Chesapeake Bay Monitoring
"Monitoring for Management Actions"

4. chemical and physical properties

The chemical and physical properties of an estuary provide many of the fundamental indicators of water quality. Chemicals such as nutrients and toxicants reflect most directly the impacts of man's activities on the Bay. Once in the Bay ecosystem, these chemicals can undergo a variety of transformations and exert a strong controlling influence on living resources and the biological food web. Physical properties such as water transparency, temperature and density stratification show both human impacts and natural processes. An understanding of these physical properties is often crucial to the interpretation of chemical and biological elements of the monitoring program.

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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.


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.


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.


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).


  • 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.

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

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