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.