Chesapeake Bay Monitoring
"Monitoring for Management Actions"

2. understanding the bay's problems

An understanding of the problems that confront today's Chesapeake Bay is central to a strategy to restore and protect this valuable resource. Without an understanding of how this complex system responds to pollutants, it will be difficult to target effective management actions. Likewise, an understanding of the physical, chemical and biological processes in Chesapeake Bay played a central role in the formulation of the water quality monitoring program. Since the program is directed at specific management issues, there is a logical basis for including particular elements in the design. While it is necessary to monitor the obvious problem, for example low dissolved oxygen, it is also necessary to monitor the principal causes such as nutrient enrichment, phytoplankton growth and deposition, and water stratification which prevents oxygen from mixing into the bottom waters. Usually, the underlying causes, such as nutrient enrichment in the example just presented, are more directly under our control than are the impacts that develop as the Bay responds to pollutants. There are also natural processes, such as water stratification, which must be examined to place an evaluation of pollutant impacts in the proper perspective.

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In this chapter, some of the major problems confronting Chesapeake Bay will be examined within the ecological context of natural physical, chemical and biological processes. This information is intended to assist in an understanding of why the monitoring program is designed as it is and to provide background information that may be helpful in the comprehension of subsequent chapters.


Most of the problems currently perceived as causing decliners in the Bay's health have a common denominator - man. Man has acted directly by adding "wastes" to the Bay and its tributaries and by withdrawing its resources. Man has acted indirectly by changing the character of the land and air that surround and interact with the Bay.

The most profound Bay-wide impacts have resulted from large inputs of nutrients and other chemicals from sewage treatment plants or industrial operations, referred to as point sources, and from stormwater running off urban or rural land, referred to as nonpoint sources. These inputs are primarily composed of natural elements which are entering the Bay in excessive quantities. These are elements that normally recycle in the environment between plant and animal or between land, water and air. Problems have been created because of major perturbations in the balance of these recycling processes due largely to high populations along the shores of Chesapeake Bay. This imbalance results in an abnormal shift of recycled products, such as nutrients, so that they now enter and accumulate in Bay waters. When the shift in the balance of recycling is considerable, as it is in some regions of Chesapeake Bay, these natural products can cause severe problems. The nature of these impacts will be discussed later.

Another type of problem confronting the Bay comes from toxic compounds, unnatural products created by man or naturally-occurring chemicals that are concentrated to levels far exceeding the trace quantities normally found in the environment. These compounds are usually discharged during the manufacture, application or disposal of various products. These toxicant problems tend to be most severe in regions of the Bay where manufacturing industries or waste disposal sites are concentrated. The problems caused by toxic compounds are difficult to predict or understand because of their extremely complex chemical properties. It is known, however, that serious human and environmental health impacts may result when these compounds enter the Bay.


Chesapeake Bay - the mainstem and tidal tributaries - constitutes an ecosystem. An ecosystem is a unit within which there is close linkage of many physical, chemical and biological processes such as water circulation, nutrient recycling and food chains. Ecosystems interact with adjacent ecosystems, making their boundaries sometimes difficult to distinguish. Chesapeake Bay is an estuarine ecosystem.

As an estuarine system, Chesapeake Bay is one of the most complete, ecosystems. Within its boundaries exist a range of aquatic environments, from fresh to nearly full-strength seawater, allowing a broad spectrum of organisms to flourish and chemical reactions to proceed. It has complex physical circulation patterns that vary with season, tide and weather. Outside of its boundaries, adjacent or sometimes remote ecosystems influence Chesapeake Bay, thereby contributing additional complexity (Figure 1). Many of these external effects are mediated through the atmosphere such as rainfall which affects freshwater, nutrient and sediment inputs to the Bay and wind patterns which exert strong influences on water circulation within the Bay. Acidic precipitation, originating from distances of hundreds of miles surrounding the Bay, is also transported by the atmosphere. Other external effects arise because several key living resources such as blue crabs and striped bass migrate to spend part of their lives outside of the Bay. Unfavorable currents in coastal waters off the mouth of the Bay and successful fishermen off the New England coast can have appreciable impacts on the abundance within the Bay of blue crabs and striped bass, respectively.

a graphic showing the influence of adjacent ecosystems on the Chesapeake Bay ecosystem.
Figure 1. Influence of adjacent ecosystems on the Chesapeake Bay ecosystem.


Estuaries, like many ecosystems, are resilient in response to natural perturbations of the environment such as large storms and droughts. The system may be knocked out of balance by these unusual events, as Chesapeake Bay was during and after tropical storm Agnes in 1972. Nevertheless, the Bay gradually returns to a state similar to that existed prior to the disturbance. Similarly, most systems can remain relatively unchanged when confronted with a modest level of anthropogenic (of human origin) influences. However, there are often thresholds beyond which resilience or assimilative capacity for a given system can be exceeded, resulting in significant changes in the balance of the ecosystem. When these thresholds are exceeded, as they appear to be in parts of Chesapeake Bay, the system responds with perceptible changes or impacts. In the Bay, these impacts can take the form of low dissolved oxygen concentrations, turbid waters and lowered abundances of fish and shellfish. Because these impacts are usually the result of substantial historical as well as ongoing anthropogenic influences that have finally overwhelmed the Bay's assimilative capacity, responses to measures taken to rectify the problems may not become apparent for many years. Thus, the effort to improve conditions in Chesapeake Bay and to monitor the progress of management actions will need to be sustained for many years before the goal of improving the Bay's health can be realized.

To develop and evaluate plans for protecting and restoring the Bay, one must have an appreciation for the complex interactions that mediate between anthropogenic influences, such as sewage treatment plant discharges or urban runoff, and measurable ecosystem impacts. The measurable impact, such as low dissolved oxygen or reduced fisheries yield, often results from a convoluted series of physical, chemical and biological events that may make the impact seem unrelated to the cause in character, time or place. For example, land use changes in Pennsylvania could cause appreciable increases in the amount of nitrogen and phosphorus entering the Susquehanna River and ultimately transported by spring runoff into Chesapeake Bay. An increase in nitrogen and phosphorus may fuel excessive summer algal growth in the upper Bay. Much of this algal production may then settle in the deep waters of the Bay in Maryland resulting in oxygen deficient waters as this algal material is metabolized by bacteria. Thus, the observed impact, which is low dissolved oxygen, is much different than the cause, which is land use practice. The impact occurs far away from the cause of the problem and it may be several months before the pollutant input is manifest as an impact.

Physical Circulation Patterns are one of the most important characteristics of the Chesapeake Bay estuary. Although quite variable, most of the mainstem and the lower reaches of the tributaries generally have a two-layer flow pattern with lighter, fresher surface waters showing a net flow seaward and heavier, saltier bottom waters showing a net flow landward (Figure 2). This circulation pattern has profound influences on both natural processes and man's disturbances. Many small but important organisms such as oyster larvae and phytoplankton (microscopic plants suspended in the water column) depend upon the deeper, landward moving waters to reach favorable areas of the Bay and complete their life cycle. At the same time, this two-layer flow tends to trap material that enters the Bay either naturally or through man's activities. Inputs to the Bay or is deposited in the sediments. Whether in the lower portion of the water column or in bottom sediments, the material can undergo chemical and biological transformations and continue through additional transport cycles between surface and bottom waters. Ultimately, however, little of the material that enters the Bay ever exits from its mouth. This concept is particularly important to understanding the effects of pollutants which are not "flushed away" as in free-flowing rivers but are "trapped" in the estuary. The "trapping" of pollutants is another reason why the restoration of Chesapeake Bay will require a long-term commitment; it will take some time to dissipate the accumulation of inputs that have entered over the past decades.

Figure 2.  Two-layered flow patterns of Chesapeake Bay waters showing net down-Bay movement of surface waters and net up-Bay movement of bottom waters.

Chemistry plays a large role in both the natural processes and anthropogenic disturbances of an estuary. All life is dependent upon numerous chemical reactions which are ultimately driven by the energy of sunlight. This energy is captured by plants during a process known as photosynthesis. This captured energy is stored in chemical bonds between elements such as carbon, oxygen, hydrogen, nitrogen and phosphorus which serve as building blocks for the complex molecules synthesized by the photosynthetic chemical reactions within plants. In turn, animals and bacteria, as they digest their food, break down the complex molecules to capture energy stored in their chemical bonds. In this breakdown process, the building block elements are released back into the water. This continuing process of synthesis and degradation is known as recycling. As noted earlier, it is often the imbalance of these normal recycling processes that have impacted the Bay's health.

In most natural systems, chemical recycling is reasonably "tight", meaning that chemical synthesis from building block elements roughly balances their release by degradation processes in Chesapeake Bay. However, the balance of recycling processes in the Bay has been affected by the unusually high import of building block elements such as nitrogen and phosphorus generated from human biological waste and runoff from altered land surfaces. One of the outcomes of this chemical imbalance is a condition known as eutrophication which will be elaborated upon below.

Biological Processes in Chesapeake Bay are the most visible and relevant to most people. The crabs and oysters on your plate or an algal bloom on the Potomac River that looks like green paint are vivid examples of the Bay's biological activity. Often it is the abundance of biological resources that are used to judge the "health" of the Bay.

Biological processes are governed by the chemical and physical environment discussed above and by biological interactions such as those occurring in the food chain (more correctly labeled the food web to indicate complex linkages). In aquatic ecosystems such as Chesapeake Bay, most biological processes are fueled by the growth of phytoplankton, the principal photosynthetic organisms. The growth, or production of phytoplankton requires light, water and the presence of several essential elements. In Chesapeake Bay, as in other aquatic systems, levels of nitrogen, phosphorus, light and temperature appear to be the major determinants of phytoplankton production. Much of the phytoplankton production is utilized to sustain the growth of higher organisms such as zooplankton (microscopic animals in the water column) and benthic organisms living within or on the bottom sediments. These consumers in turn fall prey to larger and larger animals, ultimately leading up to predatory fishes such as the striped bass and other high-level predators such as birds and humans who remove biological resources from the Bay.

At the same time that biological production is passing up the food web to higher predators, some of the production, in the form of biological wastes or dead organisms, is shunted to pathways that lead to decomposition of this matter into its basic elements. This decomposition is usually mediated by digestive enzymes in the guts of animals and by bacteria, either free-living in the environment or harbored within organisms. In the Bay these decompositional processes are actively occurring both in the water column and in the sediments. The basic elements regenerated by decomposition can then be utilized once again by phytoplankton populations and the circular pathway describing the recycling of chemical elements is completed. Many of the biological, chemical and physical interactions important to the Bay ecosystem are diagrammed in Figure 3 (below).

Figure 3.  Major ecological relationships in Chesapeake Bay

The preceding discussion has emphasized the need to understand physical, chemical and biological factors and their interactions when confronted with the task of protecting and restoring the Bay. One of the most profound conditions affecting the Bay -eutrophication- is a typical example of how physical, chemical and biological processes interact in response to anthropogenic inputs to produce a significant impact on the Bay (Figure 4). It also points out the need for a monitoring program that is comprehensive in its approach so it can yield the information necessary to guide management actions.

Figure 4.  Comparison of balanced and eutrophic conditions in an estuarine environment.
  • algal growth controlled
  • water transparency high, encouraging aquatic plant growth
  • sufficient oxygen in bottom waters support a variety of aquatic life
  • algal growth excessive
  • water transparency reduced, limiting aquatic plant growth
  • depleted oxygen in bottom waters reduces habitat for aquatic life

Eutrophication is a term used to describe the over enrichment of aquatic systems by excessive inputs of phytoplankton nutrients, typically phosphorus and/or nitrogen. In unimpacted systems, nutrients are present in such low quantities that algal growth is controlled. In systems with an oversupply of these nutrients, the growth of phytoplankton is stimulated, initiating a chain of events that leads to the symptoms of eutrophication. Usually, a major fraction of the enhanced phytoplankton growth cannot be assimilated through the normal food web. This situation occurs because there simply is too much phytoplankton being produced or because certain phytoplankton species, which are unpalatable and thus not eaten by the important small animal consumers, start to flourish under these conditions.

Sometimes, the excessive algal growth of eutrophic waters is readily visible as algal "blooms" or "scums" that form near the surface. More typically, much of the activity associated with eutrophication is hidden below the surface. The large mass of algal growth that does not enter the food web pathways leading to larger animals, enters the decomposer pathways as it sinks to bottom waters and sediments. The decomposition of this algal matter by bacteria requires large amounts of oxygen which quickly depletes dissolved oxygen from bottom waters. Physical circulation patterns in many parts of the Bay during spring and summer exacerbate the problem by restricting the mixing of oxygen-rich surface waters with bottom waters. Furthermore, there are some complex chemical conditions produced by eutrophication which cause additional phosphorus and nitrogen to be released from sediments. These additional inputs of nutrients tend to further accelerate the eutrophication process by prolonging or stimulating additional algal growth.

Low oxygen conditions produced by eutrophication result in major losses of habitat for fish and shellfish that cannot survive in these stressed environments. Eutrophication also causes much of the turbidity that affects the aesthetic appeal of Bay waters in many areas. Thus, a complex series of underlying physical, chemical and biological processes has transformed an anthropogenic influence into a wide-ranging impact. As this example demonstrates, if we are to make progress in the effort to protect and restore Chesapeake Bay, the underlying causes of each problem must continue to be monitored, understood and acted upon so that the damaging impacts can be mitigated.

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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