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U.S. Environmental
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PUBLISHED BY
THE MARYLAND DEPARTMENT OF NATURAL RESOURCES
TIDEWATER ADMINISTRATION
CHESAPEAKE BAY RESEARCH AND MONITORING DIVISION
FOR
THE CHESAPEAKE BAY PROGRAM
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CHESAPEAKE BAY
DISSOLVED OXYGEN GOAL FOR RESTORATION
OF LIVING RESOURCE HABITATS
A Synthesis of Living Resource Habitat Requirements with
Guidelines for Their Use in Evaluating
Model Results and Monitoring Information
DECEMBER 1992
Prepared by: Steve Jordan1, Cynthia Stenger1, Marcia Olson3,
Richard Batiuk2 and Kent Mountford2
Graphics and layout by: Lamar Platt1
For the Living Resources Subcommittee
and
The Implementation Committee's
Nutrient Reduction Strategy Reevaluation Workgroup
of the
Chesapeake Bay Program
Reevaluation Report #7c
CBP/TRS 88/93
'Maryland Department of Natural Resources, Chesapeake Bay Research and Monitoring Division
2United States Environmental Protection Agency, Chesapeake Bay Program Office
3Chesapeake Bay Program Office, Computer Sciences Corporation
Annapolis, Maryland
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EXECUTIVE SUMMARY
i ...:!; to, PA 1S107
Overview
The signatories of the 1987 Chesapeake Bay
Agreement pledged to manage the Chesapeake Bay
as an integrated ecosystem. To that end, goals and
commitments were established for living resources
and water quality, as well as population growth and
development, public information, education and
participation, public access, and governance. The
living resources and water quality goals of the 1987
Bay Agreement are as follows:
"provide for the restoration and protection of the
living resources, their habitats and ecological
relationships," and
"reduce and control point and nonpoint sources
of pollution to attain the water quality condition
necessary to support the living resources of the
Bay."
In support of these goals, the Chesapeake Executive
Council (CEC) made commitments to "develop and
adopt guidelines for the protection of water quality
and habitat conditions necessary to support the
living resources found in the Chesapeake Bay
system, and to use these guidelines in the
implementation of water quality and habitat
protection programs," and to "achieve by the year
2000 at least a 40 percent reduction of nitrogen and
phosphorus entering the mainstem of Chesapeake
Bay." The signatories also agreed "to re-evaluate the
40 percent reduction target based on the results of
modeling, research, monitoring and other
information."
One tool which has been developed to address the
Bay Agreement commitments is the time-variable
water quality model, which predicts the effects of
particular nutrient load reduction scenarios on water
quality. Additional tools include this report,
Chesapeake Bay Dissolved Oxygen Goal for Restoration of
Living Resource Habitats, and the companion
document Chesapeake Bay Submerged Aquatic
Vegetation Habitat Requirements and Restoration Targets:
A Technical Synthesis (Batiuk et al. 1992). These
syntheses are intended to:
• establish living resources-based water quality
goals to be used in evaluating model
simulation results;
• provide a firm ecological basis for the
reevaluation of the Baywide Nutrient Reduction
Strategy (CEC 1988a);
• provide guidelines that can be used "in the
implementation of water quality and habitat
protection programs;" and
• establish firmer connections between living
resources and restoration of water quality.
The target concentrations of the dissolved oxygen
restoration goal in this report are not meant to be
enforceable standards for either wastewater
discharge permitting or other types of regulatory
activities. A state may pursue adoption of these goals
as water quality standards using the appropriate
administrative process.
The Report
Section I provides an introduction, including
background on the need for developing this
document, report objectives, and a brief summary of
characteristics of dissolved oxygen in Chesapeake
Bay. The aquatic animals and plants which make up
the Bay ecosystem require dissolved oxygen for
respiration. Monitoring data indicates that many
areas of the Bay experience sudden or persistent
declines in dissolved oxygen, which can adversely
affect living resources. Low dissolved oxygen
reduces available habitat for all but a few species
and may cause stress and mortality in immobile
species which are unable to avoid the unsuitable
conditions. A major tenet of the Chesapeake Bay
Program is that restoring dissolved oxygen to such
areas will provide substantial habitat benefits.
Sections II and III in this document establish and
defend a dissolved oxygen restoration goal for
Chesapeake Bay, based on extensive analysis and
evaluation of research data. Dissolved oxygen
tolerance information was compiled and interpreted
for the 14 target species of fish, molluscs and
crustaceans reported in Habitat Requirements for
Chesapeake Bay Living Resources (Funderburk et al.
1991), as well as information published for other
benthic and planktonic species, and key
investigations recently completed.
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The Chesapeake Bay Dissolved Oxygen Goal for
Restoration of Living Resource Habitats is:
to provide for sufficient dissolved oxygen to support survival, growth and reproduction of
anadromous, estuarine and marine fish and invertebrates in Chesapeake Ray and its tidal
tributaries by achieving, to the greatest spatial and temporal extent possible, the following four
target concentrations of dissolved oxygen, and by maintaining the existing minimum
concentration of dissolved oxygen in areas of Chesapeake Bay and its tidal tributaries where
dissolved oxygen concentrations are above the recommended targets.
TIME AND
LOCATION
ALL TIMES, EVERYWHERE;
FOR NO LONGER THAN 12 HOURS,
INTERVAL BETWEEN EXCURSIONS AT
LEAST 48 HOURS, EVERYWHERE;
ALL TIMES, THROUGHOUT
ABOVE-PYCNOCLINE WATERS;
ALL TIMES, THROUGHOUT
ABOVE-PYCNOCLINE WATERS, IN
SPAWNING REACHES, SPAWNING RIVERS
AND NURSERY AREAS.
The pycnocline is the portion of the water column where density changes rapidly because of salinity and temperature
TARGET DO
CONCENTRATIONS
DO >1.0 mg/L
1.0mg/L 5.0 MG/L
DO >5.0mg/L
The target concentrations are based on patterns
which emerge from examining the best available
information. Although a large body of data exists,
there remain extensive gaps in our knowledge and
therefore, best professional judgement was exercised
in making decisions about the precise values of the
target concentrations. As research continues in this
area, especially on the effects of exposure to
fluctuating concentrations of DO, revision of the
target concentrations may be appropriate. Appendix
A contains details of the literature cited in this
synthesis.
Section IV provides applications of the Goal and
target concentrations to monitoring and modeling
information. Linkages are developed relating
complex variability in environmental oxygen
concentrations to data from the semi-monthly
Baywide monitoring program, and seasonally
averaged output from the Chesapeake Bay time-
variable water quality model. This section explains
the relationships developed and how to use them to
evaluate present and projected dissolved oxygen
conditions in the Bay and its tributaries. Appendix B
contains further details of the statistical approach
used in this analysis.
Interpretation
With these tools we can evaluate achievement of the
target concentrations for any model cell or
monitoring station. However, for simplicity of
presentation on a Baywide basis, it was necessary to
develop an aggregation scheme. Comparison of
habitat benefits among nutrient reduction scenarios
are based on the percentages of time that areas of
11
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bottom habitat and volumes of water are predicted
to meet or exceed the applicable target
concentrations of dissolved oxygen. For interpretive
purposes, the following table defines measures for
reaching decisions about suitability, unsuitability and
marginality of habitat:
Habitat
Condition
Percentage of Time Areas
Meet Target Concentrations
Suitable
Marginal
Unsuitable
90-100%
50-90%
Less than 50%
Suitable or acceptable habitat provides satisfactory
conditions for survival, growth and reproduction of
living resources within the constraints imposed by
the formation of density layers. Marginal habitat
provides increased opportunities for establishment of
benthic invertebrates and foraging by bottom feeding
fish. Unsuitable habitat is inhospitable to all but the
most tolerant of living resources. We note that what
is considered "acceptable habitat" for this purpose is
not necessarily fully supportive of living resources
requirements for dissolved oxygen. The uncertainties
in the analysis and natural variability of the Bay
environment compel a more flexible view of habitat
suitability than would be dictated by biological
considerations alone.
Nutrient reduction scenario results are compared to
the "base case" (i.e. existing conditions) model
scenario results using this interpretive method. The
percent of bottom area or water volume in which
habitat quality improves from unsuitable to marginal
or from marginal to suitable appears to be the most
convenient means of evaluating the living resources
benefits for a particular scenario relative to base case
or other scenarios.
This interpretive scheme was developed for the
particular needs of reevaluating the Nutrient
Reduction Strategy. There are other ways to apply
the target concentrations of the Chesapeake Bay
Dissolved Oxygen Goal for Restoration of Living
Resource Habitats to monitoring data and to model
output. Work to develop other applications is
ongoing.
Ill
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I. INTRODUCTION
Background
The living resources goal of the 1987 Chesapeake Bay
Agreement is to: "provide for the restoration and
protection of the living resources, their habitats and
ecological relationships." In support of this goal, the
Chesapeake Executive Council (CEC) made a com-
mitment to "develop and adopt guidelines for the
protection of water quality and habitat conditions
necessary to support the living resources found in
the Chesapeake Bay system, and to use these
guidelines in the implementation of water quality
and habitat protection programs."
Habitat Requirements for Chesapeake Bay Living
Resources (CEC 1988b) was published in January 1988
in response to this commitment. Thirty target species
were selected from a list of 160 representative species
and species complexes to represent, either directly or
through food chain associations, the Bay's commer-
cially, recreationally, and ecologically important
species of fish, shellfish, submerged aquatic
vegetation, and wildlife. Although this document
was a good start towards defining the conditions
necessary to provide suitable habitats for the Bay's
living resources, the information was neither
complete, nor presented in such a way that it could
be used directly in the implementation of water
quality restoration programs.
An extensive revision of the original habitat
requirements report was completed by a team of
scientists who are experts on each of the target spec-
ies. The objectives of the revised habitat re-
quirements document (Funderburk et al. 1991) were
to compile all of the available information on habitat
requirements of the designated Chesapeake Bay
target species, and to synthesize this information in
ways that would make it directly useful in water
quality management programs. Habitat requirements
for dissolved oxygen (DO) identified in Funderburk
et al. (1991) provided the starting point for the
development of the DO restoration goal.
The 1987 Chesapeake Bay Agreement also committed
the signatories to "achieve by the year 2000 at least
a 40 percent reduction of nitrogen and phosphorus
entering the mainstem of Chesapeake Bay" and "to
re-evaluate the 40 percent reduction target based on
the results of modeling, research, monitoring and
other information." The nutrient reduction com-
mitment was based upon the results of a summer-
averaged, steady-state water quality model that
predicted marginal increases in deep water DO in re-
sponse to a 40 percent reduction in nitrogen and
phosphorus loads to the Bay. As a part of the pro-
cess of reevaluating the nutrient reduction goal,
habitat requirements for nutrients (Batiuk et al. 1992)
and DO (this report) have been synthesized. These
syntheses are intended to: 1) establish living re-
sources-based water quality goals to be used in
evaluating model simulation results; 2) provide a
firm ecological basis for the revaluation of the Bay-
wide Nutrient Reduction Strategy (CEC 1988a); 3)
provide guidelines that can be used "in the imple-
mentation of water quality and habitat protection
programs;" and 4) establish firmer connections
between living resources and restoration of water
quality.
Dissolved oxygen is a major factor affecting the
survival, distribution, and productivity of living
resources in Chesapeake Bay. Much of the deep
water of the mainstem Chesapeake Bay becomes
anoxic during summer months and is therefore
nearly devoid of animal life. Many Chesapeake Bay
tributaries experience both episodic and persistent
oxygen depletion in summer that results in
significant stress to living resources. Model projec-
tions which led to the current nutrient reduction
strategy for the Bay indicated that reductions in
nutrient inputs would result in increased deep-
trough DO that would benefit the Bay's living
resources. However, neither tributaries, other areas
of the mainstem Bay, nor the specific DO
requirements of living resources were given
consideration at the time, because of the relatively
low resolution of the steady-state water quality
model, and the limited information available on
habitat requirements.
Objectives of this Report
1. Establish a restoration goal for DO with target
concentrations sufficient to protect the survival, growth
and reproduction of the Bay's living resources.
All of the aquatic animals among the Chesapeake
Bay target species - ten species of fish, blue crabs,
and three molluscs - require DO for respiration. So do
the benthic and planktonic animals and plants which
form the food base for the target species. The target
species, together with additional representative
benthic species of fish and invertebrates for which
DO tolerance information was reviewed (Saksena and
Joseph 1972; Holland et al. 1989; Stickle et al. 1989;
Stickle 1991; Houde 1991; Miller and Poucher 1991,
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1992; Breitburg 1992a, 1992b), represent a wide range
of habitats, life history patterns, and tolerances to
low DO. Therefore, habitat restoration goals designed
to protect the survival, growth, and reproduction of
these species should be sufficient to protect other
species, and by extension, the Bay's aquatic ecosys-
tem, from harm caused by inadequate concentrations
of DO. All of the information on DO tolerances (lethal,
sublethal, long term and short term) contained in
Funderburk et al. (1991) and supplementary
references (Appendix A) has been combined and
evaluated to develop the DO restoration goal. The
target concentrations of the DO Goal in this report are
not meant to be enforceable standards for either
wastewater discharge permitting or other types of
regulatory activities. A state may pursue adoption of
these target concentrations as water quality
standards using the appropriate administrative
process.
2. Provide a basis for evaluating water quality model
results.
The DO restoration goal presented here will be used
to assist in evaluating the results of nutrient load
reduction scenarios modeled as a part of the
reevaluation of the Baywide Nutrient Reduction
Strategy (CEC 1988a). The three-dimensional, time-
variable water quality model of the Bay projects
concentrations of DO for nine segments (averaged
from projections for thousands of model cells) in
three depth layers and four seasons based upon
varying amounts, timing, and geographical distri-
butions of nitrogen and phosphorus loads delivered
to the Bay and its tributaries. Sufficient concentra-
tions of DO to protect living resources will be an
important consideration in evaluating options for
nutrient reduction.
3. Ensure that the baywide DO restoration goal is
reasonable with respect to natural processes.
The restoration goal includes target DO concentra-
tions, with limits to the duration and frequency of
reoccurrence, which reflect living resources toler-
ances to low DO. In order to make these re-
quirements comparable with results from the
Chesapeake Bay time-variable water quality model,
and to ensure that the target requirements are
physically reasonable, water quality data from the
Chesapeake Bay Monitoring Program and measure-
ments made in greater temporal detail by other
sampling programs have been analyzed extensively.
In Section IV, we compare the DO target
concentrations to monitoring data from several areas
of the Bay, present a hypothetical demonstration of
how improvements in DO might translate into fulfill-
ment of the habitat goal, and describe methods for
evaluating time-variable model results in comparison
to the target concentrations.
Characteristics of Dissolved Oxygen in Chesapeake
Bay
To understand how the goal-setting decisions were
reached, and the process for making the restoration
goal useful for evaluating model results, it is
necessary to know something of the complex dynam-
ics of DO in the Bay. The following paragraphs
provide a brief overview. For detailed information on
DO processes in Chesapeake Bay, see Mackiernan
(1987) and Smith et al. (1992).
Dissolved oxygen in natural waters has two major
sources: 1) atmospheric oxygen which diffuses into
the water at the surface, and 2) oxygen which is
produced by plants (chiefly free-floating microscopic
plants, or phytoplankton) during photosynthesis.
Animals, plants and bacteria consume DO by respira-
tion. Oxygen is also consumed by chemical processes
(e.g., sulfide oxidation, nitrification). Depletion of DO
has harmful effects on animals, and can stimulate
production of hydrogen sulfide and ammonia and
the release of heavy metals and phosphate from bot-
tom sediments.
Trie amount of oxygen dissolved in water changes as
a function of temperature, salinity, atmospheric
pressure, and biological and chemical processes. The
equilibrium (or saturated) concentration of DO in
natural waters ranges from about 6 to 14 parts per
million (or mg/L). The higher the temperature and
salinity, the lower the equilibrium DO concentration.
Biological processes such as respiration and
photosynthesis can affect the concentration of DO
faster than new equilibrium can be reached with the
atmosphere. As a result, for relatively short periods
of time, or under conditions of reduced mixing, DO
concentrations can be driven far above or reduced
well below saturation. Dissolved oxygen can
decrease to near zero (anoxia), especially in deep or
stratified bodies of water, or increase as high as
about 20 mg/L (super-saturation) in dense algal
blooms.
There are seasonal considerations, as well. Low DO in
Chesapeake Bay is mostly associated with deep
water during the warm months (May-September),
when the water column is stratified into density
layers with cool salty water at the bottom, and
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warm, fresher water near the surface. The bottom
layer becomes oxygen-depleted because the oxygen
consumed by respiration and chemical oxidation
cannot be replaced through diffusion of atmospheric
oxygen and there is insufficient light to support
photosynthetic production of oxygen. Some parts of
the Bay can become anoxic for periods of days or
weeks during midsummer.
In summer, very low DO can also occur for shorter
periods of time (a few hours to a few days) in
shallow water. In these cases, DO is depleted by the
decay of large amounts of organic matter (perhaps
due to respiring or dying algae blooms or from
wastewater discharges). Deep water low in oxygen
can also be moved into shallow areas by winds.
Episodes of strong winds can transport (literally
"slosh") water with extremely low oxygen content
across the Bay bottom, up and into the habitat of
shallow-water dwelling living resources. While
strong winds persist, low oxygen waters may remain
in the shallows for 40 hours or more. During these
times inshore species are continuously exposed to
stressful or life-threatening conditions. This sloshing
of deep water is sometimes so extreme that anoxic
waters move almost to the shoreline. During the
resulting "jubilees" or "crab wars," blue crabs and
fish congregate at the water's edge attempting to
find sufficient oxygen to stay alive (Van Heukelem
1991).
In the spring, striped bass, white perch, shad,
herring, and yellow perch spawn far up the Bay's
tributaries. The eggs and larvae of these species are
quite sensitive to low DO, and could be threatened by
even moderate DO depletion associated with algal
blooms or wastewater discharges. In the fall and
winter, DO depletion is uncommon, and the most
sensitive life stages of the target species generally are
not present.
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II. PROCEDURES FOR ESTABLISHING A
BAYWIDE DISSOLVED OXYGEN RESTORATION GOAL
Because of the natural fluctuations of DO, and the
varied ability of the target species to tolerate less
than desirable concentrations, habitat requirements
for DO cannot be stated as a single, critical concentra-
tion. The sensitivity of each species to low DO de-
pends upon life stage, temperature, salinity, duration
of exposure, and perhaps other stress factors (e.g.
contaminants), in addition to the absolute
concentration of DO. Some species are more tolerant
of low DO than others. For example, adult oysters
and clams can survive anoxia for days (although
growth and reproduction may be impaired) whereas
shorter exposures to moderately low DO (below
about 3 mg/L) can severely affect the survival and
development of fish eggs and larvae.
By selecting conditions acceptable for the reproduc-
tion, growth, and survival of a variety of sensitive
species, habitat requirements can be established that
will also protect the Bay's other living resources. Dis-
solved oxygen tolerance information was compiled
and interpreted for the 14 target species of fish,
molluscs and crustaceans reported in Funderburk ct
al. (1991), as well as information reported for other
benthic and planktonic species (Appendix A).
Some of the information on the effects of low DO on
Chesapeake Bay species was essentially anecdotal, or
otherwise of limited usefulness (e.g., in some refer-
ences the duration of exposure was not reported).
Information on long-term and sublethal effects of
low DO (e.g., reduced growth and reproductive
potential) was scarce, the majority of studies having
focused on survival thresholds. Variability within
and among studies sometimes limited interpretation
of results to a range of responses to low DO for some
species. When the data are tabulated in a complete
matrix of critical DO concentrations for various life
stages of the target species (Figure II-l), there are
6.O
S.O
4.0
3.O
2.O
i.o o.s ANOHC
SUITABLE
TOLERATE
LONGTERAA
| TOLERATE
I SHORT TERAA
LETHAL
Figure 11-1. Effects of low dissolved oxygen on target species, summarized from Habitat Requirements for Chesapeake Bay Living Resources,
1991 Revised Edition (Funderburk et al. 1991). Note; does not account for temperature, salinity, dissolved oxygen interactions (Appendix A).
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many gaps. Ideally, there would be sufficient data
available to fully understand the effects of low DO
concentrations on each target species as a function of
life stage, duration of exposure, temperature, salinity,
and significant biological consequences (e.g., mortali-
ty, reduced growth and reproduction).
There was, however, enough consistency in the
results across the spectrum of target species to
identify certain patterns of responses. For example,
DO concentrations between zero and near 1.0 mg/L
were lethal to all target species that had been tested
at these concentrations, with the exception of some
molluscs, and to most of the benthic species
considered. Another pattern which became apparent
was the lack of observed deleterious effects on target
species to DO above 5.0 mg/L.
The initial approach for developing the DO restora-
tion goal presumed that DO requirements would vary
for different parts of the Bay and for each season,
because of the different distributions and tolerances
of the target species and the seasonal occurrence of
critical life stages. However, DO habitat requirements
for the target species (e.g., blue crabs and bay
anchovies) that are distributed throughout the tidal
waters of the Bay represent the needs of many of the
target species except for the eggs, larvae, and juve-
niles of anadromous fish. This outcome has made the
task of developing a DO goal for restoration of living
resources habitats more straightforward. It does not,
however, preclude regional approaches to manage-
ment of water quality. When the target concentra-
tions are compared to existing water quality and to
the results of model projections, there will undoubt-
edly be regional differences in the current attainment
of the targets and the nutrient load reductions
needed to meet them in the future.
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III. CHESAPEAKE BAY DISSOLVED OXYGEN RESTORATION GOAL
Basis for the Target Dissolved Oxygen Concentra-
tions
Four target concentrations ot DO were identified as
necessary to provide sufficient habitat for the
survival, growth and reproduction of the Bay's living
resources. These DO targets and the rationale for
their establishment are outlined below.
Decisions about the precise concentrations of DO,
durations, and frequencies included elements of
professional judgement, because of the lack of
complete information on biological effects of low DO.
However, it should be clear from the information
presented in Figure II-l and Appendix A that the
recommended concentrations and time scales are
reasonable and within the ranges dictated by the
available data. We also considered the natural
fluctuations of DO and the strong effect on DO of
uncontrollable physical processes in the Bay in
evaluating whether the recommended DO target
concentrations were reasonable as management goals
(Section IV). As a result, some of the target concen-
trations are defined separately for above-pycnocline
waters.
Below, the DO target concentrations are defined,
accompanied by selected summaries of the literature
on species tolerances that was reviewed for
development of the DO target concentrations
(Appendix A). Illustrations of tolerances are given,
with emphasis on the most widely distributed
species (Box 1) and biological effects which are likely
to be limiting to these species. At the end of this
Section, the target concentrations are consolidated
into a Chesapeake Bay Dissolved Oxygen Goal for
Restoration of Living Resource Habitats.
A. The following target concentration applies to all waters
of Chesapeake Bay and its tidal tributaries at all times: 1.0
mg/L DO.
Fjcposures to DO below 0.5-1.0 mg/L have been
found lethal, during some life stage, to all of the
target species for which this exposure information is
available, except for the molluscs (Figure II-l). Most
benthic species also succumb to DO below 0.5-1.0
mg/L eventually, although a number of benthic
species survive anoxia for extended periods (Holland
et al. 1989). Adult soft shell clams can survive near
anoxic conditions for up to 7 days (McCarthy 1969)
and adult eastern oysters have survived exposure to
DO <1.0 mg/L for up to 5 days (Sparks et al. 1958).
However, 50% of eastern oyster larvae (82 urn) died
BOX1
Target species
Shellfish
blue crab
eastern oyster
hard clam
soft shell clam
Flnflsh
alewife
American shad
bay anchovy
blueback herring
hickory shad
menhaden
spot
striped bass
white perch
yellow perch
Callinectes sapidus
Crassostrea virginica
Mercenaria mercenaria
Mya arenaria
Alosa pseudoharengus
Alosa sapidissima
Anchoa mitchilli
Alosa aestiyalis
Alosa mediocris
Brevoortia tyrannus
Leiostomus xanthurus
Morone saxatilis
Morone amerlcana
Perca flavescens
Other species
Flnflsh
naked goby
skilletfish
striped blenny
winter flounder
Invertebrates
amphipod
baltic isopod
copepod
ctenophore
grass shrimp
mud crab
sand shrimp
sea nettle
Gobipsoma bosci
Gobiesox strumosus
Chasmodes bosquianus
Pseudopleuronectes americanus
ampeliscidae
Idotea baltica
Acartia tonsa
Mnemlopsls leidyl
Palaemonetes pugio, P. vulgaris
Eurypanopeus depressus
Crangon septemspinosa
Chrysaora quinquecirrha
after 11 hours of exposure to anoxia at 22.0°C and 12
ppt salinity (Widdows et al. 1989). Temperature has
a critical role in tolerance to low DO concentrations.
Adult oysters held at 10, 20 and 30 ppt salinity, had
LTgo values (days of exposure to anoxia causing 50%
mortality) of 28 days at 10°C, 18-20 days at 20°C, and
3-8 days at 30°C (Stickle et al. 1989).
Several short term lethal values of DO (1-19 hours) for
target species fall within the range of 0.3-1.0 mg/L.
The 6-hour LC^ for adult blue crabs at 28-30°C is 0.3
mg/L (Carpenter and Cargo 1957). Concentrations of
DO below 0.5 mg/L are lethal to adult blue crabs in
4.3 hours at 25°C (Lowery and Tate 1986). The LC5
and LC^ for juvenile spot, in a one hour exposure at
28 C and 6.9 ppt salinity, are 0.6 and 0.5 mg/L DO
respectively; the 2-hour LC5 and LC^ for juvenile
menhaden, under the same conditions, are 1.0 and 0.7
mg/L (Burton et al. 1980). Juvenile white perch
-------
experience 40% mortality in 19 hours at 0.5-1.0 mg/L
(Dorfman and Westman 1970).
Although adult oysters appear to be tolerant to some
degree of anoxia, many species associated with oyster
bars are more sensitive to low DO concentrations.
Naked goby larvae exposed to =s0.15, 0.35, and 0.35-
0.86 mg/L DO for 1, 2, and 24 hours respectively, suf-
fered 100% mortality (Saksena and Joseph 1972). All
new recruits (s\7 mm TL), juveniles, and adult naked
gobies survived exposure to 0.75-0.95 mg/L at 25°C
for 7 hours; however, there was 100% mortality
among new recruits exposed to 0.35-0.60 mg/L DO
(Breitburg 1992a). The median tolerance limit (oxygen
concentration at which 50% of the larvae would be
expected to die after 24 hours) for naked goby,
striped blenny and skilletfish are 1.30, 2.50, and 0.72-
1.23 mg/L DO (Saksena and Joseph 1972). The 96-
hour LCgo for adult mud crabs (Eurypanopeus
depressus), another member of the oyster bar commu-
nity, is 0.6 mg/L DO (Stickle 1991).
Other common benthic and planktonic species are
also sensitive to DO concentrations below 1.0 mg/L.
The 6-hour LC^ for adult baltic isopods at 10°C is 0.2
mg/L (Theede et al. 1969, Theede 1973); the LC^ for
ampeliscid amphipods in a 96-hour exposure is <0.5
mg/L DO (Miller and Poucher 1991). Sand shrimp
have a 96-hour LCgo of 1.5 mg/L DO at 20°C and 31
ppt salinity (preliminary data: Miller and Poucher,
1991). The copepod Acartia tonsa has a 24-hour LC^,
of 0.8 mg/L DO (Houde 1991); while the LC^ for sea
nettles and ctenophores in a 96 hour exposure are 0.7
and 1.0 mg/L DO respectively (Houde 1991).
In addition to direct lethal effects, exposure to DO
concentrations <1.0 mg/L can adversely affect the
growth and behavior of organisms. Breitburg (1992a)
found that male naked gobies abandoned the nest or
shelter at DO concentrations of 0.15-0.6 mg/L, and
that embryo development time was significantly
slowed by repeated exposure to low DO
concentrations (Appendix A).
Although DO as low as 1.0 mg/L is never desirable,
brief excursions down to 1 mg/L in some deep areas
of the Bay should not have severe adverse effects on
populations of either target species or benthos. Even
an hypoxia-sensitive species (adult alewife) can
endure a 5-minute exposure to DO of 0.5 mg/L if
escape to an area of higher DO concentration is
available (Dorfman and Westman 1970).
B, The following target concentration applies to all waters
of Chesapeake Bay and its tidal tributaries at all times: 12-
hour maximum duration of DO between 1.0 and 3.0 mg/L,
48-hour minimum return frequency of DO «;3.0 mg/L and
zl.O mg/L
Bay anchovy eggs hatch in 18-24 hours, and hatching
success declines significantly below 3.0 mg/L DO
(Chesney and Houde 1989). Houde and Zastrow
(1991) suggested that DO <3.0 mg/L limits the
viability and productivity of bay anchovy in
Chesapeake Bay.
There was no mortality of adult blue crabs in 7-day
exposures at about 3.0 mg/L, and less than 20%
mortality in a 25-day exposure at 21-23°C (deFur et
al 1990). The blue crab is often considered an hypox-
ia-tolerant species, however, long term exposures to
mild hypoxia at high temperatures may be lethal
(Stickle et al. 1989); tolerance is very temperature
dependent (Carpenter and Cargo 1957). Crabs died
in pots at -30°C and -2.5 mg/L DO (Carpenter and
Cargo 1957).
Several target species experienced deleterious effects
in exposures to less than approximately 3.0 mg/L,
e.g., growth of yellow perch juveniles is reduced at
20°C and DO <2.0 mg/L, but is not affected at DO
>3.5 mg/L (Carlson et al. 1980). Dissolved oxygen
<3.0 mg/L caused mortality in striped bass juveniles
(Krouse 1968; Chittenden 1972) and stress in adult
striped bass (Chittenden 1972; Coutant 1985). Juvenile
blueback herring and adult alewife exposed to 2.0-3.0
mg/L DO for 16 hours experienced 33% mortality
(Dorfman and Westman 1970).
Adult white perch avoided waters with DO <35%
saturation (-3.2 mg/L), over a temperature range of
8-21 °C and salinity range of 2.5-12.5 ppt (Meldrim et
al. 1974). However, there is evidence that juvenile
blueback herring are unable to detect and avoid
waters with low DO concentrations (Dorfman and
Westman 1970). Dissolved oxygen concentrations <3.0
mg/L blocked migrations of juvenile and- adult
American shad (Miller et al. 1982).
Recent research has established 96-hour LC^ values
between 1.0 and 3.0 mg/L DO for several species
found throughout the Bay (Appendix A). For
example, the 96-hour LC^ for juvenile and adult sand
shrimp, at 20°C and 31 ppt salinity, is 1.5 mg/L
(preliminary data: Miller and Poucher 1991); the 96-
hour LCg,, values for larval, and juvenile or adult
grass shrimp (Palaemonetes pugio) are 1.9 and 1.6
mg/L DO (Stickle 1991). Winter flounder eggs, larvae,
8
-------
and juveniles have 96-hour LC^ values of 1.9, 1.5,
and 1.4 mg/L DO respectively (Miller and Poucher
1991).
All target species appear to tolerate DO of 3.0 mg/L
for short periods of time (Figure II-l; Appendix A).
The recommended return frequency, in combination
with the protection provided under the 1.0 mg/L
instantaneous target concentration, will permit ample
periods of time for hatching of anchovy eggs,
probably will protect blue crabs trapped in pots for
periods of up to a few days, and will prevent
frequent recurrences of stressful conditions for other
target species. However, recent data indicate that
excursions between 1.0 and 3.0 mg/L DO for up to 12
hours may not be fully protective of every Bay
species. The time to 50% mortality of larval grass
shrimp (P. vulgaris) exposed to 1.4 and 1.6 mg/L DO
was 2.9 and 21.6 hours respectively (preliminary data:
Miller and Poucher 1992).
C. The following target concentration applies to all above-
pycnocline waters of Chesapeake Bay and its tidal
tributaries: 5.0 mg/L DO monthly average.
This concentration appears to be protective of all
target species. Optimum DO for hard clam burrowing
rates was somewhat higher than 5.0 mg/L (Savage
1976). Growth rates of hard clams were greatly
reduced below 4.2 mg/L (Morrison 1971); DO <5.0
mg/L was considered stressful for this species
(Hamwi 1968, 1969; Roegner and Mann 1991).
One study cited found a 50% mortality of juvenile
blue crabs in a 28-day exposure to 5.65 mg/L DO at
30°C (Stickle et al. 1989). However, long-term expo-
sure to a temperature of 30 C or above is uncommon
in Chesapeake Bay. At lower temperatures (21-23°C),
there was some mortality of adult crabs in 23-25 day
exposures to DO of about 3 mg/L (deFur et al. 1990).
Dissolved oxygen &5.0 mg/L is a requirement for
several species of anadromous fish (Bogdanov et al.
1967; Miller et al. 1982; ASMFC 1987; Jones et al. 1988;
Piavis 1991). Miller et al. (1982) considered DO
concentrations <5.0 mg/L sublethal to juvenile and
adult American shad, while Piavis (1991) concluded
that a DO of 5 mg/L was the lowest average concen-
tration that sustains normal development and activity
for yellow perch. Jones et al. (1988) listed 5.0 mg/L
as the minimum DO concentration required for all life
stages of American and hickory shad, striped bass,
white perch and yellow perch; the minimum required
for eggs, larvae, subadults and adults of alewife and
blueback herring; and the probable minimum for
adult menhaden and the egg, larval and juvenile life
stages of spot.
Field observations suggest that juvenile spot prefer
DO >4.0-5.0 mg/L (Ogren and Brusher 1977;
Rothschild 1990) and adult spot are most abundant
where Do is >4.0 mg/L (Markle 1976; Chao and
Musick 1977; Rothschild 1990). Dissolved oxygen
concentrations of 4.0-5.0 mg/L appear to be a
minimum for juvenile and adult American shad
(Burdick 1954; Jessop 1975), whereas other
anadromous species prefer higher concentrations.
White perch and striped bass concentrate in areas of
at least 6.0 mg/L DO (Rothschild 1990), and adult
blueback herring were never captured at sampling
stations where DO was <6.0 mg/L (Christie et al.
1981).
In general, the 5.0 mg/L monthly mean target
concentration, in combination with target concentra-
tions A and B, should protect all species (except the
anadromous fish, see D, below) against severe long
term stress, and the monthly mean presumably
would represent substantial periods with DO above
5.0 mg/L. Dissolved oxygen <6.0 mg/L may cause
avoidance or minor sublethal stress in a few species,
and may combine with very high water temperatures
(a30°C) to cause more severe stress or mortality.
D. This target DO concentration applies to anadromous
fish spawning and nursery areas (Figure 111-1) in the
above-pycnocline waters of Chesapeake Bay and its tidal
tributaries at all times: 5.0 mg/L DO.
This target DO concentration was selected to protect
the early life stages of striped bass, white perch,
alewife, blueback herring, American shad, hickory
shad, and yellow perch. This concentration of DO will
allow eggs to hatch normally (Bradford et al. 1968;
O'Malley and Boone 1972; Marcy and Jacobson 1976;
Harrell and Bayless 1981; Jones et al. 1988;), as well
as allow survival and growth of larval and juvenile
stages of all anadromous target species (Tagatz 1961;
Bogdanov et al 1967; Krouse 1968; Bowker 1969;
Chittenden 1969, 1972, 1973; Meldrim et al. 1974;
Rogers et al. 1980; Miller et al. 1982; Coutant 1985;
ASMFC 1987; Jones et al. 1988). For example, con-
centrations of DO below 5 mg/L for any duration will
not support normal hatching of striped bass eggs
(O'Malley and Boone 1972). Although one hatchery
operation was able to maintain striped bass finger-
lings at DO concentrations of 3-4 mg/L (Churchill
-------
Figure 111-1. Habitat distribution of anadromous fish spawning
reaches, spawning rivers and nursery areas in Chesapeake Bay
(M); combined for striped bass, white perch, alewife, blueback
herring, American shad, hickory shad, and yellow perch (Source:
Funderburk era/. 1991)
1985; Loos 1991), Bowker et al. (1969) found DO >3.6
mg/L required for survival of juveniles.
This target concentration appears to be a critical value
for providing acceptable protection for anadromous
fish. Dissolved oxygen concentrations above 5 mg/L
are within the suitable range for eggs, larvae, and
juveniles of yellow perch, white perch, striped bass,
alewife, blueback herring, American shad, and
hickory shad (Figure II-l). Several authors have
indicated that DO ;>5.0 mg/L is a "suitable" or
"recommended" level for early life stages of the
anadromous species (Bogdanov et al. 1967; Krouse
1968; Miller et al. 1982; ASMFC 1987; Jones et al. 1988;
Piavis 1991). Juvenile anadromous species are no
more tolerant of low DO than eggs or larvae. Jones et
al. (1988) listed 5.0 mg/L as the minimum DO
concentration required for all life stages, including
juveniles and adults, of American and hickory shad,
striped bass, white perch and yellow perch. Miller et
al. (1982) consider DO concentrations <5.0 mg/L
sublethal to juvenile and adult American shad. A DO
concentration of 5.0 mg/L is also the minimum re-
quired for eggs, larvae, subadults and adults of
alewife and blueback herring (Jones et al. 1988).
Because the juvenile anadromous species use the
lower estuarine reaches of the spawning rivers as
nursery areas, and they are present throughout the
year, the 5.0 mg/L target concentration applies to the
entire above pycnocline tidal area of the spawning
rivers (Figure III-l) over all seasons.
Some field observations have indicated that juveniles
and adults of anadromous species prefer DO of &6.0
rng/L (Hawkins 1979; Christie et al. 1981; Rothschild
1990). However, no lethal or sublethal effects other
than possible avoidance have been documented for
DO concentrations between 5.0 and 6.0 mg/L.
Chesapeake Bay Dissolved Oxygen Restoration Goal
In combination, the four target DO concentrations
provide a DO restoration goal for Chesapeake Bay
that reflects the habitat needs of the Bay's living
resources (Box 2). Applied individually, achievement
of the target concentrations would ensure sufficient
habitat quality for survival (1 mg/L and 3 mg/L
target concentrations) and continued growth and
reproduction (5 mg/L anadromous spawning river
and 5 mg/L monthly mean target concentrations).
Applied as a single, integrated restoration goal,
achievement of all the target concentrations, along
with a provision (e.) to ensure that the target
concentrations are not construed as allowing
degradation where present conditions are adequate,
will "provide for sufficient dissolved oxygen to
support survival, growth and reproduction" of the
Chesapeake Bay's aquatic living resources.
10
-------
BOX 2
CHESAPEAKE BAY DISSOLVED OXYGEN GOAL
FOR RESTORATION OF LIVING RESOURCE HABITATS
GOAL: To provide for sufficient dissolved oxygen to support survival, growth and
reproduction of anadromous, estuarine and marine fish and invertebrates in
Chesapeake Bay and its tidal tributaries by achieving, to the greatest spatial and
temporal extent possible, the following target concentrations of dissolved oxygen;
a) dissolved oxygen concentrations of at least 1.0 mg/L at all times throughout Chesapeake Bay and
its tidal tributaries, including subpycnoline1 waters;
b) dissolved oxygen concentrations between 1.0 and 3.0 mg/L should not occur for longer than 12
hours and the interval between excursions of dissolved oxygen between 1.0 and 3.0 mg/L should
be at least 48 hours throughout Chesapeake Bay and its tidal tributaries, including subpycnocline
waters;
c) monthly mean dissolved oxygen concentration of at least 5,0 mg/L throughout the
above-pycnodine waters of Chesapeake Bay and its tidal tributaries; and
w dissolved oxygen concentrations of at least 5.0 mg/L at all times throughout the abovejpycnocline
waters of anadromous fish spawning reaches, spawning rivers and nursery areas2 of Chesapeake
Bay and its tidal tributaries as defined in Habitat Requrements for Chesapeake Bay Living Resources,
1991 Revised Edition;
and
e) by maintaining the existing minimum concentration of dissolved oxygen in areas of Chesapeake
Bay and its tidal tributaries where dissolved oxygen concentrations are above those stated in a)
through d).
"The pycnocline is the portion of the water column where density changes rapidly because of salinity and temperature differences.
See Appendix B for definitions
'Spawning reaches, spawning rivers, and nursery areas are presented in Figure 111-1.
11
-------
IV. APPLICATION OF MONITORING AND MODELING INFORMATION
Introduction
In this section, examples of DO dynamics observed in
different Bay habitats are examined in light of the
target DO concentrations. An approach is presented
for using data from the Chesapeake Bay Monitoring
Program both to monitor progress toward the
restoration goal and to evaluate improvements in DO
which are projected by the Chesapeake Bay time-
variable water quality model.
Dissolved Oxygen Variability in the Environment
Recent studies have produced relatively long-term,
semicontinuous (5 to 30-minute sampling interval)
records of bottom DO concentrations in a variety of
Chesapeake Bay environments. Data from three sites
representative of major Bay subsystems are shown in
Figure IV-1. The July-August period covered by these
records is when DO deficiency is greatest and
maximum stress to living resources typically occurs.
These data demonstrate the large, but typical,
variations in DO to which organisms are exposed over
short time periods (hours to days). These data also
show the variability in the duration and frequency of
exposure to low DO within and among sites.
Dissolved oxygen data from an 18-meter deep water
habitat in the stratified and strongly tidal lower
reaches of the York River estuary are shown in Figure
IV-1 a (data courtesy of R. Diaz, Virginia Institute of
Marine Science). Dissolved oxygen concentrations at
this site are summarized below (Table IV-1). This
deployment was below the pycnocline, so the 5-mg/L
monthly mean and anadromous fish target concen-
trations of the goal do not apply; the latter concen-
trations are shown in Tables IV-1 and IV-2 only to
illustrate the habitat conditions at these sites.
Table IV-1. Distribution of DO concentrations during July and
August 1989 in the Lower York River (18m).
July mean: 3.0 mg/L
August mean: 1.7 mg/L
<5 mg/L: 98% of the time
<3 mg/L: 74% of the time
<1 mg/L: 14% of the time
15 events <3mg/L 2! 2 hours;
2 events <3mg/L lasted for 6-7 days;
70 events where DO <3mg/L returned within 48 hrs.
10
8
6
4
2
0
10
8
6
4
2
20JUN89 02JUL 14JUL 2SJUI 06AUG 17AUG 29AUG 09SEP89
08AUG87
19AUGI7
31AUGI7
HSEP87
10
f 8
I 4
o
0 2
0
27JUN88 08JUL8B 20JUL88 31JULS8 12AUG88 23AUG38 04SEP88
Figure IV-1. Semicontinuous dissolved oxygen measurements at
three sites in Chesapeake Bay. Upper line is the saturation
concentration of dissolved oxygen: bottom line is observed
dissolved oxygen. ••••••••••••• 1 and 3 mg/L reference lines.
a. York River: sensor depth 18 m, July 1 to August 31, 1989 (Diaz
et al. in press).
b. Mainstem, near Choptank River mouth; sensor depth 13 m,
August 12 to September 9, 1987 (Sanford et al. 1990).
c. St. Leonard Creek; sensor depth 3-4 m, July 1 to August 31,
1988 (Maryland Department of the Environment).
Dissolved oxygen was monitored semicontinuously
during the summer at a depth of 13 m near the
mouth of the Choptank River (Figure IV-lb; data
courtesy of L. Sanford, University of Maryland). This
location is representative of bottom habitats near the
13
-------
Table IV-2. Distribution of DO concentrations in August and
September 1987 off the mouth of the Choptank River (13 m).
28-day mean: 4.7 mg/L
<5 mg/L: 41% of the time
<3 mg/L: 22% of the time
<1 mg/L: slightly <1 % of the time
3 events <3mg/L lasted 12 hours or longer;
8 events where DO <3 mg/L returned within 48 hrs.
usual depth of the pycnocline. The DO characteristics
are summarized in Table IV-2. This deployment was
below the pycnocline for portions of the period of
record.
Tributary creeks a few meters deep also can experi-
ence low DO conditions during summer. Figure IV-lc
shows DO concentrations in waters overlying a
natural oyster bar 3-4 m deep along St. Leonard
Creek, a tidal tributary of the Patuxent River (data
courtesy of R. Summers, Maryland Dept. of the Envi-
ronment). The DO characteristics at this site are sum-
marized in Table IV-3.
Table IV-3. Distribution of DO concentrations in July and August
1988 in St. Leonard Creek (3-4 m).
July mean: 3.2 mg/L
August mean: 3.3 mg/L
<5 mg/L: 87% of the time
<3 mg/L: 46% of the time
<1 mg/L: 9% of the time
15 events <3mg/L lasted a12 hours;
64 events where DO <3mg/L returned within 48 hrs.
Cumulative frequency (in this case, cumulative
percentage) distributions constructed from all DO
observations at a site are one means of comparing
complex patterns among different locations. Although
they are simple and instructive measures of variabili-
ty, cumulative frequency distributions do not
explicitly take into account the frequency or duration
of exposure to deleterious oxygen concentrations. The
plots in Figure IV-2 show the range of the measure-
ments and what percent of the measurements were
below a given concentration.
40
20 _
246
DISSOLVED OXYGEN IN
246
DISSOLVED OXYGEN IN
8
mg/L
2468
DISSOLVED OXYGEN IN mg/L
Figure IV-2. Cumulative frequency distributions developed from
semicontinuous dissolved oxygen data, at the three sites shown in
Figure IV-1.
a York River
b Mainstem near Choptank River mouth
c. St. Leonard Creek
The three semicontinuous DO data sets were selected
to portray DO characteristics in three different Chesa-
peake Bay habitats. One important question is
whether these patterns of variation are characteristic
of similar habitats in other parts of the Bay. That is,
do similar habitats have similar DO "signatures" and
is the pattern at a location generally similar from year
to year under similar climatic conditions? Extreme
deviations from the patterns observed so far are not
expected, but additional work in this area is under-
way and planned for the future by several cooper-
ating programs.
14
-------
The Chesapeake Bay Monitoring Program and the
Time-Variable Model
Semicontinuous DO data are valuable to our knowl-
edge of physical processes, for calibration and
interpretation of low frequency monitoring data, and
in translating habitat requirements into realistic
management goals. But these high frequency data sets
are costly to acquire and not yet available from
enough sites or time periods either for Baywide
assessment of present conditions or for measuring
progress toward the DO restoration goal. Therefore,
the primary source of DO data for these assessments
is the Chesapeake Bay Monitoring Program, which
began in 1984.
Dissolved oxygen is one of a suite of water quality
parameters measured at the network of stations
throughout the Bay. The water quality monitoring
stations and the Chesapeake Bay Program segmenta-
tion scheme are shown in Figure IV-3. Depth profiles
of DO are collected at each station twice a month from
spring through summer and once a month in fall and
winter.
Another means of evaluating DO conditions is the
Chesapeake Bay time-variable water quality model.
This computer model was developed to forecast the
effects of particular nutrient load reduction scenarios
on water quality, e.g., the effect of nutrient load
reductions on DO concentrations. For purposes of the
model, the Bay is divided spatially into several
thousand three-dimensional blocks or "cells." Tem-
porally, the mathematical modelling process makes
water quality projections for each cell at time steps
of a few hours. However, for evaluating model-
projected water quality responses, each nutrient
reduction scenario provides an estimate of the
average seasonal DO concentration for each cell.
Shorter time intervals (e.g., monthly, daily) may be
available in the future. For calibration and reporting
purposes, the cells are averaged into nine segments
along the planar (surface) axis of the Bay and into
three vertical (depth) layers (Figure IV-4). The depth
layers are defined relative to the pycnocline, the
region in the water column where separation occurs
between the more buoyant, fresher surface waters,
and denser, saltier bottom waters (see Appendix B for
further discussion of the pycnocline).
The DO restoration goal specifies limits on durations
and frequency of reoccurrence of DO below the target
concentrations; monitoring data and model projec-
tions do not have sufficient temporal resolution to
Figure IV-3. Chesapeake Bay mainstem and tributary water quality
monitoring stations (©) with the Chesapeake Bay Program
segmentation scheme shown (lines). Mainstem segments are
labeled.
evaluate conformance with these limits. Therefore,
relationships must be defined among 1) the DO
restoration goal target concentrations, 2) the real-time
semicontinuous DO measurements, 3) the twice-
monthly Bay Monitoring Program data, and 4) the
seasonal means projected by the time-variable model.
15
-------
Figure IV-4. Grid for the Chesapeake Bay time-variable model.
Only surface model cells are shown. Dark lines and numbers refer
to model segments.
Bay Monitoring and Semicontinuous Data Compar-
isons
As a first step, DO data collected through the
Monitoring Program were compared with semiconti-
nuous DO data. The Choptank River data set illustrat-
ed in Figure IV-lb is one of five such data records
that were collected simultaneously at separate sites
along a cross-Bay transect in the middle region of the
mainstem Bay (Figure IV-5). Dissolved oxygen was
monitored at approximately 6, 13, and 19 m at three
locations, and at both 6 and 9 m at a fourth location.
These sites were in the vicinity of monitoring stations
in Chesapeake Bay Program segment CB4 (Figure IV-
5) which were visited on two monitoring cruises
during the four-week deployment of the continuous
recorders.
Figure IV-5. Location of Semicontinuous dissolved oxygen sensors
(0) and selected twice-monthly bay program monitoring stations
( 0 ) in segment CB4 (enlargement). Depth of sensor records
discussed in text is indicated.
Dissolved oxygen profiles from the Monitoring
Program stations in CB4 were plotted together with
the means, ranges and standard deviations of the
Semicontinuous DO data (Figure IV-6). Because the
deployed oxygen sensors made 4-12 measurements
per hour, 24 hours a day over many days, they were
more likely to encounter and record ephemeral
extreme conditions. As expected, therefore, the
minimum and maximum values of the Semicontin-
uous data are outside the ranges of the Monitoring
Program data. The other summary statistics of the
Bay Monitoring Program and Semicontinuous data,
however, are generally comparable, with the excep-
tion of the 13-m depth (Figure IV-6 and Table IV-4).
16
-------
_. 50
E
I
15
20
25
• .....
s ••*•••*•• ••»*«« o
• • • MM* •*»•*« ••*** 0 •
• • • # • • • »»•• • • • «
• • • • •••• • • • •••«•»•• «
• • ••••••• •• * • t • • •• • •
, ZIl , I > •• • ft
•••• • ••• § •
>*• •> •• • «*• » >•
• • •* • • •
• • • •
• • •
• • • ••» • •• *
• • • « • « 0
• • « I *t t
••••* MONITORING DATA
MIN , , , MAX
BUOY DATA
-ISO
+ISD
0.0
2.0
4.0 6.0
DISSOLVED OXYGEN (mgA)
8.0
10.0
Figure IV-6. Comparison of Monitoring Program and semicontinuous dissolved oxygen data. The observed dissolved oxygen values from
mainstem monitoring program stations in segment CB4 are overlain with plots of the mean, range, and standard deviation of values recorded
during the same period by semicontinuous dissolved oxygen monitoring devices in CB4 at the depths shown.
At the 13-m depth, all the statistics are higher in the
semicontinuous data than in the twice-monthly Moni-
toring Program data. The statistics are higher even
than those of the semicontinuous data from the
shallower 9-m western shore site. The difference is
probably best explained by the bottom geometry of
the 13-m location. A shallow sill lies at the mouth of
the Choptank River inhibiting intrusion of below-
pycnocline waters from the mainstem under typical
energy conditions. Sanford et al. (1990) note that tidal
markers identified in the other buoy data records
were not seen in the 13-m data record.
The semicontinuous and Monitoring Program DO data
also can be compared by means of cumulative
frequency distributions (Figure IV-7). Cumulative
frequency plots are particularly useful in showing the
extent to which minimum levels of DO may or may
not be underestimated by the lower sampling fre-
quency of the Monitoring Program.
In these comparisons, the data collected at depths
above and below the region of the pycnocline show
close agreement in the percentage of observations at
the lower end of the distribution where the stressful
DO values lie. For example, at 6 m (Figure IV-7a,b),
the percentage of observations less than or equal to
1 mg/L was less than 1% and the percentage less
than or equal to 3 mg/L was less than 5%, in both
the Monitoring Program and semicontinuous data
sets. At 19 m, 98% of the DO observations were less
than or equal to 1.6 mg/L in both the Monitoring
Program and semicontinuous data sets, and 100% and
99%, respectively, were less than or equal to 3 mg/L
(Figure IV-7e).
Whether all Chesapeake Bay mainstem and tributary
segments are equally well represented by the number
and location of Monitoring Program stations is not
completely known. However, based on the example
above and additional monitoring and research
17
-------
Table IV-4. Comparison of semicontinuous (C) and Monitoring Program (M) dissolved oxygen data. The time period lorsemicontinuous
data was August 13 through September 6, 1987; Monitoring Program cruise dates were August 16 and September 2, 1987. N =
number of observations.
C
N
M
C
Mean
M
Std. Dev.
C
M
Minimum
C
M
Maximum
C
M
Depth (m)
61
62
9s
133
194
2270
2443
2443
2422
2435
1 Choptank River, 12-min
2
West side of main
Bay,
47
47
39
30
17
intervals
5-min. intervals
6.5
6.7
3.8
4.6
0.5
6.3
6.3
4.2
1.7
0.8
3 Choptank
0.9
1.1
2.1
1.7
0.5
River mouth,
4 Mid-channel main Bay,
1.5
1.5
1.5
1.4
0.5
5-min.
1 5-min
0.5
1.9
0.0
0.4
0.0
intervals
intervals
2.0
2.0
1.4
0.1
0.1
9.9
9.8
8.1
8.1
4.4
8.6
8.6
8.3
6.3
1.6
TOO
80
60
4O
20
O
2468
DISSOLVED OXYGEN (mg/L)
10
Figure IV-7. Cumulative frequency distributions of semicontinuous
dissolved oxygen data (—) and twice- monthly Monitoring Program
data (•"•-O for locations in Figure IV-5.
a. 6 meters - west side d. 13 meters
b. 6 meters - Choptank e. 19 meters
c. 9 meters
evidence (e.g., Sanford and Boicourt 1990a; 1990b), it
was assumed that the distribution of the Monitoring
Program data represent the range and central
tendency of real-time DO conditions and that the
distributions of the twice-monthly monitoring data
can be applied in evaluating both the Bay's current
status and progress toward the DO restoration goal.
A Method for Evaluating Progress Toward the
Restoration Goal
The DO data from the Monitoring Program were
examined to develop simple methods for evaluating
different regions of the Bay with respect to the DO
restoration goal's target concentrations. Methods that
also could be applied or adapted to evaluating time-
variable model output were most desirable. Because
model results are seasonal mean concentrations, the
seasonal mean DO was chosen as the variable of
interest. The relationships of the seasonal mean to: 1)
minimum DO concentrations, 2) the standard devia-
tion of DO measurements, and 3) the percentage of
observations above or below the restoration goal's
target concentrations were explored as possible
measures of status and progress.
The relationship between the seasonal mean and the
percentage of monitoring measurements above target
concentrations proved to be strong and widely
applicable in regions of the Bay where DO observa-
tions ranged above and below the restoration goal
target concentrations. For example, Figure IV-8 shows
the relationship for the 1 rng/L, 3 mg/L, and
monthly mean of 5 mg/L target concentrations in
segment CB4. The 5 mg/L instantaneous target
concentration for anadromous fish spawning and
nursery habitats does not apply in this segment.
18
-------
100
C 80
O
CO
o
60
40
S 20
0
100
ty 80
60
40
B
B
1 mg/L MONTHLY MEAN
Tpf
b)
£ 20 I-
0
100
t 80
£ 60
g 40
LU
^
K 20
3 mg/L MONTHLY MEAN
B
BB
P
P
5 mg/L MONTHLY MEAN
0 2 A 6 8 10 12 14
SEASONAL MEAN
Figure IV-8. Examples of the empirical relationship between
observed annual seasonal (spring and summer only) mean
dissolved oxygen concentration (mg/L) and the percent of those
observations above the restoration target concentrations for
Chesapeake Bay Program segment CB4, for the years 1984
through 1990. Spring includes the months of March, April, and
May; summer includes June through September. Letter symbols
indicate depth layer of the data from which the seasonal mean and
percent of observations were calculated. A = above pycnocline, P
= region of the pycnocline, and B = below pycnocline.
a. 1 mg/L at any time
b. 3 mg/L (without duration or return constraints)
c. 5 mg/L monthly average
In Figure IV-8, the percentage of observations above
the target concentration is plotted versus the seasonal
mean. The seasonal mean DO concentration projected
by the time-variable model for each cell is related, in
part, to whether the cell is above, in, or below the
pycnocline. Similarly, in the analysis of the Moni-
toring Program data, the seasonal mean DO concentra-
tion and the number of measurements above each
target concentration were calculated separately for
each depth layer in each segment. The points in the
plot approximate a curve, which in some segments
approaches a straight line. Equations describing the
curves for each target concentration were obtained for
each segment by regression analysis using arcsine-
transformed data. The equations and plots for all
mainstem Chesapeake Bay Program and time-variable
model segments, as well as the details of this
analysis, are given in Appendix B.
The seasonal mean concentration which will achieve
the goal can be derived from the regression equa-
tions. Conversely, given the mean DO concentration
for a season in a particular segment, the percentage
of observations that will meet the targets can be
estimated. Table IV-5 shows the seasonal mean DO
Table IV-5. Seasonal mean DO concentrations (mg/L) required in
Chesapeake Bay Program segment CB4 to achieve the indicated
percentage of observations meeting or exceeding the specified
target concentration. The 5 mg/L instantaneous target
concentration for anadromous fish spawning and nursery habitats
is not applicable to this segment.
Percent of
observations
z target
100
99
90
80
70
60
50
40
30
20
10
1 instantaneous
2 instantaneous (targe!
DO at mg/L and <3
3 monthly mean
Target concentration
1 mg/L1 3 mg/L2 5
9.1 11.2
6.7 8.4
4.5 6.1
3.6 5.1
2.9 4.4
2.4 3.8
1.9 3.3
1.4 2.8
1.0 2.3
0.5 1.8
0.0 1.3
concentration permits
mg/L for <12 hours.)
mg/L3
17.2
9.1
7.4
6.4
5.7
5.1
4.5
4.0
3.5
2.9
2.2
19
-------
concentrations required to achieve 10, 20, 30,. . ., 90,
99, and 100% of the goal in example segment CB4.
The unreasonably large increase in mean DO required
to go from 99% to 100% attainment is notable in this
example and is similarly large for other segments as
well. The 100% level requires DO concentrations
exceeding typical saturation levels in summer. The
asymptotic nature of this relationship dictates that for
physical reasons the target concentrations cannot be
achieved 100% of the time. For this reason, and
because of both variability and statistical uncertainty
in the analysis, a somewhat lower percentage should
be deemed successful in achieving the restoration
goal and providing living resources with maximum
protection from harmful effects of low DO conditions.
Knowing the seasonal mean DO concentration for an
area in the Bay, therefore, permits a good estimate of
what proportion of actual DO observations are likely
to meet, or fail to meet, each of the target concentra-
tions. For example, there was good agreement
between actual and predicted percent achievement in
example segment CB4 (Table IV-6). In regions where
the range of DO values in the Monitoring Program
Table IV-6. Summer mean DO concentrations above the pycnocline (layer A), in the region of the pycnocline (layer P), and below the pycnocline
(layer B) in Chesapeake Bay Program segment CB4,1984 through 1990, comparing observed and predicted percentages of observations a the
target concentrations. The 5 mg/L instantaneous target concentration for anadromous fish spawning and nursery habitats is not applicable to
this segment; the 5 mg/L monthly mean target concentration is not applicable below the pycnocline.
OBSERVED PERCENTAGE
Year Layer
1984 A
P
B
1985 A
P
B
1986 A
P
B
1987 A
P
B
1988 A
P
B
1989 A
P
B
1990 A
P
B
'instantaneous
Observed
Mean
65
2.6
0.7
6.5
2.9
0.8
6.6
2.9
0.9
6.9
3.2
0.6
6.8
3.6
0.4
6.8
2.9
0.9
6.8
3.5
0.7
Target
1 mg/L1
99.6
60.4
183
1000
83.1
34.0
995
73.6
31 7
100.0
84.2
20.7
99.9
81.6
12.1
99.6
672
24.6
100.0
80.1
28.1
Concentration
3 mg/L2 5 mg/L3
97 6 88.9
434
9.8
98 1 92.5
44.4
0.9
97.5 95.6
40.1
2.4
99.5 97 1
47.8
1.0
99.1 96.1
56.1
1.4
96.2 92.8
41 6
7.2
98.7 95.2
50.8
2.4
instantaneous (target concentration permits DO & 1 mg/L and s
'monthly mean
PREDICTED PERCENTAGE
Target
1 mg/L1
98.7
64.6
24.3
98.7
70.6
27.0
98.9
69.4
28.0
99.2
74.4
21.8
99.2
79.7
17.5
991
69.4
27.4
99.1
787
24.1
3 mg/L for 12
Concentration
3mg/L2 5 mg/L3
927 81.2
35.4
3.1
92.8 81.3
42.5
43
93 3 82 2
41.1
4.9
94.7 85.3
47.5
2.1
94.4 84.6
54.9
0.8
94.3 84.4
41 1
4.8
94.3 84 4
534
3.0
hours)
20
-------
data is narrow and DO has always been above the
target concentrations (as is the case in some shallow
areas of the mainstem Bay, the lower mainstem Bay
segments, and in the transition and tidal fresh regions
of most tributaries) such a relationship cannot be
established. In these cases, the goal states that DO
conditions may not become worse, e.g., seasonal
mean DO should not go below the lowest seasonal
mean recorded in the segment which had no mea-
surements below target levels.
Time-variable Model Scenarios and Progress
Toward the Restoration Goal
The Chesapeake Bay time-variable water quality
model has a major role in«reevaluating the Baywide
Nutrient Reduction Strategy. The model is being used
to test specific sub-basin scenarios for the effect of
nutrient load reductions in the surrounding Bay
tributary watersheds on DO and other water quality
parameters. Chesapeake Bay Program managers will
compare these model-simulated DO concentrations
and their spatial distributions with existing DO condi-
tions and with the DO restoration goal.
What might "improvement" look like?
Earlier Chesapeake Bay steady-state water quality
models projected that reducing basinwide inputs of
nitrogen and phosphorus from point and nonpoint
sources would improve summer-averaged DO levels
in the mainstem Bay. Improvements of this nature are
not expected to significantly change the frequency or
pattern of DO fluctuations, which are largely dictated
by physical factors. However, the entire distribution
of water column DO concentrations is expected to rise,
and the amplitude of fluctuations in concentration is
expected to decrease.
A hypothetical result of nutrient load reductions
might be to increase seasonal mean DO by 1.5 mg/L.
How this increase might be expressed in a selected
living resource habitat is illustrated in Figure IV-9a,
an adaptation of one of the semicontinuous records
presented earlier. Raising each of the DO observations
in this time series by 1.5 mg/L (with the constraint
that the new value not exceed the saturation
concentration unless the original measurement did)
has the effect of shifting the cumulative frequency
distribution to the right and reducing the number and
duration of low Do occurrences (Figure IV-9b). The
four-week mean is increased from 4.7 to 6.1 mg/L,
episodes below 1.0 mg/L are eliminated, and the
longest period of exposure to DO <3.0 mg/L is about
nine hours. Therefore, an overall increase in DO of 1.5
mg/L results in full achievement of the DO
08AUG87
19AUGB7
3UUG87
1IHP87
Figure IV-9a. Observed dissolved oxygen concentration (—) at
Choptank River mouth (see Figure IV-1b) and hypothetical
concentrations ( ) representing a 1.5 mg/L overall increase in
dissolved oxygen. ™»™ 1 and 3 mg/L reference lines.
2 4
DISSOLVED OXYGEN (ml/L)
Figure IV-9b. Cumulative frequency distributions of the observed
(—) and hypothetical ( — ) dissolved oxygen data in Figure lv-9a
showing the reduced occurrence of values at the low end of the
distribution.
restoration goal at this site. Two Gulf Coast estuaries,
one significantly more impacted by anthropogenic
effects than the other, have shown an analogous
pattern, where, with DO variability of the same order
of magnitude, the overall DO levels were higher in the
less impaired waterbody (Summers 1992).
As discussed earlier, the most reliable model
projections for DO are output as seasonal means for
all the model cells in each model segment. The model
scenario output can be linked directly to the DO
restoration goal using the same empirical relationship
developed from Monitoring Program data, between
seasonal means and the percent of observations
which achieve, or fail to achieve, the target
concentrations. Equations describing the relationship
have been determined for each target concentration
for each model segment (Appendix B).
21
-------
Evaluating the Time-Variable Model Scenario
Output
The probable percentage of observations achieving or
exceeding the target concentrations for a given model
scenario can be calculated from the projected model
cell mean using the specific equation developed for
the model segment to which the model cell belongs.
The minimum seasonal mean DO concentrations that
will assure achievement of each target concentration
in each model segment are given in Table IV-7.
Table IV-7 Minimum seasonal mean DO concentrations (mg/L)
required to achieve the DO restoration goal target concentrations
based upon 99% of observations > target concentrations. Values
with asterisks (*) are observed seasonal means for segments
where no observations were below the target concentration The 5
mg/L instantaneous target concentration for anadromous fish
spawning and nursery habitats applies only to model segments 1
and 2
Model
segment
1
2
3
4
5
6
7
8
9
1 mg/L1
5.1
6.6
6.6
6.4
6.0
4.0
5.2*
5.7*
4.5*
Target concentration
3 mg/L2 5 mg/L3 5 mg/L4
6.8
8.5
8.3
8.2
7.4
6.4
5.2*
6.1
6.2
8.5
9.4
8.0
9.2
9.2
8.9
8.8
7.4
6.4
6.6
7.0
1 instantaneous
2 instantaneous (target concentration permits DO &1 mg/L
and s3 mg/L for <12 hours, so the instantaneous values
are overestimates)
3 instantaneous - applies only above the pycnocline
(in anadromous fish spawning and nursery habitats)
4 monthly mean - applies only above the pycnocline
The 3 mg/L goal component permits excursions
below target concentration for periods up to 12 h, a
condition suggesting seasonal means somewhat lower
than those shown in Table IV-7. The seasonal means
in Table IV-7 are those that should assure DO
concentrations above 3 mg/L at all times. The season-
al mean required to meet the duration and frequency
components of this target concentration cannot be
determined from the analysis of twice-monthly
Monitoring Program data. Analysis of the semicon-
tinuous data, however, suggests that there is a relati-
onship between seasonal mean DO and the duration
as well as the frequency of low DO in particular
habitats (Appendix B). At present, however, there are
too few semicontinuous data sets from a wide
enough variety of habitats to firmly establish such a
relationship.
It is evident from Table IV-7 that there is a
"controlling" target concentration for a given layer of
the mainstem Bay segments, i.e. a seasonal mean that
once achieved, ensures achievement of all the relevant
target concentrations. In waters above the pycnocline,
the controlling target concentration is 5 mg/L in
segments where the anadromous fish spawning and
nursery habitat target applies, and the 5 mg/L
monthly mean elsewhere. Below the pycnocline, the
1 mg/L target concentration is assumed to be control-
ling, because we are not able to fully evaluate the
duration and frequency components of the 3 mg/L
target concentration at this time. Table IV-8 shows the
seasonal means and controlling target concentrations
required in each model segment to achieve the DO
restoration goal.
Table IV-8. Minimum seasonal mean DO concentration (mg/L)
required to achieve the DO restoration goal based upon 99% of
observations a target concentrations. Values with asterisks (*) are
observed seasonal means for segments where few or no
observations were below the target concentration.
Model segment
1
2
3
4
5
6
7
8
9
Below pycnocline
5.1
6.6
6.6
6.4
6.0
4.0
5.2*
5.7*
4.5*
Above pycnocline
8.5
9.4
9.2
8.9
8.8
7.4
6.4
6.6
7.0
Measuring Progress and Differentiating Between
Scenario Results
The restoration goal is to provide sufficient DO by
achieving, to the greatest spatial and temporal extent
possible, the goal's target concentrations. For evalu-
ating progress toward the goal and discriminating
between the benefits of different nutrient load
reduction scenarios, the percent achievement method
described above provides a relative measure of
improvement. But, a simpler means of comparing the
scenarios is desirable: one which can simply depict
both magnitude of difference and the geography of
difference baywide. Because physical processes can
inhibit full achievement of the goal, and because of
uncertainties in the analysis, an interpretive scheme
22
-------
to evaluate goal achievement and habitat suitability
was adopted.
Habitat is defined as "suitable" when the percentage
achievement is 90% or greater, "marginal" if it is
between 50 and 90%, and "unsuitable" if it achieves
the target less than 50% of the time. Suitable or
acceptable habitat provides satisfactory conditions for
survival, growth and reproduction of living resources
(although not necessarily fully supportive of living
resources' DO requirements). Marginal habitat
provides increased opportunities for benthic
colonization and forage feeding. Unsuitable habitat
is inhospitable to all but the most tolerant of living
resources.
For each scenario, individual model cells are evalu-
ated directly and grouped within these specific
ranges. The cells are aggregated and expressed as
volumes of water or areas of bottom habitat that meet
or exceed the DO target concentrations. Model seg-
ments can then be mapped to show where and how
much improvement or degradation is projected by
each scenario.
Similarly, model cells and segments within the
projected achievement category can be associated
with and reported for particular existing or potential
living resource habitats. The model output from the
various scenarios can then be judged in terms of the
specific gain or loss of critical living resource habitats.
Conclusion
The habitat requirements of representative
Chesapeake Bay living resources have been
synthesized to construct a DO restoration goal intend-
ed to assure the protection of most of the Bay's living
resources. A method has been developed for using
Chesapeake Bay Monitoring Program data to evaluate
current and model-simulated future conditions of the
Bay with respect to the DO restoration goal. With
these methods, Bay Program managers can assess and
map progress toward achievement of this goal in
Chesapeake Bay on a local, regional or baywide basis.
As a result of this analysis, DO concentrations
projected by the model for proposed nutrient load
reduction scenarios can be compared directly to the
DO restoration goal.
23
-------
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29
-------
APPENDIX A
EFFECTS OF LOW DISSOLVED OXYGEN
ON CHESAPEAKE BAY TARGET SPECIES
Tabulated from chapters in
Habitat Requirements for Chesapeake Bay Living Resources, 1991 Revised Edition,
and other sources
31
-------
32
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DO down to 1 .4 mg/L; whereas YOY exposed
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APPENDIX B
TECHNICAL ADDENDUM
Bl. Definition of Pycnocline
B2. Technical Description of Data Reduction and Analysis of Semicontinuous and Chesapeake Bay Program
(CBP) Monitoring Data
B3. Plots of Percent Above Target versus Seasonal Mean DO
a) for mainstem model segments
b) for mainstem CBP segments
B4. Coefficients and R2s of Regression Equation for calculating Percent Achievement
a) for mainstem model segments
b) for mainstem CBP segments
B5. Minimum Seasonal Mean Dissolved Oxygen Concentration required to Achieve the DO Restoration Goal.
a) for mainstem model segments
b) controlling concentrations for mainstem model segments
c) for mainstem CBP segments
d) controlling concentrations for mainstem CBP segments
B6. Steps for Determining Water Quality Status Relative to the Dissolved Oxygen
Restoration Goal
a) Calculating Percentage Achievement From Time-Variable Model Output
b) Calculating Percentage Achievement From Monitoring Program Data
51
-------
Bl. Definition of Pycnocline
In a partially mixed estuary like Chesapeake Bay,
water density is typically heterogeneous from surface
to bottom, with fresher, less dense water from the
tributaries overlying saltier, denser water from the
ocean. Temperature also has an effect on density.
Other things being equal, warm water is less dense
than cool water. Under certain physical and climatic
conditions, the water column can become stratified
into two or more layers of distinctly different densi-
ty. The region of the water column where the density
discontinuity occurs is called the pycnocline.
Certain components of the dissolved oxygen restora-
tion goal are applied only above-pycnocline:
"c) monthly mean dissolved oxygen
concentration of at least 5.0 mg/L
throughout the above-pycnocline
waters of Chesapeake Bay and its
tidal tributaries; and
d) dissolved oxygen concentration of
at least 5.0 mg/L at all times
throughout the above-pycnocline
waters of anadromous fish spawning
reaches, spawning rivers and nurs-
ery areas of Chesapeake Bay and its
tidal tributaries...".
For the purpose of evaluating time-variable model
output or Monitoring Program data with respect to
these components, the regions of the water column
relative to the pycnocline are defined as follows.
Chesapeake Bay Time-Variable Model Output
In the model, the water column is divided into three
density regions: 0 to 6.7 m, 6.7 to 12.7 m, and greater
than 12.7 m (CBP 1992). The middle portion (6.7 to
12.7) is the region of the pycnocline. For purposes of
evaluating model output relative to the DO restora-
tion goal, the above-pycnocline region is defined by
the upper boundary of the pycnocline and includes
model cells in layers 1 through 4. The below-pycno-
cline region includes model cells in layers 5 through
14.
Chesapeake Bay Monitoring Program Data
When using Monitoring Program data, if the dis-
solved oxygen measurement is made at a time and
place where at least one pycnocline exists, then the
actual depth of the uppermost pycnocline marks the
boundary between above-and below-pycnocline
waters. Where no pycnocline exists, the boundary is
arbitrarily set at 6.7 m, to be consistent with the
model.
The current Chesapeake Bay Monitoring Program
shipboard protocol for determining the pycnocline,
using specific conductivity as the substitute measure
of density, is as follows:
A computed threshold value is cal-
culated from two times the mean
change in conductivity per meter
between the surface and bottom. A
pycnocline exists if the threshold
value is greater than 500 micromhos-
/cm per meter. The upper and low-
er boundaries of the pycnocline are
the first depth interval from the
surface and the first from the bottom
with a change in conductivity that
exceeds the threshold value.
52
-------
52. Data Reduction and Analysis
Background
The DO restoration goal specifies target DO
concentrations, some of which are applicable over
the entire water column and some which are
applicable only above-pycnocline. The goal also
specifies the duration and frequency of reoccurrence
of DO below target concentrations. Monitoring data
and model projections do not have sufficient
temporal resolution to evaluate conformance to these
limits. Therefore, relationships had to be defined
among the DO target concentrations, the real-time
semicontinuous dissolved oxygen fields, the twice-
monthly Chesapeake Bay Monitoring Program data,
and the seasonal means projected by the time-
variable model.
Using Semicontinuous Data to Characterize
Dissolved Oxygen in the Environment
In-situ recording devices allow the collection of close-
interval (semicontinuous) DO measurements at a
single point over relatively long periods.
Semicontinuous data from a number of sources were
used to provide examples of real-time variability in
DO in several different estuarine environments. Data
demonstrating summer conditions in the York River
(provided by Dr. Robert Diaz, Virginia Institute of
Marine Sciences) were collected at 20-minute
intervals from June 21 to October 15, 1989. A subset
of the complete record was used in the time-series
plot (Figure IV-la) covering July and August only.
Data from the mainstem Bay near the mouth of the
Choptank River (provided by Dr. Lawrence Sanford,
University of Maryland) were collected at 5-minute
intervals from August 12 to September 9, 1987. The
entire period was used in the time-series plot
characterizing the general environment (Figure IV-
Ib). Data from St. Leonard Creek (provided by Dr.
Robert Summers, Maryland Dept. of the
Environment) were collected between April 29 and
October 7, 1988. Measurements were taken at 30-
minute intervals. The data used in the time-series
plot (Figure IV-lc) were from July and August 1988
only.
Comparing Semicontinuous and Monitoring
Program Data
In the Chesapeake Bay Monitoring Program, DO
measurements are made vertically and horizontally
at many points in the mainstem Bay. During the
summer, when sampling is most frequent,
measurements are made twice a month. To evaluate
the applicability of Monitoring Program DO data in
assessing status and progress with respect to the DO
restoration goal, the multi-location, low frequency
Monitoring Program data were compared with
single-location, high frequency semicontinuous data.
The semicontinuous monitoring site near the mouth
of the Choptank River mentioned above was one of
four sites in the middle region of the Bay that were
monitored simultaneously over the four-week period.
At each location, a sensor was fixed approximately
1 meter off the bottom. Dissolved oxygen was
measured at three sites at 6, 13, and 19 m,
respectively. At the fourth site, one sensor measured
DO near the bottom at 9 m and one near mid-depth,
at 6 m. These sites were in the vicinity of several
monitoring stations in Chesapeake Bay Program
segment CB4 (Figure IV-5) which were visited on
two monitoring cruises during the deployment of the
sensors. Estimates of dissolved oxygen concentration
are commonly made for a station or segment by
averaging the values of the two cruises within a
calendar month and reported as "monthly" averages.
Because of slight differences in the dates and times
of deployment, the data from each of the sensors
were equalized by using only data collected from
August 13 through September 6, 1987, exactly 25
days. The time interval between measurements also
differed among sensors: three sensors measured at
intervals of 5 minutes, one at 12 minutes, and one at
15 minutes. To adjust for this difference, only every
third measurement in the 5 minute-interval records
was included, and in the 12 minute-interval record,
every 5th measurement was dropped. The 15 minute-
interval record was not adjusted.
53
-------
The Monitoring Program stations were sampled 15
days apart within the period of the sensor
deployment, on August 17-18 and on September 1-2,
1987. The length of time between the two cruises was
typical for the summer sampling schedule. The
Monitoring Program stations included in the
comparison were the stations in segment CB4:
stations CB3.3C, CB3.3E, CB3.3W (which are, indeed,
in segment CB4), CB4.1C, CB4.1E, CB4.1W, CB4.2C,
CB4.2E, CB4.2W, CB4.3C, CB4.3E, CB4.3W, and
CB4.4.
To display the data from the two sources together
(Figure IV-6), all DO profile data at all thirteen
stations on both cruise dates were pooled and
plotted as depth versus concentration. The mean,
standard deviation, minimum and maximum of each
of the five adjusted semicontinuous DO data sets
were superimposed on the plot at the appropriate
depth.
To compare means and other summary statistics
(Table IV-4), DO data from between 5 and 7 m at the
thirteen Monitoring Program stations were pooled
for comparison with the 6-m semicontinuous data,
data between 8 and 10m were compared with the 9-
m semicontinuous data, data between 12 and 14 m
were compared with the 13-m semicontinuous data,
and data between 18 and 20 m were compared with
the 19-m semicontinuous data. The data were
rounded to the nearest 0.1 mg/L.
Cumulative percent distributions of these depth-
specific groups (Figures IV-7a,b,c,d,e) were obtained
by adapting the SAS "PROC FREQ" procedure. This
SAS computer programming function provides the
percentage of observations at each value for any
variable in a dataset.
Using Monitoring Program Data To Determine a
Relationship Between Seasonal Mean DO and the
Percent of Observations Above Goal
An analysis of the Monitoring Program data was
performed to determine the relationship between
seasonal mean DO and the percentage of observations
(and, by extension, the percentage of time) above
target concentrations. The analysis was performed on
dissolved oxygen data collected between June 1984
through September 1990 in the Chesapeake Bay
Monitoring Program. Samples are routinely collected
at a Baywide network of stations, twice a month in
spring and summer, once a month in fall emd winter.
Described simplistically, the sampling protocol for
profiling dissolved oxygen at Monitoring Program
stations is to measure DO at the surface and at one-
meter intervals to the bottom. Water temperature,
salinity, and conductivity are also measured
concurrently. This protocol, however, has been
executed slightly differently between data collection
institutions and over time. For example, if DO doesn't
change with depth, measurements may not be
recorded until a depth is reached where a change in
DO is detected. Also, early in the Monitoring
Program, some institutions made measurements at
two-m depth intervals, others at 1-m. Another
problem, although much rarer, is the inequality
caused by missing stations or cruises or blocks of
missing data due to faulty instruments.
To equalize station profiles, missing values were
estimated where possible. For each station profile,
sample depths greater than 0.5 m were rounded to
the nearest meter. Infrequently, this resulted in more
than one measurement per depth. In that case, the
average concentration was used at that depth. For
any meter interval missing a value in this skeleton
DO profile, DO concentration was linearly
interpolated from adjacent values above and below
the depth of the missing value.
Prior to rounding the depths to whole meters, each
depth was assigned a layer code to indicate whether
it was above (A), in the region of (P), or below (B)
the pycnocline. The presence or absence of a
pycnocline was checked, and if one or more existed,
the depth of the upper pycnocline was used. Depths
between the surface and the pycnocline were
assigned to the A-layer, depths below the actual
pycnocline down to 12.7 m were assigned to the P-
layer, and depths below 12.7 meters were assigned to
54
-------
the B-layer. If a pycnocline was absent, the
pycnocline was arbitrarily set at 6.7 m. The A-layer
was then 0-6.7 m, the P-layer was 6.8 m to 12.7, and
the B-layer was any depth greater than 12.7 m. At
those stations whose total depth was near a
boundary depth, i.e., stations about 7-8 or 13-14 m
deep, the few observations falling in the lower layer
were assigned to the layer above. The data were then
grouped by layer.
An alternative choice to grouping within depth layer
would have been to determine the relationship by
calculating the seasonal mean DO and the percentage
of observations above target for each sampling point
in the Monitoring Program, then pooling these. But,
because the maximum number of observations at any
point was so small (2 cruises/month x 4 months = 8
observations), the percentage of observations above
or below a target concentration would only be eight
possible values and very sensitive to missing values.
Therefore, it was decided to group data within depth
layers.
Because water quality varies in different areas of the
Bay, it was expected that the relationship between
seasonal mean DO and the percentage of observations
above goal would also vary over the Bay and, thus,
the relationship should be described separately for
each area. The data were therefore grouped spatially
according to two different schemes: one appropriate
to the spatial scheme of the time-variable model, and
the other conforming to the spatial aggregation units,
or "segments", usually used in analyzing or
evaluating Monitoring Program data.
The CBP segmentation scheme divides the Bay and
its tributaries into 45 separate areas (Figure IV-3).
Segments CB1 through CBS and segment EE3 are the
areas which most closely correspond to the areas of
the main Bay addressed in the current time-variable
model. The modelers use a different segmentation
scheme for aggregating model output: they also have
nine segments, segments #1 through #9, but with
somewhat different internal boundaries (Figure IV-4).
To determine the relationship for these areas, then,
each Monitoring Program data point was assigned its
appropriate CBP and model segment designation,
then grouped by segment (either CBP- or model-
defined segment) and by depth layer (above, below,
or in the region of the pycnocline) within segment.
The data were then further grouped within season
within year. The four seasons were defined as
follows: winter included January and February;
spring, March through May; summer, June through
September; and fall, October through December. If
more than one cruise was missing within a season,
the data from that layer/season/year/segment were
dropped from the analysis. Because segment CB1
contains only one station, approximately 6 m deep,
segments CB1 and CB2 were combined for the
analysis (indicated as segment CB1-2 in the plots).
To reduce the effect of supersaturated DO conditions
(often caused by undesirable excess phytoplankton),
DO measurements that were above the saturation
concentration were set down to the saturation
concentration. Saturation concentration was
calculated for each DO data point using the
concurrent temperature and salinity measurements
collected with each sample.
The seasonal mean was then calculated for each
valid layer/season/year/segment group, as was the
percentage of all observations within the group that
were above the DO target concentrations: 1 mg/L, 3
mg/L, 5 mg/L, monthly mean of 5 mg/L. In the fall
and winter seasons, dissolved oxygen was almost
always above target concentrations, and, as expected,
a strong relationship between seasonal mean and the
percentage of observations above or below target
was not evident. Fall and winter seasons were,
therefore, dropped from the analysis. Plots of the
percentages as a function of spring and summer
mean DO concentrations were made for each
segment for each target concentration (Appendix B3).
Regression analysis was used to obtain the equation
that would describe the relationship between the
percentage above target and the seasonal mean DO.
55
-------
As is commonly done for percentage data, an arcsine
transformation of the data was used in the analysis
(SAS "PROC REG"). A quadratic model obtained the
best fit, where
coefficients for time-variable model and CBP
segments are given in Appendix Tables B4.(a) and
(b), respectively.
r = the ratio of the number of
observations above target
concentration to the total number of
observations,
and
arcsine(square root
A*(seasonal mean)2 +
mean) + C.
of r) =
B*(seasonal
Because the objective was to describe this
relationship within the range of seasonal means at
which DO was problematic, i.e. when DO was less
than 100% above target concentrations, the data for
conditions supporting 100% achievement were
censored: of these "100 percent" data points, the
lowest seasonal mean in each season (spring and
summer) each year which had 100 percent of the
observations above target was included. The others
were omitted from the analysis. The regression
Using Semicontinuous Data to Explore Relation-
ships Between Seasonal Mean DO and Duration of
Low DO Events
An important component of the DO Restoration Goal
is the allowable duration of excursions below 3
mg/L. Excursions between 1 and 3 mg/L must be
under twelve hours and the interval between such
excursions should be at least 48 hours. The
sernicontinuous data records provide empirical
observations on the frequency and duration of low
DO events in particular habitats. For example, Figure
B2 shows the number and duration of events where
DO fell below 3 mg/L at St. Leonard Creek.
Improving conditions should lead to a reduction in
the number and duration of these events. We have
not yet sufficiently analyzed the return frequency
component of this target concentration. Further work
on measuring status and progress toward this goal
component is planned as additional semicontinuous
data from various habitats and water quality
conditions become avcdlable.
100
LU
LL
O
ce
in
m
10
WORSENING
CONDITION
IMPROVING
CONDITION
o
OO
0.1
10
EVENT DURATION, HOURS
100
1000
Figure B2. Number of events where DO fell below 3 mg/L at St. Leonard Creek, July and August, 1988.
56
-------
B3.(a). Figures B3-a1 through B3-a9 present plots of the percent of Monitoring Program observations above target
DO concentration (percent above target) versus annual seasonal mean DO concentration (seasonal mean), by
mainstem model segments, for the years 1984 through 1990. Observations are grouped by segment, depth layer,
season (spring and summer), and year. Letter symbols indicate depth layer of the data from which the seasonal
mean and percent of observations were calculated. A=above pycnocline, P=region of the pycnocline, and B=below
pycnocline. Target DO concentrations are 1, 3, and 5 mg/L (instantaneous), and 5 mg/L monthly mean. The 5 mg/L
target applies to anadromous fish spawning and nursery areas and therefore does not apply to model segments
3 through 9.
57
-------
mg/L
3 mg/L
100
90
80
P 70
50
Q)
fc
Q_
40
30
20
10
0
1 1 1 1——I 1—
0 2 4 6 8101214
Seasonal Mean
s
fc
Q_
100-
90
80
70
60
50
40
30
20
10
0
A0&F> A-»^
P P
P FR
P
F=>
P
Fp-
T
0 2 4 6 8 101214
Seasonal Mean
-4-*
0)
1_
O
h-
0)
o
c
0)
u
(D
Q_
5 mg/L monthly mean
100
90
80-
70
60
50
40
30
20
10
0
p
p
p
p
p
p
&
1 i t I i 1 1 l
0 2 4 6 8 10 12 14
5 mg/L
1
-t-i
(D
CT>
O
! —
Q)
O
_Q
-*-*
C
0)
(J
a.
00
90
80
70
60
50
40
30
20
10
0
>¥>. Altti
A*
p
pp
p
p
p
p
'1
Seasonal Mean
0 2 4 6 8 1012 14
Seasonal Mean
Figure B3-a1. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within model segment 1. Letter
symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
58
-------
1 mg/L
3 mg/L
1
"CD
en
D
t—
o
_Q
C
CD
O
u
Q_
00
90
80
70
60
50
40
30
20
10
0
/4WP •***
B
B
BB
B
P
P
B
m
B
1
-M
(D
CP
O
t-
0)
o
_Q
"c
CD
O
CD
0.
00
90
80
70
60
50
40
30
20
10
0
If P **
A
BP
^
B
B
1"
P
P
P
B
JF
0 2 4 6 8 10 12 14
Seasonal Mean
0 2 4 6 8 10 12 14
Seasonal Mean
5 mg/L monthly mean
5 mg/L
1
-4-*
CD
C7>
a
1—
CD
>
o
c
CD
o
CD
a.
00
90
80
70
60
50
40
30
20
10
0
rp-* f>i
\ r _A,_
^
A R3
P
B
^
B
B
Fp
P
P
«B
0 2 4 6 8 10 12 14
Seasonal Mean
1
CD
D1
D
h-
CD
O
.£)
C
CD
O
CD
a.
00
90
80
70
60
50
40
30
20
10
0
,6ft
P TOL
fc P
B^
PP
%
B
B
B
p
Fp.
FF3
P
P
B
tf3
0 2 4 6 8101214
Seasonal Mean
Figure B3-a2. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within model segment 2. Letter
symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
59
-------
1 mg/L
3 mg/L
100
90
80
^ *
P 70
L-
O
u 60
>
|50
c 40
Q)
£ 30
Q_
20
10
0
sapFfppj*
p
P B
P B
P
P B
P
B
B
B
%
B
100
90
80
-+-*
Q)
p 70
D
l__
>
5 50
1 40
o
fc 30
Q_
20
10
0
^PP^-
pp
m R
B
B
B
P
P
£
E$
0 2 4 6 8 10 12 14
Seasonal Mean
0 2 4 6 8 10 12. 14
Seasonal Mean
5 mg/L monthly mean
100i
90
80
ET 70
o
1 6°
I 50
"c 40
CD
/ \
30
D
a.
20
10
0
P
IF1
PP
P
P
0 2 4 6 8101214
Seasonal Mean
Figur* B3-a3. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within model segment 3. Letter
symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
60
-------
1 mg/L
3 mg/L
1
Ol
L_
O
1—
Q)
O
_Q
-+-*
C
o
-Q
~t_*
C
o
l_
(D
0.
100
90
80
70
60
50
40
30
20
10
0
to &*™^
B ^E..,
B
f?
P
(P5
P
B
B
B3
B
0 2 4 6 8 10 12 14
Seasonal Mean
0 2 4 6 8 10 12 14
Seasonal Mean
5 mg/L monthly mean
o
_o
"c
o
1_
Q.
100
90
80
70
60
50
40
30
20
10
0
jfc gp^aniAA
A p
A
P
B
B ^
P
P
P
H»
B
Ii'i i ' »-"<
2 4 6 8 10 12 14
Seasonal Mean
Figure B3-a4. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within model segment 4. Letter
symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
61
-------
1 mg/L
3 mg/L
-4— '
O1
1
o
Q)
O
_Q
<£
Percent ,
100-
90
80
70-
60
50
40
30
20
10
0
BJ»- ^OMNP,
P%
B
B&
B
1
-4— '
O
t-
1D
O
_Q
"^
C
ID
O
0)
Q_
00
90
80
70
60
50
40
30-
20
10
0
d**W*^
P B
P B
f53
p
B
B
B
B
B
0 2 4 6 8 10 12 14
Seasonal Mean
0 2 4 6 8 101214
Seasonal Mean
5 mg/L monthly mean
1
-t-J
0>
O
1—
-------
1 mg/L
3 mg/L
100-
90
80
§> 70
D
'" 60
jj 50
c 40
Q)
O
0) 30
Q.
20
10
0
0r
100
90
80
70
D
60
0)
o
-Q
0 50
c 40
-------
1 mg/L
ID
L»
a
h-
O
1—
0
-4-J
C
Q)
o
0)
a.
100
90
80
70
60
50
40
30
20
10
0
t*fM»tWH*
B
P
B"
B
B
0 2 4 6 8 10 12 14
Seasonal Mean
Figure B3-a7. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within model segment 7. Letter
symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
64
-------
1 mg/L
3 mg/L
1
-4-1
CD
o>
D
-------
1 mg/L
3 mg/L
1UO
90
80
-±->
CD
01 70
D
j> 50
c 40
CD
fe 30
CL
20
10
0
B
100
90
80
-4->
^70
o
| 5°'
•c 40
CD
fc 30
0.
20
10
0
tp>
pp
B
B
0 2 4 6 8 10 12 14
Seasonal Mean
0 2
I I I I I I
4 6 8 10 12 14
Seasonal Mean
5 mg/L monthly mean
1
0)
a>
L
D
1—
CD
O
J3
00
90
80
70
60
50
40
30
20
10
0
Aj£, J^HBI"
A B
fi1
CD
FP3
P
P
f
B
B9
B
0 2 4 6 8101214
Seasonal Mean
Figure B3-a9. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within model segment 9. Letter
symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
66
-------
B(3).b. Figures B3-b1 through B3-b8 present plots of the percent of Monitoring Program observations above target
DO concentration (percent above target) versus annual seasonal mean DO concentration (seasonal mean), by
mainstem CBP segments, for the years 1984 through 1990. Observations are grouped by segment, depth layer,
season (spring and summer), and year. Letter symbols indicate depth layer of the data from which the seasonal
mean and percent of observations were calculated. A=above pycnocline, P=region of the pycnocline, and B=below
pycnocline. Target DO concentrations are 1, 3, and 5 mg/L (instantaneous), and 5 mg/L monthly mean. The 5 mg/L
target applies to anadromous fish spawning and nursery areas and therefore does not apply to CBP segments CB4
through CBS, and EE3.
67
-------
1 mg/L
3 mg/L
100
90-
80
*-^
p 70
o
!~ 60
1 50
•c 40
CD
fc 30
rx
20
10
0
H-IHW*^ FWWMfc.
P
100
90
80
4—*
(D
p 70
D
0 60
5 50
c 40
0
g ^50
Q_
20
to
0
H-IWW* HW*W«.
P
1 1 1 i r 1
0 2 4 6 8 1012 14
Seasonal Mean
0 2 4 6 8 1012 14
Seasonal Mean
CD
C
CD
O
CD
Q_
5 mg/L monthly mean
1001
5 mg/L
90]
80
70
i_
a
^ 60
I 50
40
30
20
10
0
FR
0 2 4 6 8 10 12 14
Seasonal Mean
'
-4-J
-------
1 mg/L
3 mg/L
CD
0
'~
>
0
XI
-*-J
C
CD
o
l_
Q_
100
90
80
70
60
50
40
30
20
10
0
/yjeet? 4fe
F^
BB
m s
p
B
H
B
1
CD
i_
O
1
CD
O
-Q
C
CD
o
CD
Q_
00
90
80
70
60
50
40
30
20
10
0
*pp 4*
p
B
R
B
P B
•P»
P
P
B
g
S
0 2 4 6 8 10 12 14
Seasonal Mean
0 2 4 6 8 10 12 14
Seasonal Mean
5 mg/L monthly mean
5 mg/L
1
4— '
CD
Oi
a
i —
CD
^>
0
_Q
"c
CD
O
CD
Q_
00
90
80
70
60
50
40
30
20
10
0
•440k
B
^p"
P
BPR
D
B
B
pP
P°
(P>
P
KE3
0 2 4 6 8 10 12 14
Seasonal Mean
CD
O
CD
Q_
100
90
80
70-
60
50
40
30
20
10
0
^
p
^B
PP
P
B
B
B
P
P
*
IWB
0 2 4 6 8 10 12 14
Seasonal Mean
Figure B3-b2. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within CBP segment CBS.
Letter symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated, A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
69
-------
1 mg/L
3 mg/L
(D
L_
a
l—
CD
o
_Q
c
0)
o
Q_
100
90
80
70
60
50
40
30-
20
10
0
t*W="PF*»*
B
B
P
R
B
P
P
£3
B
B
I
B
1
(D
Oi
D
>
O
_Q
-4_J
C
(D
o
Q_
00
90
80
70
60
50
40
30
20
10
0
^E%£P xm
<<^P
Pp
p
%
B
B
P
R
P
f
B
B
#
0 2 4 6 8101214
Seasonal Mean
0 2 4 6 8 10 12 14
Seasonal Mean
5 mg/L monthly mean
a>
CD
o
*
0)
>
o
C
-------
1 mg/L
3 mg/L
1
0
01
O
CD
0
o
1
CD
O
C
CD
CJ
CD
Q_
00
90
80
70
60
50
40
30
20
10
0
^BBtpKk
B
eP
B
P B
B
P
Ffc
P
B
B
B
B
B
0 2 4 6 8 101214
Seasonal Mean
0 2 4 6 8 10 12 14
Seasonal Mean
5 mg/L monthly mean
1
t-j
CD
01
0
I—
CD
b
J3
"c
CD
O
CD
CL
00
90
80
70
60
50
40
30
20
10
0
£ BF**V
B
B
B
B B
P
f?
P
P
B
0 2 4 6 8 10 12 14
Seasonal Mean
Figure B3-b4. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within GBP segment CBS.
Letter symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
71
-------
1 mg/L
3 mg/L
100
90
80
CD
o> 70
o
« 60
CD
>
S 50
c 40
CD
«S 30
a.
20
10
0
BBCTR jmnrmaMv
Rp?3
B
B
100
90
80
0
? 70
D
« 60
)>
5 50
c 40
Q)
u
CD" 30
a.
20
10
0
B JH'I) ^ W^
R
re=
^
R
9
^
B>
C3
^3
E3
0 2 4 6 8 10 !2 14
Seasonal Mean
0 2 4 6 8 10 12 14
Seasonal Mean
5 mg/L monthly mean
1
-+_ '
CD
CT>
O
I—
CD
b
-W
C
CD
o
Q_
00
90
80
70
60
50
40
30
20
10
0
lyt^^MOR.
&• R
PP
P
R
R
^
R
B
R
B3EB
0
2 4 6 8 10 12 14
Seasonal Mean
Figure B3-b5. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within CBP segment CB6.
Letter symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
72
-------
1 mg/L
3 mg/L
too
90
80
4— •
O
_Q
-*-^
C
o
Q-
100
90
80
70
60
50
40
30
20
to-
0
^^^A
P
cP
?.
P
e
^
B
0 2 4 6 8 10 12 14
Seasonal Mean
Figure B3-b6. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within CBP segment CB7.
Letter symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
73
-------
1 mg/L
3 mg/L
(D
55
a.
90]
80
70]
60
50
40
30
20
10
0
1
-4—1
(D
CP
O
1—
(D
~!>
o
_Q
<•
^^
-i-j
C
(D
o
i_
CD
Q.
00
90
80
70
60
50
40
30
20
10
0
ESVBK@>SW>
0
B
0 2 4 6 8 101214
Seasonal Mean
0 2 4 6 8 10 12 14
Seasonal Mean
5 mg/L monthly mean
1
+-j
0)
CT>
l_
O
0
^>
O
J3
<£
t~>
C.
CD
O
L_
B
0 2 4 6 8101214
Seasonal Mean
Figure B3-b7. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within CBP segment CBS.
Letter symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
74
-------
1 mg/L
3 mg/L
100
90
80
4 — '
CD
g. 70
o
^ 60
5 50
c 40
CD
fc 30
Q_
20
10
0
milHilKV EEBBX
. . — — . . — — — i 1 ' — • t — • • i
100
90
80
(D
o. 70
o
« 60
.£ 50
c 40
CD
£ 30
Q_
20
10
0
FFH^ HtfflBX
^
13 P
B
S3
0 2 4 6 8101214
Seasonal Mean
0 2 4 6 8 10 12 14
Seasonal Mean
5 mg/L monthly mean
1
-4-J
O>
O
o
.Q
~C
a)
o
CD
a.
00
90
80-
70
60
50
40
30
20
10
0
&£• tp*^
I=A B
P
P
P
E&
B
B
B
B
0 2 4 6 8 10 12 14
Seasonal Mean
Figure B3-b8. Percent of observations above target concentration versus seasonal mean DO concentration (mg/L) within CBP segment EE3.
Letter symbols indicate depth layer of the data from which the seasonal mean and percent of observations were calculated. A=above pycnocline,
P=region of the pycnocline, and B=below pycnocline.
75
-------
B4.(a). Regression coefficients and R-square values for the equation predicting the arcsine(square root of the ratio
of the number of observations above the indicated target concentration to the total number of observations) as a
function of seasonal mean DO within each mainstem model segment. The equation used in the regression analysis
is:
arcsine[sqrt(ratio)] = A*(seasonal mean)2 + B*(seasonal mean) + C.
A. 1 mg/L target concentration
Model Segment
1
2
3
4
5
6
7
8
9
A
-0.0336
-0.0156
-0.0143
-0.0150
-0.0148
0.0023
0.0000
0.0000
0.0085
B
0.4670
0.2797
0.2661
0.2613
0.2520
-0.0037
0.0000
0.0000
-0.1086
C
-0.0322
0.3024
0.3366
0.4114
0.4954
1 .4497
1 .5708
1 .5708
1 .8650
R-Square
0.8854
o.gsig
0.9324
0.9070
0.8820
0.3518
.
6.1262
B. 3 mg/L target concentration
Model Segment
1
2
3
4
5
6
7
8
9
C. 5 mg/L monthly
Model Segment
1
2
3
4
5
6
7
8
9
A
-0.0250
-0.0130
-0.0130
-0.0155
-0.0146
-0.0251
0.0000
-0.0495
-0.0444
mean concentration
A
-0.02gg
-0.0068
-0.0070
-0.0091
-0.0187
-0.0524
-0.1111
-0.1220
-0.0678
B
0.4360
0.2819
0.2904
0.3153
0.3121
0.4082
0.0000
0.7801
0.7090
B
0.5746
0.2420
0.2436
0.2791
0.4147
0.9096
1.7381
1 .9405
1.1203
C
-0.3392
0.0128
-0.0473
-0.0718
-0.0451
-0.1103
1 .5708
-1 .4466
-1 .2260
C
-1.2151
-0.1802
-0.1779
-0.2892
-0.7257
-2.3889
-5.1132
-6.0291
-3.0530
R-Square
0.7041
0.9477
0.9563
0.9428
0.9442
0.7821
t
0.5729
0.8601
R-Square
0.7266
O.g700
0.9564
0.9322
0.9104
0.9720
0.8182
0.7472
0.9071
D. 5 mg/L target concentration for anadromous fish
spawning and nursery areas.
Model Segment
B
R-Square
1
2
-0.0221
-0.0040
0.4572
0.2012
-0.8170
-0.0690
0.8128
0.9725
76
-------
B4.(b). Regression coefficients and R-square values for the equation predicting the arcsine(square root of the ratio
of the number of observations above the indicated target concentration to the total number of observations) as a
function of seasonal mean DO within each mainstem GBP segment. The equation used in the regression analysis
arcsine[sqrt(ratio)] = A*(seasonal mean)2 + B*(seasonal mean) + C.
A. 1 mg/L target concentration
CBP Segment A
CB1 -0.0267
CB2 -0.0267
CB3 -0.0164
CB4 -0.0148
CBS -0.0136
CB6 0.0054
CB7 0.0088
CBS 0.0000
EE3 0.0000
B
0.4185
0.4185
0.2864
0.2682
0.2463
-0.0237
-0.1093
0.0000
0.0000
C
-0.0032
-0.0032
0.3216
0.3367
0.4455
1 .4398
1 .8599
1.5708
1.5708
R-Square
0.4958
0.4958
0.9425
0.9418
0.9255
0.1915
0.1056
B. 3 mg/L target concentration
CBP Segment A
CB1 -0.0267
CB2 -0.0267
CBS -0.0124
CB4 -0.0126
CBS -0.0150
CB6 -0.0527
CB7 -0.0362
CBS -0.0460
EE3 -0.0567
B
0.4185
0.4185
0.2797
0.2831
0.3068
0.7459
0.5790
0.7254
0.8950
C
-0.0032
-0.0032
0.0179
-0.0131
-0.0280
-1 .0806
-0.7240
-1 .2357
-1 .9027
R-Square
0.4958
0.4958
0.9443
0.9596
0.9284
0.8094
0.8795
0.5625
0.7004
C. 5 mg/L monthly mean concentration
CBP Segment A
CB1 -0.0419
CB2 -0.0419
CB3 -0.0056
CB4 -0.0072
CBS -0.0142
CB6 -0.0357
CB7 -0.0591
CBS -0.1075
EE3 -0.0813
D. 5 mg/L target concentration for
spawning and nursery areas.
CBP Segment A
CB1 -0.0434
CB2 -0.0434
CBS -0.0041
B
0.7127
0.7127
0.2367
0.2490
0.3432
0.6865
1.0013
1.7198
1 .3241
anadromous fish
B
0.7533
0.7533
0.2112
C
-1 .4039
-1 .4039
-0.2001
-0.1964
-0.4664
-1.6835
-2.6720
-5.2148
-3.7892
C
-1.6714
-1.6714
-0.1336
R-Square
0.5003
0.5003
0.9646
0.9622
0.9137
0.8888
0.9506
0.7551
0.8234
R-Square
0.6421
0.6421
0.9709
77
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B5.(a). Minimum seasonal mean DO concentration (mg/L), by model segment, required to achieve the DO restoration
goal, i.e. 99% of observations will equal or exceed the specified target concentration. (-) not applicable.
Target Concentration
Model Segment
1
2
3
4
5
6
7
8
9
1 mg/La
5.1
6.6
6.6
6.4
6.0
4.0e
5.2e
5.7*
4.5e
3mg/Lb
6.8
8.5
8.3
8.2
7.4
6.4
5.2
6.1
6.2
5 mg/Lc 5 mg/Ld
8.5 8.0
9.4 9.2
9.2
8.9
8.8
7.4
6.4
6.6
7.0
a Applied at all times to all depths.
b Applied at all times to all depths. The seasonal mean DO concentrations shown do not take into account the
duration and return frequency of excursions between 1 and 3 mg/L allowed under this goal component. The
seasonal mean required to attain the formal goal component would be lower than the concentrations shown here.
0 Applied at all times above the pycnocline in anadromous fish spawning and nursery habitats.
d Applied above the pycnocline (monthly mean).
e Dissolved oxygen never, or rarely, went below the target concentration in this segment. The seasonal mean shown
is the lowest seasonal mean recorded in any depth category with 100% of the observations above the target
concentration.
B5.(b). Minimum seasonal mean DO concentration (mg/L), by model segment, required to achieve the DO restoration
goal, i.e. 99% of observations are above the applicable target concentrations.
Model
Segment
1
2
3
4
5
6
7
8
9
Below
Pycnocline
5.1"
6.6"
6.6s
6.4a
6.0s
4.0a'd
5.2*d
s.rd
4.5*d
Above
Pycnocline
8.5b
9.4b
9.2°
8.9C
8.8°
7.4C
6.4C
6.6C
7.0C
a Controlling target concentration is 1 mg/L
"Controlling target concentration is 5 mg/L
c Controlling target concentration is 5 mg/L monthly mean
d Dissolved oxygen never, or rarely, went below the target concentration in this segment. The seasonal mean
shown is the lowest seasonal mean recorded in any depth category with 100% of the observations above the target
concentration.
78
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B5.(c). Minimum seasonal mean DO concentration (mg/L), by CBP segment, required to achieve the DO restoration
goal, i.e. 99% of observations will equal or exceed the specified target concentration. (-) not applicable.
Target Concentration
CBP segment
CB1
CB2
CBS
CB4
CBS
CB6
CB7
CBS
EE3
1 mg/La
5.3
5.3
6.3
6.7
6.5
3.3e
4.4e
5.6e
4.9e
3mg/Lb
5.3
5.3
8.1
8.4
8.1
5.8
6.2
6.0
6.2
5mg/Lc
7.0
7.0
9.2
-
-
-
-
-
~
5 mg/Ld
6.6
6.6
8.9
9.1
9.0
7.6
7.2
6.7
6.9
a Applied at all times to all depths.
b Applied at all times to all depths. The seasonal mean DO concentrations shown do not take into account the
duration and return frequency of excursions between 1 and 3 mg/L allowed under this goal component. The
seasonal mean required to attain the goal component as actually defined would be lower than the concentrations
shown here.
c Applied at all times above the pycnocline in anadromous fish spawning and nursery habitats.
d Applied above the pycnocline (monthly mean).
eDissolved oxygen never, or rarely, went below the target concentration in this segment. The seasonal mean shown
is the lowest seasonal mean recorded in any depth category with 100% of the observations above the target
concentration.
B5.(d). Minimum seasonal mean DO concentration (mg/L), by CBP segment, required to achieve the DO restoration
goal, i.e. 99% of observations are above the applicable target concentrations.
CBP Segment
CB1
CB2
CBS
CB4
CBS
CB6
CB7
CBS
EE3
Below Pycnocline
5.3s
5.3"
6.3"
6.7s
6.5a
3.3a'd
4-4a,d
5.6"'d
4.9a'd
Above Pycnocline
7.0b
7.0b
9.2b
9.1C
9.0C
7.6°
7.2C
6.7°
6.9°
a Controlling target concentration is 1 mg/L
b Controlling target concentration is 5 mg/L
0 Controlling target concentration is 5 mg/L monthly mean
d Dissolved oxygen never, or rarely, went below the target concentration in this segment. The seasonal mean shown
is the lowest seasonal mean recorded in any depth category with 100% of the observations above the target
concentration.
79
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B6.(a). Steps for Determining the Status of Water Quality
Relative to the Dissolved Oxygen Restoration Goal
from Time-Variable Model Output
To calculate Percentage Achievement:
1. Obtain the estimate of the seasonal mean dissolved
oxygen concentration for the particular model cell of
interest. Model results are seasonal mean DO
concentrations. Seasonal mean concentrations are
estimated for each of several thousand model cells.
For this application, the four seasons are defined as
follows: winter includes January and February;
spring, March through May; summer, June through
September; and fall, October through December.
Example: summer mean DO concentration =
5.5 mg/L
2. Identify the model segment and depth layer to
which the model cell belongs. For purposes of the
time-variable model, the Bay is divided, in planar
view, into nine segments (Segments 1-9, Figure IV-4).
Model estimates for each cell are related to the
location of the cell above, at, or below arbitrary
depth boundaries. Surface to 6.7 m is defined as the
region above the pycnocline, 6.8 m to 12.7 m is the
region of the pycnocline, and greater than 12.7 m is
the region below the pycnocline. For this application,
model cells above 6.7 m (cell layers 1 through 4) are
considered above pycnocline and model cells below
6.7 m (cell layers 5 through 14) are considered below
pycnocline.
Example: model segment #2, below pycnocline
3. Identify the controlling goal component relevant to
the model segment and depth layer [Table IV-7 or
Appendix Table B5.(b)]. Note that the 3 mg/L target
concentration cannot be applied in this context.
Example: the 1 mg/L target concentration is
controlling for below pycnocline cells in model
segment 2. The minimum seasonal mean DO
required for 99% achievement is 6.6 mg/L.
4. Compute the value for T in equation (a) below.
(a) T = A*(conc)2 + B*(conc) + C, where
T = predicted arcsine transformation of the square
root of the ratio of the number of observations above
target concentration to the total number of
observations;
cone = the mean dissolved oxygen
concentration for the cell obtained in step 1,
above; and,
A, B, and C are the regression coefficients
specific to the relevant goal component
determined in step 3, above, and found in
Appendix Table B4.(a).
Example: T = (-0.0156 x 5.5Z) + (0.2797 x 5.5) +
0.3024 = 1.3689
5. Compute the Percentage Achievement (percent
above target) using equation (b) below, where the
percent above target is obtained by back
transforming the result of step 4; i.e, by taking
sine(T), squaring the result, then multiplying by 100.
(b) Percent Achievement = [sine(T)]2 x 100.
Example: Pet = [sine(1.3689)f x 100 = (0.9797)2
x 100 = 96% (rounded to nearest percent)
80
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B6.(b). Steps for Determining the Status of Water Quality
Relative to the Dissolved Oxygen Restoration Goal
from Chesapeake Bay Monitoring Program Data
To calculate Percentage Achievement:
1. Obtain the estimate of the seasonal mean dissolved
oxygen (DO) concentration at the location of interest.
The four seasons are defined as follows: winter
includes January and February; spring, March
through May; summer, June through September; and
fall, October through December. For this application,
the seasonal mean is calculated separately for above-
and below-pycnocline strata for each Chesapeake Bay
Program segment (Figure IV-3).
In the Bay Monitoring Program, DO is measured at a
Baywide network of stations, twice a month in
spring and summer, and once a month in fall and
winter. Dissolved oxygen is measured at one- to two-
meter intervals from surface to bottom at each
station, and corollary data for characterizing water
column stratification are collected simultaneously.
From these data, the depth of a pycnocline, if any
exists, can be determined. The shipboard protocol for
determining pycnocline is described in Appendix Bl.
2. The seasonal mean is the average of all
observations within the segment/depth stratum
within the season (winter, spring, summer, or fall).
Identify the CBP segment and depth stratum of the
location of interest, and identify data for calculating
the mean. Measurements are included in the
calculation depending on the actual depth of the
pycnocline, if one was present. If multiple
pycnoclines existed, use the depth of the uppermost
pycnocline. Otherwise, use 6.7 m as the arbitrary
boundary between above- and below-pycnocline
strata.
Example: segment CB4, below pycnocline,
summer mean DO concentration = 5.5 mg/L
3. Identify the controlling goal component relevant to
the CBP segment and depth stratum [Appendix
Table B5.(d)]. Note that the 3 mg/L target
concentration cannot be applied in this context.
Example: the 1 mg/L target concentration is
controlling for below-pycnocline areas in segment
CB4. Minimum seasonal mean DO concentration
required for 99% achievement is 6.7 mg/L
4. Compute the value for T in equation (a) below.
(a) T = A*(conc)2 + B*(conc) + C, where
T = predicted arcsine transformation of the
square root of the ratio of the number of
observations above target concentration to the
total number of observations;
cone = the mean dissolved oxygen
concentration for the segment obtained in
step 1, above; and,
A, B, and C are the regression coefficients specific to
the relevant goal component determined in step 3,
above, and found in Appendix Table B4.(b).
Example: T = (-0.0148 x 5.52) + (0.2682 x 5.5) +
0.3367 = 1.3641
5. Compute the Percentage Achievement (percent
above target) using equation (b) below, where the
percent above target is obtained by back
transforming the result of step 4; i.e, by taking
sine(T), squaring the result, then multiplying by 100.
(b) Percent Achievement = [sine(T)]2*100.
Example: Pet = [sine(1.3641)f x 100 = (0.9787)2
x 100 = 96% (rounded to nearest percent).
81
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FOR ADDITIONAL COPIES OF THIS REPORT, CONTACT
CHESAPEAKE BAY PROGRAM OFFICE
410 SEVERN AVENUE, SUITE 109
ANNAPOLIS, MARYLAND 21403
410-267-0061
O
PRINTED ON RECYCLED PAPER
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