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Estuaries: where the river meets the sea.
Abstract:
Scientists have discovered that the physical, chemical and biological factors operating in a complicated and indecipherable balance have enriched estuaries such as Chesapeake Bay, and that this balance can be altered and drastically changed by humans, most often to their disadvantage. An estuary, a semi-enclosed coastal formation of water which possesses a free bond with the open sea, usually contains seawater that is noticeably diluted with fresh water from land drainage.

Subject:
Estuaries (Environmental aspects)
Author:
Boicourt, William C.
Pub Date:
06/22/1993
Publication:
Name: Oceanus Publisher: Woods Hole Oceanographic Institution Audience: Academic Format: Magazine/Journal Subject: Earth sciences; Environmental issues Copyright: COPYRIGHT 1993 Woods Hole Oceanographic Institution ISSN: 0029-8182
Issue:
Date: Summer, 1993 Source Volume: v36 Source Issue: n2
Accession Number:
14234458
Full Text:
As the world's population has grown and moved toward the sea, the oceanic effects of this progression have been felt first and most acutely in the estuary. Perhaps this should come as no surprise, for the estuary is where the river, with all its waterborne materials draining off the altered landscape, meets the sea. But there is more than proximity at work here. The estuary is nearly a world unto itself, buffered from a strong marine influence by a controlled communication with the ocean, and protected by enclosing coastal boundaries. Within this domain, the estuary's unique water motion retains and recycles nutrients essential to living organisms, inducing the richest productivity per square kilometer on the earth's surface. Labeled the "protein factory" by writer H.L. Mencken, an estuary such as Chesapeake Bay is made bountiful through the workings of physical, chemical, and biological engines in complex and sometimes mysterious balance. We are learning that these balances can be very delicate, and that humans can tilt and dramatically shift the outcome, usually to their own detriment. We are also learning that the estuary forms only a fragile line of defense for the coastal ocean.

The gradual rise in sea level that created estuaries from ancient river valleys, glaciated grooves in the landscape, or faults in the earth's crust also produced natural harbors. These were fortuitously situated where maritime commerce could connect to the hinterlands via river highways. Cities grew, creating "urban estuaries" threatened not only by unnatural amounts of nutrients, sediments, and toxic materials drained from the watershed, but also by the concentration of human activity on the edge of major fish and waterfowl habitats. These estuarine habitats support both native and transient populations. Because their shallow marshes and seagrass beds serve as spawning grounds and nurseries for mobile fish as well as havens for migrating birds, their influence extends far beyond the confines of the local estuary.

A quick tour of the coastal US reveals how many of the larger estuaries have become urban. Starting in the northeast, the large estuaries of the Gulf of Maine, such as Penobscot Bay, are relatively rural. Boston Harbor represents both an urban water body and an environmental cause celebre. It is, however, just barely an estuary. Narragansett Bay, not far to the south, is a genuine estuary flowing in a series of glacial grooves, loosely separated by islands, with the city of Providence located at its head. The rivers that drive Long Island Sound's estuarine circulation discharge along its broad northern flank. At its western end, Long Island Sound is connected to the most urban of estuaries, the Hudson River, via the East River. The Hudson River and most of the larger estuaries to the south are river valleys drowned by the rise in sea level that followed the last ice age.

From Delaware Bay south, the river valleys were cut from low-lying, coastal-plain sediments. Delaware Bay seems placidly nonurban in its lower reaches, where it opens like the bell of a trumpet toward the sea. Extensive salt marshes border the broadening estuary. Immediately upstream, the Delaware River connects the Bay to Wilmington, Philadelphia, and Trenton. In Chesapeake Bay, the largest estuary in the coastal US, there are locales where the view in all directions is nearly as unspoiled as in the days when Captain John Smith first set eyes on these waters. But the bay and its tributaries reach to Baltimore, Washington, Norfolk, and nearly to Richmond. The bay is so large that it is not just one estuary, but a 300-kilometer-long backbone connecting a series of tributaries such as the Potomac, Rappahannock, York, and James rivers, each robust estuaries in their own right. South of Chesapeake Bay, the broad North Carolina sounds and the estuaries and tidal marshes of the South Atlantic Bight are comparatively nonurban. The estuaries of Wilmington (North Carolina), Charleston (South Carolina), Savannah (Georgia), and Jacksonville (Florida) are modest exceptions. On the Gulf Coast, long stretches of shallow, barrier-island lagoons are punctuated by truly urban estuaries, such as Tampa Bay, Mobile Bay, the lower Mississippi River, and Galveston Bay (connected to Houston via the Ship Channel).

On the West Coast, the steep, geologically young, California coastline has precluded the development of estuaries. The major exception is a notable one, both for its preeminent status as an urban estuary, and for its manner of creation. San Francisco Bay, crossed by the San Andreas fault (which parallels nearby Tomales Bay estuary), was formed primarily by movements of the earth's crust. To the north, the estuary of the mighty Columbia River, vigorously stirred by strong tides, appears nearly wild, despite commercial navigation upstream to Portland. Human control of natural river flow, however, has dramatically altered annual cycles. Our tour of the coastal US ends in a water body that is both urban and a fine example of a fjord-type estuary, Puget Sound. The glaciers that carved the deep channels now connecting Seattle and Tacoma to the sea left mound of rubble at their feet. Not far seaward from Seattle, this mound acts as a sill, and salt water entering from the ocean must flow over it.

It is now clear that not just urban estuaries, but also wild and apparently pristine estuaries, face accelerating human stress. In the few locations where this threat is being met with action to restore and preserve the resource, costs to society are proving very dear. For instance, the US Environmental Protection Agency estimates that over $600 million is spent annually toward cleaning up Chesapeake Bay. Developing and maintaining the political will to dedicate these kinds of monies for environmental remediation has not been easy. In the process, the role of science and scientists has at times been central, while at other times, it has seemed peripheral or even irrelevant. Regardless of this history, scientists are now increasingly challenged to identify key environmental problems, help formulate solutions, and provide ongoing guidance to restoration efforts. Billion-dollar decisions often hang in the balance. In the face of this challenge, scientists seldom feel comfortable with the present state of knowledge, nor are they encouraged by the complexity of the system revealed by recent research. Fortunately, in their attack on the multifaceted interrelationships, they are empowered by new tools, not only to observe the estuary, but also to separate, analyze, and recombine the individual processes that make up the working whole.

What is an Estuary?

Before discussing estuaries further, perhaps we should define the term "estuary." Dictionaries are of little help here. They talk of tides, and rivers, and the sea, but the definitions range widely and never converge on unifying characteristics. Scientists have embraced the definition offered by D.W. Pritchard (Professor of Oceanography at Johns Hopkins University and Director of Hopkins' former Chesapeake Bay Institute, and later a professor at State University of New York at Stony Brook):An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which seawater is measurably diluted with fresh water derived from land drainage.

Now this may sound a bit intricate and overly precise, but it is carefully crafted to reflect the fundamental conditions that lead to a unique pattern of estuarine flow. Partial enclosure is necessary to provide a constrained pathway along which fresh water and salt water mix. In a coastal embayment lacking these side boundaries, buoyant river water spills out and spreads broadly over the denser seawater, at the mercy of the winds and forces arising from the earth's rotation. If seawater is not diluted by fresh water, then there is no mixing, and without mixing there are no spatial differences in density to drive the characteristic estuarine circulation.

When these conditions are satisfied, river water entering the estuary moves seaward, floating over heavier salt water entering from the adjacent ocean. As it moves, the fresh water mixes with seawater in the lower layer, and becomes progressively saltier toward the mouth of the estuary. Conversely, seawater moving toward the head of the estuary loses some of its salinity in the process. This interplay of lighter fresh water and heavier salt water seems intuitively easy to comprehend. But there is more. As the fresher water moves seaward in the upper layer, the initial river flow is joined by more and more water moving up from the lower layer. By the time it reaches the mouth of the estuary, the flow discharged by the upper layer to the sea may be six to ten times the flow of the river discharge at the head. In the lower layer, the flow of seawater entering the estuary at its mouth is nearly as strong as the outgoing flow above. How can this be? What drives this dynamic circulation? It almost sounds as if we are getting something for nothing. It turns out that the energy for this amplification comes from the winds and tides. These forces create turbulence, which vigorously mixes fresh water and seawater, thereby creating spatial gradients in water density that drive the two-layer estuarine motion.

As might be suspected from the discussion of influences on estuarine circulation, the global variety of land forms and rivers produces a broad spectrum of estuarine types, with remarkably different realizations of the defining circulation. At one end of the spectrum is the salt-wedge estuary, where wind and tidal mixing are relatively weak, and where the water entering from the sea forms a "wedge" of undiluted salt water under the outflowing fresh water. Salt transfer is primarily one way, from the lower layer to the upper layer. The lower Mississippi River is an example of a salt-wedge estuary. At the other end of the spectrum is the well-mixed estuary, where tidal and wind mixing are sufficiently active to nearly eliminate top-to-bottom differences in salinity. In the middle range of estuarine types is the partially mixed estuary, where wind and tidal mixing transfers salt water and fresh water in both directions, but still allows significant stratification, or vertical changes in salinity. The Chesapeake Bay and most of its tributaries are partially mixed estuaries.

The consequences of this somewhat mysterious estuarine circulation to the plants and animals that inhabit the estuary are manifold. First of all, most of them owe their very existence to this water motion because the flow helps retain nutrients and sustain the remarkable productivity typical of estuaries. Many estuarine species that have weak-swimming life stages rely on horizontal flow to move great distances, from spawning grounds to nurseries within the estuary, or from far reaches of the continental shelf into the estuary. The blue crab Callinectes sapidus, for instance, releases its eggs in late spring near the mouth of Chesapeake Bay. All summer, crab larvae drift over the continental shelf, growing from stage to stage. In early fall, some are returned to the estuary, carried into and moved up the estuary by the lower-layer estuarine circulation. The eastern oyster is often found far upstream from its spawning grounds. Oyster larvae, like most planktonic organisms, can't swim very far, but they make the most of their abilities by swimming or sinking, thereby taking advantage of the upstream flow in the lower layer of the estuary.

In the upper reaches of many estuaries there is a region called the turbidity maximum, where sediment suspended in the water reduces clarity to the point where a diver literally cannot see a hand at arm's length. This feature is a result of the estuarine circulation, where fine-grained sediment delivered by the river is retained and resuspended many times by the ebb and flow of tidal currents. Because many toxic substances are attracted to the surfaces of these sediment particles, some fear that such trapping processes created by the two-layer flow might produce local regions with unacceptably high levels of contamination.

Assessing Human Impact on the Estuary

In order to restore and preserve our estuaries, we must be able to detect and evaluate changes wrought by human activity. In the face of accelerating environmental stress, our ability to assess trends in the health of estuarine waters has been woefully inadequate, and almost always retrospective. We are learning that human impacts on the estuary, which should be among the most detectable in all the ocean, do not appear as sudden, obvious jumps in measured signals. They are typically subtle, creeping changes in sometimes unexpected indicators, slowly manifest over many decades.

In the presence of "noise" from the more vigorous short-term variations produced by storms and the seasonal progression, many of these impacts go undetected. For instance, scientists have known that the deeper waters of Chesapeake Bay are regularly depleted in oxygen each summer. A decade ago, some suspected that the intensity and areal extent of this anoxic region was increasing, and that this increase was caused by overstimulation of the food chain by introduction of more and more nutrients--especially nitrogen and phosphorous--from municipal sewage and water draining off farmlands. But though the historical record of anoxia seemed long (30 years), it proved so noisy and full of gaps that scientists could neither agree on its interpretation, nor unambiguously detect a trend. As a result, more than a decade of further measurement and analysis has been necessary to form a consensus that man has indeed deleteriously altered the balance in this estuary.

Perhaps an equally disturbing trend in Chesapeake Bay has been the wholesale decline of seagrasses, which provide nurturing sanctuaries for juvenile fish and crabs, and food for waterfowl. Because the start of this decline was coincidental with the introduction of agricultural herbicides in the early 1970s, the cause seemed obvious to scientist and layman alike. However, research showed that it was probably not herbicides, but a surfeit of nutrients that provoked destructive algal growth on the surface of seagrasses, robbing them of light necessary for survival.

In San Francisco Bay, the primary problem is less what the river is bringing to the bay than what it is not. Wholesale diversion of the flow from the Sacramento and San Joaquin rivers for municipal and agricultural uses has, over the past century, progressively decreased the amount of fresh water delivered to the bay. This diversion has changed the estuarine circulation within the bay, allowing deeper penetration of salt, and markedly reducing the habitat available for its living resources. In the Columbia River estuary, human control of river flow is also leading to uncertain changes. The grand dams and storage reservoirs that hinder the upstream migration of salmon also greatly alter the seasonal progression of river flow to the estuary.

Unfortunately, these human-induced changes are proving difficult to document, and even more difficult to ascribe to any particular cause. But it is not always the multiplicity of smoking guns that proves most frustrating. Detective work cannot proceed very far without clues, and the trail of clues to environmental change gets cold very quickly. Few monitoring records extend sufficiently far back in time to cover the period of change. Even if changes were clearly understood, linking these changes to effects on fish and waterfowl is often beyond the state of the science. The task of separating the effects of environmental change on fish from great natural year-to-year population variations is further complicated by resource harvest (or overharvest) unpredictability. What is even more worrisome is that it is not a one-way street from an altered environment to effects on fish populations. For instance, some have estimated that there were enough oysters in Chesapeake Bay at the turn of the last century to filter the entire bay water volume through their siphons in less than a day, clarifying the water in the process. With overharvesting and disease bringing the oyster industry to near collapse, this same feat takes almost a year. The resulting turbidity in the water reduces the amount of sunlight penetrating to the depths of the bay, and greatly limits the amount of light available for plants and animals.

Is Science Up to the Challenge?

In the past two decades, scientists have used such classical concepts as the two-layer flow pattern to significantly advance our understanding of the motion of estuarine waters and the interactions among its living organisms. As observational techniques improved, they began to reveal a rich variability and complexity that sometimes appeared at odds with conventional paradigms. A divide-and-conquer approach has been employed during this exploration in an attempt to isolate mechanisms and define their individual contributions to the circulation and to the ecosystem. Important examples are the surprising, sudden changes in flow forced by winds and surges of entering rivers. Subtle residual circulations were discovered, generated by the ebb and flow of tidal currents over an uneven bottom. In larger estuaries, long wavelike oscillations of the water surface, called seiches, akin to water sloshing in a dishpan, were revealed as an important mode of motion (see Oceanus, Spring 1993). Many biological and chemical processes, once thought to be steady or slowly varying, were found to change rapidly and over small spatial scales. Although the divide-and-conquer approach has served us well in shedding light into some of the dark corners of estuarine processes, the revealed complexity has been daunting. As more and more intricate detail is discovered in the myriad components of the system, the task of combining these individual descriptions into a working, interconnected whole seems increasingly difficult. In the face of these complexities, it is all too easy to view estuaries as chaotic, hopelessly unpredictable, and ultimately unmanageable systems. Perhaps worst of all, this view only strengthens the opinion of some who feel that estuarine scientists cannot see the forest for the trees, but see each estuary as unique unto itself, specific to the local situation and sharing few processes in common with other estuaries.

Fortunately, this extreme view, though not altogether unfair, is changing. Scientists, armed with the knowledge gained from focusing on the trees, are beginning to look beyond their own estuaries, to form underlying principles, and to place order in this perceived chaos. This process has been greatly aided by some new weapons, such as acoustic current measurement techniques, remote sensing from satellites, and laboratory simulations of the larval transport of bottom-dwelling organisms. Perhaps the most important development has been the maturing of computer modeling techniques, for these models' ability to combine the multiplicity of individual mechanisms into an accurate description of the entire system is crucial for the support of efforts to restore and preserve our estuaries.

Restoration and preservation of coastal-ocean water bodies is clearly not solely a scientific issue. By the same token, neither is the translation of scientific results into public action solely the province of managers or politicians. Although the communication among these groups has not traditionally been easy, there are some encouraging developments in such urban estuaries as Chesapeake Bay and San Francisco Bay, where the urgency for action is very clear.

The demonstrated success of a few scientific efforts in identifying problems and potential solutions has fostered a sense that scientists care about the application of their results to fulfill societal needs. Scientists, in turn, are beginning to appreciate the workings of managers, politicians, and public advocacy groups to educate and convince the public that action, sometimes expensive action, is required. Finally, both scientists and managers are developing an unaccustomed, but necessary, longer-term perspective on environmental changes in the estuary, which usually span many decades. Evidence for this movement is the cooperation shown in carrying out long-term monitoring of estuarine processes. In Chesapeake Bay, for instance, two complementary monitoring programs are under way, including shipboard surveys and an observing system of sensor buoys and tide-level recorders reporting in real time via radio telemetry. Although the motivation and goals of these efforts are clear, the high costs of maintaining quality measurements and the need to continue them far into the indefinite future require strong support from the community. Scientists, managers, and environmental advocacy groups have worked together to inform the public of the state of the bay, and the necessary action to restore and preserve the resources. In turn, this awareness has been translated through the political realm into support for the monitoring programs. Such partnerships are essential if there is any hope of controlling the environmental consequences of the increased urbanization of our estuaries.

William C. Boicourt is a physical oceanographer at the Horn Point Environmental Laboratory of the University of Maryland's Center for Environmental and Estuarine Studies. He was raised in western Massachusetts, spending summer vacations on Cape Cod, near a large oceanographic institution (from which he later received the B.H. Ketchum Award). For graduate work, he abandoned the rocky coasts and clear waters of New England for the murky waters and salt marshes of Chesapeake Bay. His primary research interests are estuarine and continental shelf circulation, and the interactions between these two water bodies.
Gale Copyright:
Copyright 1993 Gale, Cengage Learning. All rights reserved.