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The success or failure of efforts to restore the Bay will first be evident
in the chemical and physical data from this monitoring program. Indicators
such as nutrient and dissolved oxygen concentrations will signal important
changes in the Bay. The response of other biological indicators and living
resources will require more time as the perturbed food web readjusts and
pollutants stored in the system are dissipated. This rapid and
interpretable response in chemical and physical indicators is vital to the
success of management strategies. These strategies must be evaluated as
quickly as possible to insure that they are properly targeted and
cost-effective.
DESIGN CONSIDERATIONS
Because chemical and physical measures provide the most fundamental and
interpretable indicators of water quality, they were given the broadest
spatial and temporal coverage within the overall program. The chemical and
physical indicators also provide a baseline of information for the
interpretation and extrapolation of biological and process measurements
made at a subset of stations.
In order to characterize water quality in the Chesapeake Bay system,
virtually every significant tributary to the Bay as well as the mainstem
has been included in the sampling design (see
Map 1). All stations are
located in tidal waters with salinity regimes ranging from completely
freshwater in the upper tributaries and mainstem to over 20 parts per
thousand (60% seawater) in bottom waters of the mainstem near the mouth of
the Potomac.
In smaller tributaries, where only one site was sampled, stations were
located near the upper extent of the main channel regions. These areas are
generally representative of local water quality conditions in each of the
basins and avoid the often overwhelming influence of mainstem waters near
river mouths.
In larger tributaries and the mainstem, multiple stations were sited along
their length to characterize the strong spatial gradients that usually
occur from upper, freshwater reaches, to lower, high salinity estuarine
reaches. In the Potomac and Patuxent Rivers, two of the largest tributaries
in Maryland, ongoing intensive sampling networks were incorporated into the
present program design. In the deep-trough region of the mainstem, where
low oxygen problems are most severe, four lateral transects were
established to characterize west to east differences in water quality.
In the siting of station locations within representative regions, other
considerations such as important habitats (for example striped bass
spawning reaches), historical records and local influences such as sewage
treatment plants, basin morphometry, and hydrodynamics, were incorporated
into station siting decisions.
At each station, measurements of salinity, dissolved oxygen, temperature,
pH and water transparency, are made. Water samples are collected at one of
two depths in both surface and bottom waters to provide representative
samples from the water column. These water samples are then chemically
analyzed for nitrogen, phosphorus, carbon and silica constituents, total
suspended solids and chlorophyll, which is a measure of phytoplankton
biomass.
The frequency of water column sampling is up to 20 times per year - 2 times
per month from March through October and once per month from November
through February. In smaller tributaries, the frequency of sampling is
intended to provide reliable estimates of seasonal changes in water quality
and provide a measure of variability within seasonal time frames.
Monitoring for toxicants is focused primarily on the analysis of sediments.
This strategy is followed because most toxicants that are released into the
water become associated with particulate material, settle and thus become
concentrated in bottom sediments. Higher concentration of toxicants in
sediments aid in the detection and quantification of these compounds.
Toxicants deposited in the sediments also provide an integration of
exposure over time within a region.
Sediment samples are collected once-yearly at most of the water quality
stations for the analysis of organic compounds and heavy metals. These
compounds represent likely contaminant inputs from point and nonpoint
sources such as industries, croplands and sewage treatment facilities.
Associate measurements such as sediment grain size and organic matter
content are made to aid in the interpretation of toxicant concentrations.
In addition, at selected sites, the same compounds measured in sediments
are also measured in the tissues of benthic organisms that are abundant in
these locations. These tissue levels provide an assessment of contaminant
levels entering the food chain and the relationships between contaminants
in the sediments and in organisms living on or in those sediments.
RESULTS
Physical Characteristics
During the initial two years of the program, the Bay exhibited very
different physical characteristics resulting from large differences in
freshwater flows. These contrasting years provide an indication of how much
natural variability we might expect to encounter on a year-to-year basis.
In 1984, most basins draining into Chesapeake Bay experienced flows 30% to
100% above normal during the summer while flows in the summer of 1985 were
20% to 50% below normal. These differences in freshwater flow produced
noticeable differences in the physical structure of Bay waters. For
example, in the deep waters of the mainstem, high flows in the summer of
1984 reduced surface and bottom salinities by approximately 6 parts per
thousand (ppt) and 3 ppt, respectively, relative to 1985 (Figure
6, A and B). This resulted in a larger salinity difference from
surface to bottom in 1984. These surface to bottom changes in salinity
restrict the mixing of surface and bottom waters and therefore mixing was
more restricted in the summer of 1984. The strength of this barrier to
mixing can have important implications for water quality conditions; its
effect on dissolved oxygen concentrations will be discussed below.
Figure 6. Longitudinal profiles
for average salinity and dissolved oxygen in the Chesapeake Bay mainstem
during the summers of 1984 and 1985.
Suspended solids concentration in the water column is an important water
quality indicator because of its effect on water transparency. These
suspended solids consist of inorganic material such as clays and organic
material such as living phytoplankton. Suspended solids reduce the depth to
which sunlight can penetrate, thereby reducing the habitable zones for
phytoplankton and submerged aquatic vegetation which depend upon light to
grow. Water transparency, or turbidity, is considered the primary limiting
factor to phytoplankton growth in some regions of the Bay. Reduction in
light penetration has also been implicated as a major cause of declines in
submerged aquatic vegetation in recent decades.
As with many other quality constituents, suspended solids in the Bay's
tidal waters are influenced by river inputs. Management actions in the
Bay's watersheds that stabilize soil and shoreline erosion will help to
reduce the river loadings of suspended sediments and in turn increase water
transparency. Natural processes also contribute to the amount of suspended
solids in the water column. In areas of high mixing such as near the
upstream extent of salinity intrusion in the mainstem and tributaries,
there is usually a peak in suspended solids called the "turbidity maximum".
In this region, trapping of river-derived inputs and resuspension of bottom
sediments contributes to high levels of suspended solids. This turbidity
maximum region occurs between Pooles Is. and Turkey Pt. in the mainstem of
Chesapeake Bay. Here, turbidity is highest and total suspended solids are
generally 15-25 mg/l (mg = milligrams of mass and l = liters of volume) in
surface waters.
Below the influence of river-borne inorganic sediment loads and the
turbidity maximum region, turbidity generally declines and phytoplankton
populations become a more important factor affecting water clarity. These
areas are expected to show decreases in turbidity in response to nutrient
control measures which limit phytoplankton growth. In the lower Maryland
Bay, turbidity declines and total suspended solids decrease to about 5-10
mg/l. Turbidity in the mainstem is shown in
Figure
7. This relative measure of turbidity was derived from Secchi
disc readings which measure water transparency. The up-Bay to down-Bay
gradient from high to low turbidity is evident at all times of year.
 |
Dissolved Oxygen
Depletion of oxygen in bottom waters is one of the more devastating
symptoms of stresses being placed on Chesapeake Bay. The increased
severity of dissolved oxygen depletion in Chesapeake Bay has caused
declines in habitat for living resources as well as causing direct
mortalities of fish and shellfish. |
|
Figure 7. Turbidity levels in the
Chesapeake Bay mainstem for June, 1984 through December 1985. |
It is therefore very important that we monitor dissolved oxygen conditions
to determine with better certainty where problem areas occur and how they
might respond to management initiatives. It is also important to understand
both the natural and anthropogenic contribution to this problem so that
these two factors can be differentiated in our analysis of the data. The
importance of natural processes is evident when comparing the summers of
1984 and 1985.
Low dissolved oxygen develops in bottom water as a result of the interplay
between two opposing processes. High levels of organic material in Bay
bottom waters and sediments decompose and in the process consume large
amounts of oxygen. At the same time, oxygen from the atmosphere and from
phytoplankton photosynthesis in surface waters is mixing downward to
replenish oxygen levels in bottom waters. In the spring and summer, due to
high levels of organic material (largely derived from algal growth) and
high temperatures which enhance decompositional processes, the oxygen in
bottom waters is consumed at very rapid rates. This is also a time of year
when mixing of surface and bottom waters is inhibited by large differences
in surface to bottom salinity. Thus, it is in the spring-summer period when
the interplay between consumption and re-oxygenation causes oxygen levels
in bottom waters to reach a yearly minimum.
As mentioned previously, (and shown in
Figure 6,
A and B) the higher flows from the Susquehanna River in the summer of 1984
caused stronger vertical salinity gradients in the mainstem when compared
to summer of 1985. The average dissolved oxygen profiles, as shown in
Figure 6, C and D, for the 1984 and 1985
summer periods illustrate the influence of restricted mixing of surface and
bottom waters brought about by the higher salinity gradients in 1984. There
were several differences between average conditions in the two years,
especially in the areal extent of hypoxic waters (here defined as waters
with dissolved oxygen levels of less than 1.0 mg/l). In 1984, hypoxic water
extended to well below the mouth of the Potomac River and into Virginia
waters while in 1985, hypoxic waters did not reach past the mouth of the
Patuxent River. Periodic reoxygenation events, brought about by mixing of
surface and bottom waters, were more frequent in 1985 due to the lower
salinity gradients. The identification of these periodic reoxygenation
events and the dynamic nature of bottom water dissolved oxygen
concentrations with the present monitoring program has greatly expanded our
knowledge of this phenomenon and will permit a much more reliable
assessment of its year to year changes than ever possible before.
In general, the lower reaches of many larger Bay tributaries have deep
water dissolved oxygen concentrations that follow a pattern similar to the
mainstem. However, in the lower Patuxent River, where surface to bottom
salinity gradients were also more pronounced in 1984 than in 1985, the
differences in dissolved oxygen levels between the two summers were not as
obvious as in the mainstem. The observed discrepancy between the dissolved
oxygen conditions in the Patuxent and the Bay during the summers of 1984
and 1985 is indicative of the influence of factors other than
stratification. System specific characteristics such as river bottom
topography, localized storm events and periodic exchanges with mainstem
waters also affect dissolved oxygen conditions.
An indication of the influence of mainstem dissolved oxygen conditions on
dissolved oxygen in the lower Patuxent can be seen in
Figure 8. The 1984 mean summer profile suggests that deeper
Bay waters with low dissolved oxygen concentrations my be intruding into
the lower Patuxent and producing an area of low oxygen concentrations just
inside the mouth. However, in the 1985 mean summer profile, this phenomenon
is not apparent. Because of the shallow sill separating deeper Bay waters
from deeper Patuxent waters, the exchange appears to be restricted much of
the time. The intermittent nature of the deep water exchange makes it
difficult to quantify its effect. This exchange phenomenon and related
issues in the Patuxent estuary are the subject of detailed monitoring and
modeling study by OEP.
Figure 8. Longitudinal profiles
for average dissolved oxygen in the Patuxent River during the summers of
1984 and 1985.
Nutrients
Concentrations of nutrients are critical to defining the "health" of
Chesapeake Bay waters and the degree of impact imposed by man's activities.
Nutrients, primarily nitrogen and phosphorus, are necessary for algal
growth and their excessive inputs into Bay waters are the primary cause of
eutrophication problems. The nutrient status of Chesapeake Bay will be a
key determinant in the targeting of management actions and the evaluation
of their success.
Nitrogen: Levels in Chesapeake Bay are generally elevated in the
upper reaches of the mainstem and tributaries, exhibiting the influence of
inputs of this nutrient from rivers and point sources (Map
2). In tidal fresh areas of the mainstem and Patuxent, Potomac
and Choptank Rivers, the total nitrogen concentration is high, with values
between 1.5 and 2.5 mg/l. In the Patuxent and Potomac Rivers, sites with
high point source inputs which do not vary much on a seasonal basis, the
total nitrogen values do not show appreciable seasonal changes. However, in
the Choptank River where nonpoint sources dominate, there is a distinct
nitrogen peak in winter and spring when high freshwater flows increase
nonpoint source inputs.
Map 2
Total nitrogen declines downstream in the mainstem and each of the
tributaries as high nutrient inputs at the head of the estuaries are
diluted. The Patuxent, Potomac and Choptank Rivers all exhibit lower total
nitrogen levels in their downstream, high-salinity reaches relative to
upstream locations. These levels are similar between the three systems,
ranging between 0.6 and 0.9 mg/l. The Baltimore Harbor (lower Patapsco
River) station, which has a salinity regime comparable to the lower
stations in the three tributaries just discussed, exhibits nitrogen
concentrations almost twice what is found in those systems. The mainstem,
on the other hand, exhibits the lowest concentrations, about 0.5 mg/l, in
its saltier reaches.
Tangier Sound, an area generally considered more pristine than most in
Maryland's Bay, also shows the influence of nutrients transported by
several rivers draining the lower Eastern Shore. Total nitrogen is
approximately 50% higher than comparable areas in the adjacent mainstem.
Figure 9. Simplified cycle of
inorganic and organic forms of nitrogen in Chesapeake Bay. Total
nitrogen is the sum of inorganic and organic forms. Phosphorus is
similarly recycled through inorganic and organic forms.
A closer examination of nitrogen in the mainstem reveals distinct spatial
patterns in the different forms of nitrogen.
Figure 9 (above) shows a simplified diagram of the major relationships
between the different forms of nitrogen. The strong gradient of declining
total nitrogen from the head of the Bay can be seen to be driven
principally by changes in dissolved inorganic nitrogen (nitrate, nitrite,
ammonium), which ranges from around 1 mg/l in the upper Bay to less than
0.25 mg/l in the lower Bay (Figure 10,
A and B). Most of this dissolved inorganic nitrogen is in the form of
nitrate delivered by the Susquehanna River.
Figure 10. Dissolved inorganic
nitrogen and ammonium concentrations in surface and bottom waters of the
Chesapeake Bay mainstem from June, 1984 through December, 1985
Another dramatic pattern is brought about by the recycling of nitrogen by
the Bay's microorganisms. During the summer, recycling rates of nitrogen
from biological activity in the sediments and the water column are high.
This nitrogen recycling produces ammonium, an inorganic form of this
nutrient. In the upper mixed layers of the water column this recycled form
of nitrogen is actively consumed by growing phytoplankton and thus the
ammonium does not accumulate (Figure 10,
C). In bottom waters, where the utilization by organisms is relatively low,
this form of nitrogen does accumulate during summer months (Figure
10, D). This high concentration of ammonium in bottom waters can
serve as a source of nitrogen for summer algal growth as it mixes upward
into surface waters. The nitrogen patterns observed in mainstem waters are
also found in the major tributaries which generally function as
smaller-scale analogues of the Bay itself.
Phosphorus: Similar to patterns observed for nitrogen, total
phosphorus concentrations are highest in the upper reaches of the Bay and
its tributaries (Map 3). Unlike
nitrogen, however, concentrations in the tributaries are generally
significantly higher than in the mainstem and point to some appreciable
differences among rivers. The Patuxent River has the highest total
phosphorus concentrations, reaching 0.35 mg/l, in its upper tidal reaches
when compared to similar areas of the Potomac and Choptank Rivers which
usually exhibit concentrations below 0.2 mg/l. The highest total phosphorus
concentrations in the mainstem are about 0.1 mg/l. Anthropogenic loadings,
from point and nonpoint sources, lead to the high phosphorus concentrations
observed in the tributaries. Differences in point source loadings of
phosphorus among tributaries, relative to river volume and flushing,
probably accounts for much of the differences in phosphorus concentrations
among the rivers.
Map 3
Total phosphorus in the high salinity, lower estuarine zones of the
Patuxent, Potomac and Choptank Rivers show declines from upstream
locations. However, these tributary concentrations remain appreciably
higher, about 0.1 mg/l, than comparable areas of the mainstem at less than
0.05 mg/l. Baltimore Harbor exhibits one of the highest concentrations of
phosphorus in the Bay's higher salinity zones, being similar to levels
found in the lower Patuxent. Tangier Sound exhibits total phosphorus
concentrations approximately twice that found in adjacent regions of the
mainstem. As with nitrogen, inputs from the watersheds surrounding Tangier
Sound are the likely explanation for this finding.
 |
Recycling of phosphorus is also evident in the monitoring data from the
mainstem. Like nitrogen, phosphorus is recycled actively during summer
months in both the water column and sediments. Sediment release of
phosphorus into the overlying water column is especially strong when oxygen
levels are low, as they are in summer. This phosphorus from the sediments
and lower water column, recycled as dissolved inorganic phosphorus, builds
to high concentrations in bottom waters during spring and summer months (Figure
11, B left) because of restricted mixing between surface and bottom
waters at this time of year. Furthermore, phytoplankton, which would
typically consume this form of phosphorus, cannot grow at these depths
because insufficient light penetrates the turbid waters. |
| Figure 11.
Dissolved inorganic phosphorus concentrations in surface and bottom
waters of the Chesapeake Bay mainstem from June 1984 through December
1985. |
In surface waters, growing phytoplankton consume inorganic phosphorus.
This uptake keeps inorganic phosphorus concentrations lower in surface
waters relative to bottom waters during the summer (Figure 11, A
above).
Toxic Chemicals
Organic Chemicals: Polynuclear aromatic hydrocarbons (PAH's) are the
most abundant class of organic toxicants detected in bottom sediments in
the Chesapeake Bay. Detectable concentrations were found at every station
sampled (Map 4). The occurrence of
PAH's in sediments is of concern because several PAH compounds have proven
to be potent carcinogens in mammalian and aquatic test animals. PAH
contamination originates primarily from the atmospheric deposition of
particles formed from the combustion of all forms of fossil fuels, and from
the leaching of organic material from coal particles that are commonly
found in upper Bay sediments. The upper Bay stations in the mainstem and at
the mouth of the Patapsco River are the most heavily contaminated with
PAH's, with concentrations approaching 10 parts per million (ppm). It
appears that much of the PAH's that enter the Bay from the Susquehanna and
Patapsco Rivers remain in that vicinity. Not all PAH's in sediments are
hazardous, since background or natural PAH concentrations always occur in
the estuarine environment. The Tangier Sound station, a relative unimpacted
site, exemplifies low-level PAH concentrations and is a reference site with
which the contaminated areas can be compared. The mouth of the Patuxent
River shows unusually high sediment PAH concentrations which can be
attributed to the occurrence of coal particles on the river bed in this
area.
 |
PAH concentrations in the tissues of the Macoma clams (Map
4), showed that the tissue levels were directly related to the
degree of sediment contamination (Figure 12
left).
PAH concentrations in the tissues of clam worms (Nereis species) were also
related to sediment PAH concentrations (Figure 12
left). It appears that the relationship between sediment and tissue
levels might be different for the two organisms, |
| Figure 12.
Relationship between polynuclear aromatic hydrocarbon (PAH)
concentrations in sediments and in tissues of Macoma clams, Macoma
balthica, and polychaete worms, Nereis sp. Concentrations in sediments
are in parts per million (ppm) based on weight of sediment organic
carbon. Concentrations in organisms are in ppm based on total wet
weight. |
probably due to differences in feeding and metabolism. The natural organic
carbon content of sediments also appears to play an extremely important
role in the retention of toxicants, such as PAH, in sediments and in the amount of sediment-associated PAH that
accumulates in sediment-dwelling organisms. While the relationships
presented here are still preliminary, they point to the link between
sediments and organisms dwelling there. These relationships and others
throughout the food chain will help us to assess and understand the
potential threat posed by toxicants in the Bay.
Map 4
Metals: Metal concentrations in sediments and biota were determined
at the same sites where analysis was performed for the organic toxicants
discussed above. The concentrations of four metals - zinc, copper, chromium
and lead - are presented in Map 5
(below).
Clearly, higher sediment concentrations of trace metals are associated with
heavily industrialized areas such as Baltimore Harbor. Additionally,
broader spatial trends exist, such as a high to low gradient north to south
due in part to input from the Susquehanna River and atmospheric input of
such metals as lead from industrial areas to the north.
Map 5
From both an ecological and human health standpoint, it is particularly
important, as with organic toxicants, to assess the degree to which metals
are accumulating in organisms. Although in general terms one can see that
the highest metal concentrations in Macoma clams are associated with the
most contaminated sediments, the relationship between metals in the biota
and sediment is apparently complex. Factors affecting this relationship
probably include the sediment characteristics as well as the organisms'
physiology and feeding habits. The concentrations of metals in Macoma clams
vary less than the levels in associated sediment samples and there appears
to be different proportions of the 4 metals in sediments as compared to the
clams (Map 5 above).
CONCLUSIONS
- The areal extent of low dissolved oxygen in bottom waters of the
mainstem was much greater in 1984 than in 1985. This difference was due
to natural physical factors which can be accounted for in the present
program relative to pollutant impacts.
- Elevated nitrogen levels of over 1 mg/l are consistently found in the
upper mainstem and tributaries of Chesapeake Bay and in Baltimore Harbor
due to the proximity of high point and nonpoint loads of this nutrient.
- Phosphorus concentrations, like those for nitrogen, are highest in
the upper reaches of the mainstem and tributaries. There are, however,
major differences between the mainstem and the different tributaries,
indicating large differences in the proportions of phosphorus loadings
between systems; much of this difference appears to be related to point
source inputs.
- Spatial distributions of organic and metal toxicants in sediments and
biota show the effects of high industrial activity in the upper Bay.
- Results from the first two years indicate that with the spatial and
temporal resolution presently incorporated in the monitoring program, the
objectives of characterization, trend and understanding water quality
processes as they relate to chemical and physical conditions are
achievable.
Contents
a.
Preface
b.
Acknowledgements
1. Introduction
2. Understanding The Bay's Problems
3. Program Description
4. Chemical and Physical Properties
5. Plankton
6. Benthic Organisms
7. Ecosystem Processes
8. Pollutant Inputs
9. Management Strategies and the Role
of Monitoring
10. Glossary |
