Scientific knowledge and/or understanding is not static. The scientific community is continually advancing its understanding of fundamental principles or processes that will eventually have impact on decisions of societal importance. Our knowledge of the complex Chesapeake Bay system is no exception.
The scientific community has made marked progress in understanding fundamental processes underlying management approaches to the restoration of the Chesapeake Bay. Many of the individual studies have not, however, been synthesized with other studies in a context that disseminates their results to scientists or technical experts from other disciplines. In addition, as is to be expected in the study of complex systems or processes, the results raise additional questions.
This publication reviews five areas of estuarine research that we believe have critical importance to the protection and restoration of the Chesapeake Bay. The particular areas were selected because of their broad implications beyond the immediate disciplines in which the studies are conducted.
The Chesapeake Bay Program has already recognized the critical limitations in our knowledge of two of the areas: BENTHIC AND PELAGIC COUPLING IN THE CHESAPEAKE BAY and FACTORS DRIVING CHANGES IN THE PELAGIC TROPHIC STRUCTURES OF ESTUARIES, WITH IMPLICATIONS FOR THE CHESAPEAKE BAY, by initiating plans to develop new information on these areas for use in the next generation of models.
We have known for several decades that the life histories and distribution of living resources within the estuary are intimately tied to estuarine circulation, as is the distribution of toxic compounds introduced into these systems. It is only recently, however, that we have recognized that our general knowledge of circulation has not been sufficient to address the questions raised about many specific issues. It is hoped that the review of the topic: PHYSICAL PROCESSES THAT CONTROL CIRCULATION AND MIXING IN ESTUARINE SYSTEMS will assist in elucidating processes dependent on circulation and will stimulate much needed additional research in this area.
The principal underlying goals of the Chesapeake Bay Program are to restore, protect, and ultimately manage the living resources of the Bay. The main thrust at present is to provide appropriate water quality to sustain these resources. A lesser thrust has been to develop a better assessment of stocks of commercial, recreational, and ecological importance to the system. Explosive advances in methodology applicable to genetic studies are now being applied to estuarine species.
The implications of the fourth review topic: GENETICS AND THE CONSERVATION OF ESTUARINE SPECIES may open new approaches to restoration and manipulation of stocks and must be considered in any emergent plans for living resource management.
The EPA-funded Chesapeake Bay Study provided us with the first complete benchmark inventory of toxic compounds in Chesapeake Bay. Additional efforts since the termination of the formal study have extended the inventory up the tributaries. Identification of toxic "hot spots" and our ability to measure compounds at ever lower concentrations have led to the realization that the simple presence of a compound may not be sufficient to define a toxic problem. Our final paper: CHEMICAL AND PHYSICAL PROCESSES INFLUENCING BIOAVAILABILITY OF TOXICS IN ESTUARIES addresses many of the complex phenomena that must be considered when trying to evaluate the impact of specific compounds or classes of compounds.
The following capsule summaries of the full reviews only touch upon the full ramifications of each topic. The reader is encouraged to read the reviews. The authors were requested to write the reviews for an audience of scientists in their own disciplines, scientists who require knowledge of the particular processes for understanding other phenomena, and technically conversant resource managers wishing to incorporate state-of-the-art scientific knowledge in their long-term planning. The authors were also requested to briefly touch on potential management implications and additional research needs related to their topics.
Although the importance of benthic-pelagic interactions in aquatic systems has been recognized for half a century, it is evident from this review that research in the field has been active for little more than a decade. Scientific studies in the Chesapeake Bay and elsewhere have provided initial descriptions of the complex patterns and mechanisms involved in connecting ecological processes in the water with those in the sediment. While many unresolved questions remain, a few preliminary conclusions can be drawn from this review.
Summary of Major Findings
Various methods have been employed for measuring both particle deposition and benthic fluxes of oxygen, ammonium, phosphate, and other metabolites across the sediment-water interface. It is encouraging to see that particle deposition rates estimated by geochemical (and paleobotanical) tracer techniques tend to converge with rates obtained from sediment-trap deployments, even though the two approaches measure this process on very different time scales. Direct measurements of benthic fluxes of nutrients and oxygen tend to agree well with observations made using in situ chambers and those involving intact sediment cores. Indirect estimates of nutrient flux based on diffusion modeling of porewater concentrations are, however, often 2- to 10-fold lower than rates obtained from direct measurements, especially for productive systems with active macrofauna burrowing in sediments.
Seasonal cycles of particle deposition in the Chesapeake Bay and other coastal systems closely follow trends in phytoplankton production. Generally, 40-60% of the organic production by phytoplankton settles to the benthos. Much of the total mass of deposited material, however, appears to be terrigenous inorganic sediment, suggesting an important linkage between biotic processes and sedimentological transport.
The importance of particle deposition and burial in the overall input-output balance of nutrients (nitrogen, phosphorus, and silicon) for the Chesapeake Bay has recently been questioned. Whereas earlier reports indicated that most of the nutrient inputs from the Bay's watershed were retained and buried in its sediments, a revised analysis indicates that this may not be the case. This unresolved question has significant management implications for control of nutrient wastes.
Sediment oxygen consumption is an important sink for oxygen pools in the Chesapeake Bay and other estuaries. Over 50% of the total oxygen consumption in Bay regions less than 10 m deep occurs through benthic processes, which remove 1020% of the water column oxygen pools per day in late spring. Nitrogen and phosphorus regenerated by benthic processes can satisfy about 20-50% of the phytoplankton demand in the main Bay and 50-100% in the shallower tributaries. These relative rates are similar to those reported for other coastal systems. Recent work in the Bay region has also indicated that benthic-pelagic interactions may provide a mechanism for temporary retention of nutrients delivered from the watershed in winter to support production in spring and summer.
Research efforts have identified a wide variety of physical, chemical and biological factors affecting benthic fluxes of nutrients and oxygen. Temperature and deposition of particulate organic matter are primary variables regulating benthic fluxes. Fluxes of nitrate and oxygen into sediments are directly related to concentrations and turbulent mixing in the overlying water. Oxygen concentration (especially as it approaches zero) also exerts profound, but as yet poorly described, effects on most benthic processes. Similarly, burrowing and feeding activities of macrofauna appear to significantly influence benthic fluxes, although exact mechanisms remain to be explained.
Relevance to Restoration and Protection Activities
Many of the processes involved in benthic pelagic coupling will directly influence the outcome of proposed management actions to restore and protect water quality and living resources in the Chesapeake Bay. Several relevant questions are given below.
the Bay, been diminished due to eutrophication, and will management strategies restore these natural mechanisms of nutrient waste assimilation to effectively accelerate the benefits of clean-up actions?
- What is the role of benthic-pelagic coupling in the fate and transport of toxic substances entering the Bay?
In addition, if water quality modeling is to be a key component in management of Bay resources, it is particularly important that benthic-pelagic coupling processes be sufficiently well described to allow careful model calibration. The factors regulating these processes must also be understood and incorporated into such models to enable accurate projection and assessment of proposed strategies (e.g., 40% reduction in nutrient inputs) for restoration and protection of Bay water quality.
Future Directions
Conceptualization, calibration, and validation of rigorous mathematical water quality models will require an expanded scientific information base including improved descriptions of:
and
Ultimately, for these data collection activities to contribute to the development of effective Bay management strategies, an administrative mechanism must be established to communicate scientific findings to the managers and their constituents. Conversely, management questions and priorities must be articulated to the scientific community so that study designs and approaches can be adjusted to meet the management needs. The ongoing Bay monitoring program may offer a model for efficient bidirectional communication.
-W. MICHAEL KEMP
Processes governing the relative abundance of organisms ultimately determine the composition of biological communities and thus how energy and material is transferred among components of the ecosystem. In marine and estuarine environments, the pelagic food web plays a dominant role in these transfers. For this reason, changes in the composition of this trophic network, or in the direction or magnitude of flows, could have significant impacts on fishery yields, water quality, or other factors of concern to scientists and managers. This chapter reviews the factors that may influence the composition of the trophic network.
Summary of Major Findings
The biological success of any organism reflects both its physiological tolerances and controlling factors such as food supply. If growth is limited by quality or quantity of food, then population size may be regulated by factors controlling prey organisms in lower trophic levels; this has been termed "bottomup" or source control. If nutrition is adequate, however, population size may be limited by predators in the next higher trophic level; this is "top-down" or sink control. The available evidence suggests that pelagic trophic structure in estuaries is controlled by a combination of these processes, the relative importance of which constantly changes in response to environmental fluctuations.
Estuaries such as the Chesapeake Bay are characterized by environmental variability of greater magnitude and frequency than other aquatic habitats; factors such as light, salinity, and nutrient availability change over time scales ranging from seconds to decades or more. This great variability is reflected in the system's biological components as well. Species composition and relative abundance typically change throughout the year, and, although repeating seasonal signals are strong and often important indicators, yearto-year variability may be significant. As a general rule of thumb, the effects of environmental change can be scaled to body size; that is, the smaller the organism and the faster its growth rate, the shorter the relevant time scale for response. Thus changes in phytoplankton community structure may occur within a few days, whereas those of fish communities may extend over years or decades. One of the most difficult tasks for managers is to separate changes due
to natural variability from those due to human activity (which are potentially controllable), and to predict response to natural or anthropogenic environmental perturbation.
In most estuarine systems, phytoplankton are the dominant primary producers and the principal source of food for both the zooplanktonfish trophic chain and the bacterial-based microbial food web. For this reason, changes at the phytoplankton level can have wide ranging effects on pelagic trophic structure. Elevated primary production stimulates increased microbial decomposition, with accompanying demands on dissolved oxygen. Resulting hypoxia and anoxia restricts habitat for plankton, fish, and shellfish, with consequent mortalities in many species, and enhances release of nutrients from the sediments. Reduction in water transparency due to increased algal biomass has also been linked to recent losses of submerged vegetation in the Chesapeake Bay.
Long-term changes in the relative dominance of various phytoplankton groups affect higher trophic levels and the direction (and efficiency) of transfers within food webs. Very small cells, which predominate in the Chesapeake Bay, cannot be readily ingested by many grazers (such as oysters or copepods). It has been hypothesized that nutrient enrichment may selectively enhance these small forms, and that this increased dominance by nanoplankton in turn favors gelatinous zooplankton (comb jellies and medusae). Because growth efficiencies of these "jellies" are so low, much of their ingested material is released as dissolved nutrients. In this respect they may represent a trophic "dead-end", with little material being passed on to higher levels. As eutrophication increases algal biomass and production while decreasing its usability by larger zooplankton or benthic species, more organic material will be cycled through the microbial loop. There is growing evidence that this process may now be operating in the Chesapeake Bay, but whether this represents a recent change in trophic structure is still uncertain.
When food limitation is less significant, predation may be the major process regulating population abundance. Predators ranging in size from small protozoans to large carnivorous fish have specific prey preferences. Selection can influence the species composition of prey communities, and thus the structure of pelagic food webs. Although the importance
Executive Summary 5
of this process in marine ecosystems is uncertain, the concept of cascading trophic interaction as a dominant force regulating fresh water systems is well established. For example, removal of piscivorous fish can lead to increases in planktivorous species, declines in zooplankton biomass, enhanced phytoplankton densities, and decreased water clarity.
Relevance to Restoration and Protection Activities
There is a superficial similarity between this scenario and that of the Chesapeake Bay. Reduction in carnivorous species such as striped bass coupled with apparent increases in plankton-feeders such as menhaden is suggestive of top-down control. Estimates based on filtering capacity of both menhaden and benthic suspension feeders such as oysters indicate that at high population densities these planktivores can significantly alter phytoplankton abundance and even species composition. For example, oyster numbers present in the early 1900's may have been capable of filtering the entire volume of Chesapeake Bay in three days. During summer months, gelatinous predators significantly reduce zooplankton populations and may also affect larvae of species such as oysters. Because nutrients released by these forms can stimulate phytoplankton growth, their role in structuring pelagic food webs may be considerable. Selective predation by both crustacean and microzooplankton grazers also exerts control on abundance and composition of lower trophic levels.
Thus each marine pelagic trophic level is implicated in regulating the abundance and structure of one or more adjacent levels. Whether long-term changes observed (or postulated) in Bay pelagic communities are due to natural causes such as climate or to anthropogenic impacts is not readily determined, in part because of lack of sufficient long-term quantitative data. Human activities perturb natural controlling mechanisms from below through nutrient loading or from above by harvest of predatory or planktivorous species. It might therefore be hypothesized that eutrophication and a collapsed predator base are both driving changes in pelagic trophic structure of the Chesapeake Bay, and that these processes work in concert with natural fluctuations in the estuarine environment to produce observed trends.
Future Directions
These conclusions rely heavily on inference and emerging new ideas rather than rigorous scientific examination, and are thus best regarded as hypotheses in need of further testing. Important questions include:
Several of these questions represent generic research needs, but they are particularly applicable to processes regulating food webs in the Chesapeake Bay. The biota of the Bay will change as human beings continue to modify the environment. As management and restoration of the Bay proceeds, a unique opportunity exists to study and document the interactive effects of physical and biological factors controlling pelagic trophic structure. This will require a multifaceted approach incorporating data collection and modeling. Previous studies have demonstrated that results from one part of the estuary may not be valid in other portions. Intensive field sampling and in situ experimentation should be conducted on pertinent temporal and spatial scales to determine biological response to environmental variation. Baywide monitoring programs should continue to provide the long-term data necessary to identify the cause and significance of trends. Finally, these efforts should be coupled to multi-trophic level studies (e.g., mesocosms) and simulation models to test hypotheses in detail. This approach, which will require significant financial and managerial commitment, will quantify the functional relationships between nutrient supply and fish production. Thus we will increase our understanding of the factors that regulate pelagic trophic structure and improve our ability to predict changes in that structure.
GAIL MACKIERNAN
Summary of Major Findings
This chapter surveys the literature on circulation and mixing as it applies to the Chesapeake Bay. The approach is to consider processes in terms of their time scale: seasonal, short-term, or short-period. The seasonal processes are predominantly long-term fluctuations (greater than one month) in heating and fresh-water runoff. The short-term processes (from a tidal cycle up to one month) include fronts, wind, tides, and their variations, as well as the interactions with the shelf circulation. The short-period processes (less than a tidal period) are the dominant mixing mechanisms that effect the vertical exchange of properties in the estuary. There is a tendency for long-period processes to have large spatial scales and for shortperiod events to be smaller. Hence this organization by time scales also tends to sort by size as well. Often larger-scale, slower processes are easier to study, so we have more knowledge about the mean and seasonal circulation and some fairly good insight into the short-term processes. The shortperiod mixing has only recently been observed, and is still not quantified.
All the different processes interact. The mean circulation moves the fresh water down the estuary, setting up the stratification, which in turn affects the amount of vertical mixing. The vertical mixing is driven by the energy that is available from the tides, wind, internal waves, and mean circulation. The amount of vertical mixing helps determine the vertical density profile, which affects the downstream pressure gradient and hence the intensity of the mean circulation.
Relevance of Restoration and Protection Activities
Surprisingly little work has been performed in the Chesapeake proper. Although much of the fundamental work on estuarine circulation is associated with the Bay, it was predominantly performed in the tributaries. The Bay is large enough to have modes of motion that are not possible in the more restricted rivers. Our picture is incomplete, and we must understand the circulation and mixing in the Bay better before we can model the system satisfactorily. Even the classic two-layer circulation is now coming
into question as better instrumentation becomes available to profile the currents.
Our needs for understanding of the circulation and mixing of the Bay are rather serious. We have the basic understanding of the circulation that was fundamental for the initial planning of the Bay cleanup. There is also a need to understand the causes and intensity of the fluctuations in the circulation. There is some insight into this problem, but we are a long way from being able to anticipate climatic variations and predict their effect in the Bay. Finally, the most stringent demand will be the coupling of the physical and biological systems to predict the fluctuations in both water quality and biological productivity.
Future research on the physical oceanography in the Chesapeake Bay must focus on identifying and understanding the processes that drive the circulation and mixing. Towards this end two types of work are needed.
Future Directions
Long-term measurements with modern remote sensing and profiling instruments are needed. New techniques for remotely measuring the surface currents with radar backscatter from shore-based stations should be combined with new acoustic profiling current meters mounted on the bottom of the Bay. This type of information, in conjunction with satellite remote sensing and data gathered on the many research cruises in the Bay, could start to quantify the physical processes over a wide range of scales.
Second, multi-disciplinary studies of specific processes should be performed. Specific experiments should focus on distinct problems such as the development of anoxia, the interaction of physical processes and larval recruitment, and the influence of wind on primary productivity.
Although much past work in this region has focused on estuarine circulation, there is at present insufficient work, and insufficient support, for the basic research into the physical processes in the Bay. Our ignorance will limit our ability to plan wisely, to monitor the situation, to interpret the observed variations, and hence to maximize the return on our investment in the Bay.
-THOMAS OSBORN
Recent advances in scientific methodology, especially in the area of genetics, are providing researchers with the tools to analyze heritability of traits in greater detail than ever before. New technologies are also bringing the ability to modify, or "engineer", the genetic composition of organisms. These new sources of information are greatly expanding our understanding of the structures of populations, isolation or mixing of genetic materials, species complexes, the influence of environment upon genetic expression, and many other aspects of relationships at the organism and population level.
This chapter presents a comprehensive review and synthesis of the emerging place for molecular genetics as a fundamental tool in the evaluation and management of biotic systems such as the Chesapeake Bay. Of particular interest is the discussion of approaches to conservation and resource management through the application of genetics. The authors recognize the potential for protection and enhancement of stocks while citing the need for caution and consideration of the resulting impacts of such manipulations on the ecosystem. The use of genetic monitoring in hatchery or breeding programs can be effective in detecting deleterious changes or traits in organisms being developed for release into natural systems.
The chapter also addresses various aspects of methods for genetic assessment, including selective breeding and molecular techniques such as DNA sequencing and mitochondrial DNA analysis. Brief descriptions are given of the approach and rationale for using each of the methods presented.
Summary Of Major Findings
The bulk of the chapter concerns the status of genetic knowledge for selected species of recognized importance to the Chesapeake Bay. Finfish examples stress the striped bass; invertebrates selected include the blue mussel, hard clam, and oyster. Notable by its absence is the blue crab, for which very little genetic analysis has been conducted.
The authors conclude that species in the Bay have apparently undergone some differentiation in response to varying conditions from the upper reaches to the Bay mouth but none of the species studied to date have produced highly localized populations. Thus it appears that the various species may be treated as single units for management purposes, and reliance on captive broodstock for enhancement programs should probably not introduce defective traits.
The authors suggest approaches for the enhancement of striped bass and oysters while emphasizing that less-recognized species provide food or other important ecological linkages.
Relevance to restoration and protection activities
Ultimately, genetic information and the technology for manipulating the genetic composition of organisms will be considered as tools for modifying the structure of aquatic communities, including the Chesapeake Bay. In fact, such modification is under way in the case of the striped bass and its hybrids, which are being created with either white bass or white perch.
The potential exists for resource managers to have both positive and negative effects upon the structure and functioning of a system such as the Chesapeake Bay. Because molecular geneticists have only recently begun to delve into genetic modifications on a large scale, there is much undiscovered information concerning the biotic and abiotic interactions of manipulated organisms, especially as they might affect the stability of an ecosystem. With this cautionary note in mind, we should nevertheless continue to explore the potentials presented by this emerging technology so that the ecosystem may be better able to withstand perturbations. Questions include:
The rapid expansion in genetic technology suggests that within the next decade we will be able to manipulate the genetic composition of most species found in the Bay. The manner and degree to which this capability is applied to managing the dynamics of the stocks is a matter of growing concern, which should be addressed in anticipation of the desire to use the emerging technology. At the very least, fisheries
8 Executive Summary
management plans should begin to take into account the likelihood that genetically manipulated striped bass and oysters will be available for release into the Bay.
Necessary research and/or activities
The chapter authors point to the need for a variety of research efforts as well as genetic monitoring of hatchery-produced organisms. Also, readers of the
chapter will readily recognize that the body of literature upon which the synthesis is based is not extensive. Thus it is obvious that genetic technology is opening many avenues of research and potential applications in management. Perhaps some of this chapter's readers will build upon its information to enhance their use of genetics in the development of Chesapeake Bay resources.
-WILLIAM RICKARDS
Summary of Major Findings
This chapter summarizes many of the processes in estuaries affecting bioavailability of compounds present in estuaries. Chemical factors discussed include ionic strength, inorganic and organic speciation and complexation, redox reactions, and production of organo-metallic compounds. A second area of discussion includes contaminant sources and processes affecting distribution including the microlayer, adsorption to particles, flocculation, sedimentation, and remobilization from sediments. A third area is biological processes affecting bioavailability, including uptake routes, food chain magnification, organism reaction through metallothionen production, biological transformation, degradation through biological processes, and chemical and physical modification of the environment by biota. The summary delineates six areas for further research including partitioning, the role of biota in transfer of pollutants, the effect of seasonal anoxia, sediment flux and movement, the role of communities, and descriptive studies of the effects of toxics.
Relevance to restoration and protection activities.
The subject of toxics in estuaries is controversial and complicated, and this chapter correctly states the difficulties of knowing all the factors affecting transport, availability, uptake and impact of pollutants. It answers in part the question why the many toxic materials detectable in estuaries are not more effective.
Because the areas of ignorance greatly exceed those of knowledge at present, restoration and protection activities concerning toxics in estuaries must be confined to the few specific cases where toxic effects have been found. Unfortunately, although chronic, sublethal, and synergistic effects of toxics on estuarine biota are suspected to be important, little is known about them. To be realistic and cost effective we must take a scientifically broad approach towards deciphering the actual role of toxics in estuaries. This will require basic, biologically-oriented research by experts, which is not favored by managers but is the best way to discover the principles permitting effective detection and control of toxics in estuaries.
Necessary research and/or activities
Questions to be considered in future research activities include the definition of an estuarine toxic material. Are estuarine toxic materials those chemicals known to be toxic to humans who come into contact with them through food or water activities? Or are estuarine toxics defined through their effects on estuarine biota? This distinction is critical for determining the nature of the research and mediating activities to be done. If the definition is the latter, then the response of the biota (molluscs to organotin, fish to creosote, plankton communities to copper, etc.) must be examined in detail through sophisticated biomonitoring. Life cycles must be analyzed to detect impacts on early life-history stages. Physiological responses of estuarine animals must be studied to determine chronic and sublethal effects attributable to toxics. Monitoring of the health of estuarine biota must include the incidence of susceptibility to parasitism and other effects possibly synergistic with the presence of toxics.
If an estuarine contaminant is defined as that which is toxic to human beings, this initial research should establish the routes from the estuary to human populations and the probabilities of exposure as well as toxicological effects.
Once a substance has been defined as toxic in the estuarine system, the chemical and physical mechanisms governing bioavailability can be researched by using present or as-yet-undeveloped methods of chemical analysis. Natural mechanisms of reducing bioavailability and toxicity of the substance, such as capping of contaminated sediments, must be studied. Finally, laws should be drawn up and put into place to control or mediate the toxic. This has been the successful strategy in the past for handling of toxics in estuaries.
The full cost and time needed in the past for discovering and remedying problems due to estuarine toxics such as power plant chemicals, Kepone, TBT, and DDT must be examined and studied in detail if we are to make any serious recommendations about the handling of future and potential toxics.
With a careful combination of basic and applied research, admitting our ignorance and working from historical examples, perhaps we will achieve a successful strategy for detection and control of toxics in estuaries, our most impacted ecosystem.
-HARRIETTE PHELPS
10 Executive Summary
CONCLUSION
In order for research activities in estuarine processes to contribute to the development of effective Chesapeake Bay management strategies, the scientific findings must be communicated to managers and the public. It is equally important that the scientific community understand management priorities and concerns.
The Chesapeake Bay Program is the most ambitious estuarine management program ever attempted. The Chesapeake Bay system is hydrologically complex and contains a wide diversity of habitats. The land uses of the watershed cover the full range of
human activity. In addition, management is complicated by different political jurisdictions with differing philosophical approaches to resource management.
The scientific and management communities in the Bay region are faced with a strong public and political desire to "do something about the Bay." To respond to this desire, the scientific community must be prepared to address management questions with state-of-the-knowledge scientific insight. This series of papers, each in an area directly relevant to processes critical to our understanding Cheapeake Bay, should provide some of that required insight.
-MAURICE P. LYNCH