OctB? OC186 Oc!89 Oct93 Oct91 Oc!92
Oct84 Oct85 Ocl86 Oc187 CW88 Oct89 Oct90 Oct91 0«92
WE4
CONC
DELTA
OC184 Oct85 OC186 OC187 Oct88 CW89 Ocl90 CW91 Oct92
Figure 14. Average monthly concentrations of dissolved oxygen and dissolved oxygen delta (1984 to
1992) (continued).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
43
-------
140-
«
Fso-
DO Concentration Range
3.0-5.0 n 0.2-1.0
1.0-3.0
0.0-0.2
140^
Total Bay
1985 1986 1987 1988 1989 1990 1991 1992
CB1
CB2
140 -i
•5
|3CH
19S5 1986 1987 1988 1989 1990 1991 1992
CBS
1985 1986 1987 1988 1989 1990 1991 1992
CB4
Ian
10-j
B S H
B B
1965 19S6 1967 1988 1989 1990 1991 1992
1985 1986 1987 1988 1989 1990 1991 1992
Figure 15. Total volumes of water with dissolved oxygen concentrations below 0.2, 1, 3, and 5 mg/L
(June through September, 1985 to 1992).
44
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
DO Concentration Range
3.0-5.0 n 0.2-1.0
1.0-3.0
0.0-0.2
CBS
it
rso-i
•5 :
S«H
!«
CB6
pnP PPPDP
1985 1986 1987 1988 1969 1990 1991 1992
1985 1986 1987 1988 1989 1990 1991 1992
140
30-
10-i
CB7
n
1985 1986 19E7 1988 1989 1990 1991 1992
01 50-
|20i
| 10-
CBS
r , — ,
1985 1986 1987 1988 1989 1990 1991 1992
WE4
_ ?e
I" 50-
^20^
1985 1986 1987 1988 1989 1990 1991 1992
Figure 15. Total volumes of water with dissolved oxygen concentrations below 0.2,1, 3, and 5 mg/L
(June through September, 1985 to 1992) (continued).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
45
-------
usually had the lowest total volume of water below 5 mg/L. This is probably due to re-
duced stratification in low flow years.
Table 10. Trend results for interpolated monthly mean dissolved oxygen deficit by segment (4 warm
weather months, June through September).
Segment
(CBP)
All
CB1
CB2
CBS
CB4
CBS
CB6
CB7
CBS
WE4
Slope
kg/yr*
-
-
-
-
-
-
-
-
143.4
-
ZTrend P
NS
NS
NS
NS
NS
NS
NS
NS
2.04 0.041
NS
X2 %
Seasonal P Change
_ _
_ _
_
_
_
- - -
_ _
- - -
0.17 0.98 88
_ _ •
'Units are kgx 1011.
Legend and note:
Dissolved oxygen deficit is dissolved oxygen delta converted to a mass of oxy-
gen. The total number of months (N) for all segments was 32 (June 1985
through September 1992); all depths; results shown for marginally significant
(P<0.05) trends only. A positive (up) trend shows a movement toward less de-
sirable conditions (degradation); NS—Not significant (P>0.05). x2 seasonal
and its P value (last two columns) are a test for homogeneity of the trend over
different months. A P value of more than 0.05 indicates the trends were homo-
geneous. DO data were not analyzed in EE3.
The marginally significant trends in two DO metrics at the mouth of Chesapeake
Bay (segment CBS) have no obvious potential causes. DO concentrations are generally
high in CBS (see Figure 14) and DO delta is quite low, so these trends are unlikely to
ha\e any negative impact on living resources in CBS. The high percent change values (77
and 88 percent) appear to be partly due to abnormally low values in the 1985 WY.
PLANS FOR FUTURE TREND ANALYSES
INTERPOLATING ABOVE AND BELOW PYCNOCLINE LAYERS AND SURFACE
AND BOTTOM LAYERS SEPARATELY
Several enhancements to the trend analysis methods are planned for the next trend
analysis update. The enhancements that may be implemented include interpolating above
and below pycnocline layers and surface and bottom layers separately. This will make it
possible to perform trend analyses of water quality in separate water layers. Trend analy-
ses of DO concentrations will focus on the bottom layer and the region below the
pycnocline, where almost all of the low DO concentrations occur. Trends in nutrient pa-
46 Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
rameters affecting SAV growth, DIN, and DIP will focus on the surface mixed layer,
since SAV habitat requirements are defined only for surface concentrations.
ACCOUNTING FOR INTERANNUAL CHANGES IN FLOW
The seasonal Kendall test accounts for seasonal changes in flow within years, but
not for changes in flow between years. Interannual flow differences could be estimated
from fall line flow data; however, this is difficult in the main stem, where the fall line
may be quite far from the segment and flow from more than one river affects some seg-
ments. Flow effects will probably be estimated indirectly from the degree of stratification
within each main stem segment, especially for DO.
ADDING PARAMETRIC TREND TESTS
Software is currently being developed to streamline the autoregressive parametric
trend tests that were used in two of the previous trend reports.3'4 The advantages of these
tests are that they account for serial correlation in the data; therefore, the significance lev-
els are more accurate and they can account for changes in detection limits.
ADDING TREND TESTS ON INTERPOLATED TRIBUTARY DATA
The volumetric interpolate! >es not currently operate in the tributaries, but there
are plans to develop this capability This would permit the analysis of tributary water
quality trends using the same methods and time periods used to analyze main stem trends.
SUMMARY
The trend results for phosphorus, nitrogen, Secchi depth, and DO are summarized in
Table 11. Because eutrophication is "one of the main causes of low DO in Chesapeake
Bay. there should be improvements in DO where there are improvements (declines) in
nutrient levels. Improving trends in DO would increase the amount of living resource
habitat in Chesapeake Bay. Table 11 shows that there were statistically significant de-
clines (improvements) in TP and DIP in some segments but no corresponding
improvements in any of the DO metrics. This lack of improving trends in DO could be
due to two factors:
1. The nutrient declines, although statistically significant, may not have been
large enough to improve DO conditions or they may not have affected
enough of Chesapeake Bay. Also, nitrogen has not shown significant im-
provements in any segments. Summer hypoxia and anoxia may be more
affected by the freshet and its nitrogen supply than by phosphorus levels.
2. There has not been enough time for DO responses. There may be a time lag
between nutrient reductions and DO improvements. Nutrients stored in sed-
iments from previous years may promote DO depletion,8 although some
authors have argued that the timing and extent of summer anoxia are gov-
erned mainly by climatic conditions in that year.14 Upbay and downbay
transfer of nutrients, organic matter, and DO also complicate any responses
of DO levels to nutrient reductions.
Improving trends in dissolved inorganic nutrients and Secchi depths can also lead to
attainment of SAV habitat requirements if the requirements are not currently met. This
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992 47
-------
should promote SAV restoration. Both of the segments that had improving trends in DIP
are already in attainment for those requirements, so these trends should have little impact
on SAV restoration. However, the improving trend in Secchi depth in one upper Chesa-
peake Bay segment (CB2) may lead to an increased frequency of attainment of the Secchi
depth requirement in that segment in the future.
Table 11. Summary of trend results (October 1984 through September 1992).
Main Stem CBP Segments
Parameter
No. oi
Months All CB1 CB2 CBS CB4 CBS CB6 CB7 CBS WE4 EE3
TP
DIP
DIP
TN
DIN
DIN
Secchi Depth
Secchi Depth
DO Concentration
DO Delta
DO Deficit
DO<0.2
DO<1.0
DO<3.0
DO<5.0
12
12
7
12
12
7
12
7
4
4
4
4
4
4
4
I IM I IM - - I IM I - -
________ | __
____ | ______
_________ DM
------ + + - + +
------ + + - + +
___________
-IMIM --------
-----------
________ DM --
__._ _ _ _ _ _ DM -
-----------
-----------
___________
-----------
Legend and note:
I—Significant improvement (P<0.01).
IM—Marginally significant improvement (P<0.05).
DM—Marginally significant degradation.
Dash—No significant trend (P>0.05).
H—DIN trends could not be assessed in these segments because detection limits did not stop declin-
ing untilJuly 1990.
7 months—April through October only, same as SAV growing season in lower salinity zones; 4
months—June through September only, used as period of anoxia in three-dimensional model analy-
ses.
DO data were not analyzed in EE3.
See text for explanation of DO delta, DO deficit, and DO volumes below the four concentrations.
48
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
REFERENCES
1. "Progress Report of the Baywide Nutrient Reduction Reevaluation: 1991 Reeval-
uation Report No. 5," CBP/TRS 92/93, CBP, Annapolis, Md. (1993).
2. "Dissolved Oxygen Trends in the Chesapeake Bay (1984-1990)," CBP/TRS
66/91, CBP, Annapolis, Md. (1991).
3. "Trends in Phosphorus in the Chesapeake Bay (1994-1990)," CBP/TRS 67/91,
CBP, Annapolis, Md. (1991).
4. "Trends in Nitrogen in the Chesapeake Bay (1984-1990)," CBP/TRS 68/92,
CBP, Annapolis, Md. (1992).
5. Batiuk, R.A., R.J. Orth, K. Moore, W.C. Dennison, J.C. Stevenson, V. Carter, N.
Rybicki, R. Hickman, S. Kollar, S. Bieber, and P. Heasly, "Chesapeake Bay Sub-
merged Aquatic Vegetation Habitat Requirements and Restoration Targets: A
Technical Synthesis," CBP/TRS 83/92, CBP, Annapolis, Md. (1992).
6. Reynolds, R., and L. Bahner, "A Three-Dimensional Interpolator for Estimating
Water Quality Conditions in the Chesapeake Bay: Description and Preliminary
Application to Dissolved Oxygen," Computer Sciences Corp., Annapolis, Md.
(1989).
7. Schubel, J., and D. Pritchard, "Responses of Upper Chesapeake Bay to Varia-
tions in Discharge of the Susquehanna River," Estuaries, 9:236-249 (1986).
8. Taft, J., E. Hartwig, and R. Loftus, "Seasonal Oxygen Depletion in Chesapeake
Bay," Estuaries, 3:242-247 (1980).
9. Gilbert. R., Statistical Methods for Environmental Pollution Monitoring, Van
Nostrand Remhold Co., New York (1987).
10. SAS Procedures Guide, Version 6, Third Edition, SAS Institute, Inc. Gary, N.C.
(1990).
11. Jordan, S., C. Stenger, M. Olson, R. Batiuk, and K. Mountford, "Chesapeake Bay
Dissolved Oxygen Goal for Restoration of Living Resource Habitats: A Synthe-
sis of Living Resource Habitat Requirements With Guidelines for Their Use in
Evaluating Model Results and Monitoring Information," CBP/TRS 88/93, Mary-
land Department of Natural Resources, CBP, Annapolis, Md. (1993).
12. "Guide To Using Chesapeake Bay Program Water Quality Monitoring Data,"
CBP/TRS 78/92, CBP, Annapolis, Md. (1992).
13. "Discussion Paper: Reducing Nutrients in Virginia's Tidal Tributaries," Virginia
Department of Environmental Quality, Richmond, Va. (1993).
14. Boicourt, W., "Influences of Circulation Processes on Dissolved Oxygen in the
Chesapeake Bay," in: Oxygen Dynamics in the Chesapeake Bay: A Synthesis of
Recent Research, D. Smith, M. Leffler, and G. Mackiernan, Eds., pp. 7-59,
Maryland Sea Grant College, College Park, Md. (1992).
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992 49
-------
50 Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
APPENDIX
FREQUENCIES OF BELOW DETECTION LIMIT VALUES FOR
DISSOLVED INORGANIC PHOSPHORUS AND DISSOLVED INORGANIC NITROGEN
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992 51
-------
Table A.1. Percent of observations with below detection limit values for dissolved inorganic phosphorus
by segment, laboratory, and water year.
Segment Laboratory Year % BDL Mean MDL
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB3
CBS
CBS
CBS
CBS
CB3
CBS
CBS
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CBS
CBS
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL7CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
L/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
85
86
87
88
89
90
91
92
85
86
87
88
89
90
91
92
85
86
87
88
89
90
91
92
85
86
87
88
89
90
91
92
85
86
19.4
0
7.9
0
0
0
0
0
3.8
0
0
0
0
0
0
0
8.6
1.3
2.5
0
0
0
0
0
22.2
6.8
11.4
0.1
0
0
0.6
0
30.7
21.5
0.0055
-
0.0016
-
-
-
-
-
0.007
-
-
-
-
-
-
-
0.007
0.0016
0.0016
-
-
-
-
-
0.006
0.0016
0.0016
0.0006
-
-
0.0006
-
0.0061
0.0016
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Segment Laboratory Year % BDL Mean MDL
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
EE3
EE3
EE3
EE3
EE3
EE3
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
v'IMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
87
88
89
90
91
92
85
86
87
88
89
90
91
92
85
86
87
88
89
90
91
92
85
86
87
88
89
90
91
92
85
86
87
88
89
90
15.2
0.9
0.6
0
0
0.3
81.8
89.4
75.6
37.6
27.4
62
18.1
7.8
85.1
85.4
70.8
37.8
30.5
62.5
18.1
10.5
83.5
83.8
75.8
34.2
25
55.6
19.5
10.6
100
100
94.9
36.1
47.4
69.4
0.0016
0.0006
0.0006
-
-
0.0006
0.01
0.0105
0.0101
0.0016
0.002
0.0028
0.0006
0.0006
0.01
0.0105
0.0104
0.0016
0.002
0.0027
0.0006
0.0006
0.01
0.0105
0.01
0.0016
0.0022
0.0028
0.0006
0.0006
' 0.01
0.0105
0.0094
0.0019
0.0016
0.0024
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
53
-------
Segment Laboratory Year % BDL Mean MDL
EE3
EE3
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
91
92
85
86
87
88
89
90
91
92
85
86
87
88
89
90
91
92
85
86
87
88
89
90
91
92
85
86
87
88
89
90
91
92
27.8
5.6
90.7
84.9
71.7
40.3
28.3
58.3
11.9
2.8
36.2
63.3
50
47.2
25.7
36.1
44.4
58.3
32.3
52.7
46.7
32.7
17.8
23.1
23.6
44.4
16.1
39.9
29.2
26.1
9
7.2
15
35.8
0.0006
0.0006
0.0099
0.0105
0.0109
0.0016
0.0022
0.0028
0.0006
0.0006
0.01
0.01
0.0053
0.005
0.005
0.005
0.005
0.005
0.01
0.01
0.0052
0.005
0.005
0.005
0.005
0.005
0.01
0.01
0.0051
0.005
0.005
0.005
0.005
0.005
54 Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Table A.2. Percent of observations with below detection limit values for dissolved inorganic nitrogen by
segment, laboratory, constituent parameter, and water year.
Segment Laboratory Parameter Year % BDL Mean MDL % of DIN
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB1
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRUCBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
Both
NH4
N023
Both
NH4
NC-23
Both
NH4
N023
Both
NH4
NO23
Both
NH4
NC-23
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
NO23
Both
NH4
N023
Both
85
85
85
86
86
86
87
87
87
88
88
88
89
89
89
90
90
90
91
91
91
92
92
92
85
85
85
86
86
86
87
87
87
88
0 - -
13.9 0.0166 1.3
0
0 - -
2.8 0.003 0.3
0
0 - -
0 - -
0 - -
0 - -
5.3 0.003 0.5
0
0
10.5 0.003 0.2
0
0 - -
5 0.003 0.3
0
0 - -
15.4 0.003 0.3
0 - -
0 - -
2.5 0.003 0.3
0
0
0
0
0
0.9 0.003 1.4
0 - -
0
0 - -
0 - -
o - • -
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
55
-------
Segment Laboratory Parameter Year % BDL Mean MDL % of DIN
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CB2
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
NH4
NO23
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
NO23
Both
NH4
N023
Both
88
88
89
89
89
90
90
90
91
91
91
92
92
92
85
85
85
86
86
86
87
87
87
88
88
88
89
89
89
90
90
90
91
91
91
92
0.9 0.005
0
0
0.9 0.003
0
0
5 0.003
0
0
3.3 0.003
0
0
1.7 0.003
0
0
2.9 0.0257
0
0
0
0
0
0
0
0
0
0
0
3.9 0.003
0
0
4.4 0.003
0
0
4.3 0.003
0
0
0.7
-
-
0.2
-
-
0.3
-
-
0.9
-
-
0.3
-
-
6.7
-
-
-
-
-
-
-
-
-
-
-
0.5
-
-
0.7
-
-
7.6
'
56 Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Segment Laboratory Parameter Year % BDL Mean MDL % of DIN
CBS
CBS
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB4
CB5
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL -
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
NH4
N023
Both
NH4
N023
Both
NH4
NC-23
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
92
92
85
85
85
86
86
86
87
87
87
88
88
88
89
89
89
90
90
90
91
91
91
92
92
92
85
85
85
86
86
86
87
87
87
88
2.5
0
0
7.5
3.8
0
0.7
0.1
0
0
0.1
0
5.7
0
0
7.8
0
0
10.3
6
0
13.9
0
0
11
0
3.8
10
14.1
0
0.6
0
0
0
0
0
0.0097 1.7
-
-
0.0293 19.2
0.04 22
-
0.003 0.6
0.0009 0.3
-
- -
0.0009 0.3
-
0.0046 25.9
-
- -
0.003 2.3
- -
-
0.003 4.8
-
-
0.003 13.1
-
-
0.003 13.4
-
0.06
0.0242 30
0.04 34
-
0.003 0.9
-
- -
-
-
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
57
-------
Segment Laboratory Parameter Year % BDL Mean MDL % of DIN
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CBS
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
CRL/CBL
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
3th
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
NO23
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
88
88
89
89
89
90
90
90
91
91
91
92
92
92
85
85
85
86
86
86
87
87
87
88
88
88
89
89
89
90
90
90
91
91
91
92
14.7
0
0
15.9
0
0
13.8
0
0
20.6
0
0
14.7
0
22.3
25.9
33.1
18.1
35.7
16.6
8.1
12.1
31.8
4.2
12.2
9.5
6.5
28.8
1.6
5.6
36.3
6.1
7.9
9.6
14.7
5
0.0043
-
-
0.003
-
-
0.003
-
-
0.003
-
-
0.003
-
0.0397
0.0198
0.0199
0.0418
0.0215
0.0198
0.0254
0.0155
0.0104
0.0161
0.0133
0.0033
0.0121
0.012
0.0021
0.0075
0.0094
0.0024
0.0045
0.0038
0.0024
0.0046
22
-
-
5.2
-
-
11.2
-
-
20.7
-
-
22.8
-
-
19.1
22.8
-
13
26.7
-
14.7
21.2
-
53.5
9.6
-
23
7.2
-
23.5
19.9
-
8
24.5
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Segment Laboratory Parameter Year % BDL Mean MDL % of DIN
CBS
CBS
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
NO23
DOth
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
92
92
85
85
85
86
86
86
87
87
87
88
88
88
89
89
89
90
90
90
91
91
91
92
92
92
85
85
85
86
86
86
87
87
87
88
16.7
6.7
28.9
24.5
29.6
16.8
33.5
19.2
8.8
11.3
36.8
6.3
10
14.3
9
28.1
2
19.6
28.4
3.5
8.7
11.1
11.8
7
25.5
8
28.4
23.3
31.5
16.3
32.9
20.8
10.8
9.2
39.6
6.8
0.0039
0.0023
0.04
0.0193
0.0199
0.042
0.0215
0.0198
0.0212
0.0119
0.0112
0.0162
0.0124
0.0047
0.0121
0.0121
0.0021
0.0102
0.0096
0.0024
0.005
0.0038
0.0024
0.0046
0.0039
0.0023
0.0398
0.0197
0.02
0.0415
'0.0215
0.0199
0.0202
0.0147
0.0106
0.0183
31.2
31.2
-
25
29
-
14.9
26.6
-
12.8
24.5
-
56.1
16
-
26.2
8.1
-
36.2
22
-
9.6
28.7
-
43.6
34.6
-
27.4
25.2
-
18.5
30
-
25.5
21.5
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
59
-------
Segment Laboratory Parameter Year % BDL Mean MDL % of DIN
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
EE3
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
NO23
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
88
88
89
89
89
90
90
90
91
91
91
92
92
92
85
85
85
86
86
86
87
87
87
88
88
88
89
89
89
90
90
90
91
91
91
92
10.5
16.4
7.2
31.4
0.9
14.4
28.2
4.2
9.3
7.4
9.3
7.4
25.9
8.3
37.1
5.7
25.7
21.1
44.7
2.6
7.7
2.6
43.6
2.8
13.9
19.4
13.5
27
0
19.4
36.1
5.6
11.1
11.1
25
13.9
0.0129
0.0068
0.0122
0.0121
0.0021
0.0103
0.0092
0.0023
0.0053
0.0036
0.0024
0.0045
0.0039
0.0023
0.0398
0.02
0.02
0.0407
0.0214
0.021
0.02
0.021
0.0099
0.023
0.013
0.0054
0.0126
0.0121
-
0.0105
0.0095
0.0024
0.0049
0.004
0.0024
0.0045
67.9
20.4
-
37.3
8.1
-
45
12.9
-
9.1
27.6
-
48.2
40.5
-
30.8
28
-
23.1
28
-
21.2
26.6
-
77.2
18.9
-
50.4
-
-
50.3
22.1
-
5
28.5
60
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Segment Laboratory Parameter Year % BDL Mean MDL % of DIN
EE3
EE3
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
WE4
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
ViMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
VIMS
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
NH4
N023
Both
NH4
N023
Both
NH4
'^23
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
NO23
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
92
92
85
85
85
86
86
86
87
87
87
88
88
88
89
89
89
90
90
90
91
91
91
92
92
92
85
85
85
86
86
86
87
87
87
88
11.1
11.1
59.7
8.1
19.5
29.5
32.5
15.1
18.9
5
44
12.1
2.8
21.5
10.8
31.8
4.7
33.3
16.7
14.6
16.7
7.6
20.1
5.6
22.9
12.5
20.8
14.6
47.9
10
3.3
18.3
33.8
5.6
19.7
22.9
0.004
0.0024
0.0397
0.02
0.02
0.0412
0.022
0.0198
0.0236
0.0124
0.01
0.0226
3.0129
0.0073
0.0124
0.0124
0.0021
0.0113
0.01
0.0023
0.0052
0.004
0.0024
0.0045
0.0039
0.0022
0.0174
0.01
0.01
0.0106
0.0056
0.0064
0.0106
0.0056
0.005
0.0101
62.6
40.5
-
39.1
31.1
-
30
29.3
-
27.7
24.3
-
79.6
21
-
48.4
12.1
-
62.3
16.2
-
14.2
24.3
-
60.1
29.3
-
24.6
28.1
-
6
24.7
-
36.8
17.2
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
61
-------
Segment Laboratory Parameter Year % BDL Mean MDL % of DIN
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB6
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
CB7
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
NH4
NO23
Both
NH4
N023
Both
NH4
NO23
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
NH4
N023
Both
88
88
89
89
89
90
90
90
91
91
91
92
92
92
85
85
85
86
86
86
87
87
87
88
88
88
89
89
89
90
90
90
91
91
91
92
7.1
22.9
8
10.7
6.7
20.8
22.2
4.2
6.9
22.2
0
34.4
21.9
4.7
35.4
10.8
36.9
20.4
4.1
30.6
45.4
4.6
23.7
29.1
4.1
37.2
18.4
10.5
18.4
31.3
23.6
14.6
16.7
22.2
6.3
44.4
0.0056
0.0039
0.0081
0.0056
0.0025
0.0081
0.0056
0.0025
0.0081
0.0056
-
0.0081
0.0056
0.0025
0.0172
0.01
0.01
0.0118
0.0056
0.0062
0.0106
0.0056
0.005
0.01
0.0056
0.004
0.0081
0.0056
0.0025
0.0081
0.0056
0.0025
0.0081
0.0056
0.0025
0.0081
48.1
23.3
-
28.6
19.8
-
38.1
26.8
-
17.1
-
-
38.3
17
-
26.6
33.3
-
6.9
28.1
-
33
24.8
-
56.1
21.8
-
46.1
15.3
-
48.6
17.1
-
26.4
19.7
62
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
-------
Segment Laboratory Parameter Year % BDL Mean MDL % of DIN
CB7
CB7
CB8
CBS
CBQ
CBQ
CBQ
CBQ
CBQ
CBQ
CB8
CBQ
CBQ
CBQ
CB8
CBQ
CBQ
CBQ
CBQ
CB8
CBQ
CBQ
CBQ
CBQ
CBQ
CBQ
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
ODU
NH4
N023
Both
NH4
N023
Both
NH4
NO23
Both
NH4
N023
Both
NH4
NO23
Both
NH4
N023
Both
NH4
N023
Both
NH4
NT023
Both
NH4
N023
92
92
85
85
85
86
86
86
87
87
87
88
86
88
89
89
89
90
90
90
91
91
91
92
92
92
22.9
9
29.7
4.2
35.8
11.3
4
28.8
29.8
3.7
28.8
25.9
3.2
33
13.8
4.2
12.2
19.1
19.1
9.6
13.3
16.7
6.1
39.9
12.9
5.6
0.0056
0.0025
0.018
0.0094
0.01
0.0113
0.0056
0.0059
0.0106
0.0056
0.005
0.0103
0.0056
0.0042
0.0081
0.0056
0.0025
0.0081
0.0056
0.0025
0.0081
0.0056
0.0025
0.0081
0.0056
0.0025
55.3
19.6
-
33.6
37.9
-
13.8
23
-
38.5
28.3
-
56.1
25.5
-
40.4
15.7
-
50
19
-
32.9
17.2
-
53
18.6
Trends in Phosphorus, Nitrogen, and Dissolved Oxygen, 1984 to 1992
63
-------
------- |