Download Estuaries are also a focus for impacts from human activities, like

Environment on the Web
Essay: Changing Trophic Structure in Estuaries Affected by Human Activities
Estuaries—the zones where rivers empty into the ocean—are among the most productive ecosystems on Earth, with the most
complex food webs. The combination of terrestrial habitat, fresh water and saltwater, the tidal flux of nutrients and other materials,
and the flushing action of ocean currents create a system in which many habitat types can coexist. This diversity of habitat in turn
allows the coexistence of a wide range of species that would not normally be found together.
Estuaries are also a focus for impacts from human activities, like dam building, in upstream areas. Such practices can fundamentally
alter downstream ecosystems through habitat degradation, blockage of migratory routes, and increased pollution levels. Think, for
example, about the food web of the harp seal. Although it is not strictly speaking an estuarine species, the harp seal feeds on a wide
range of organisms at several trophic levels, some of them estuarine in adult or juvenile forms. (A migratory species, the harp seal
spends time both in near-shore waters and in offshore waters and thus is tied to different marine ecosystems.) This food web is
complex, and perhaps because of its complexity is able to withstand minor stresses imposed by fluctuating water-quality conditions,
temperature, and flows. However, when major estuarine flow and sedimentation changes occur, as would be the case when a dam
is built in a river that flows to the coast, habitat alteration may be so significant that some species no longer thrive and may die out
locally. In particular, as the reduction of sediment loads to downstream estuaries decline, so do the number of organisms that
depend on it for habitat through some or all of their life cycles. These can include sediment-dwelling worms and other invertebrates,
but also higher level consumers, such as fish, which require a particular type of substrate for effective spawning and rearing of
young. Even modest changes in the number and abundance of sediment-dwelling polychaete worms, for example, can have a
“snowball” effect, reducing the number of flatfish and sand lance (a small fish). As species at higher trophic levels fail to find enough
prey, their own numbers will decline, while competing species, such as capelin and redfish, flourish. The net result is a simplified
food web, now dependent on a few “keystone” species. And although the harp seal itself, which feeds on a variety of prey, may be
largely unaffected in such a system, the food web overall is now much more vulnerable to the effects of other temperature, climatic,
or chemical stressors.
Case Study: The Three Gorges Dam
The essay on page 50 describes the challenges of ecosystem restoration following disruption by human influences. Dam-building is
one such disruptive influence. Dams are built for several main reasons: (1) to control flooding (by containing excess flows until it is
safe to release them); (2) to store water for irrigation; and (3) to generate hydroelectric power. (Hydropower has long been a
preferred method of generating electricity, because it requires no fuel and can tap the energy stored in flowing water.) Historically,
it has been believed that hydroelectric power also has minimal impacts on the environment, compared, for example, to coal-fired
thermal power production. More recently, however, scientists have become aware of a number of environmental effects associated
with dam building, including release of toxic methyl mercury into food chains and production of greenhouse gases.
The most obvious and dramatic impact of dam building relates to habitat destruction, which occurs when large areas of terrestrial
habitat are inundated by the newly created reservoir behind a dam. Associated with this inundation, and the reduced flows
downstream of the dam, are changes in river and coastal erosion processes. As flows are reduced, the natural processes of eroding
and redepositing sediment are also altered. Sediment that would normally flow down the river and into downstream deltas and
estuaries is trapped behind the dam. As a result, natural wetlands in the delta region are starved of sediment and may shrink and
gradually disappear. As wetlands and deltas shrink, longshore coastal currents can move faster, increasing coastal erosion and
carrying eroded sediments out to sea.
The impacts of dam and reservoir building have been particularly devastating for anadromous (ocean-going) fish such as salmon.
These fish spawn in fresh water, then return to marine systems to grow to maturity. Many species have very specific requirements
for spawning habitat, particularly the coarseness of river sediments. Without appropriate habitat, these species will not spawn or
rear young successfully, and populations will gradually decline. Recent evidence suggests that over 100 western U.S. populations of
commercially valuable salmon and steelhead are now extinct, and many others are in decline. A major cause is thought to be dam
construction, with associated modification of flow and sedimentation regimes, reduction in nutrient inputs (because nutrients also
are trapped behind dams), and increased levels of toxic contaminants such as mercury. Physical habitat alteration, as wetlands dry
out and deltas shrink, is another key factor.
Dam building has, of course, also had many benefits for human society, including flood control and reliable generation of electric
power. Dams can also be used to store water during wet periods and release it in droughts, allowing farmers to continue with
irrigation and protecting downstream biota that would otherwise have been affected by dry conditions. Our growing understanding
about the impacts of dam building can be used to create dam management regimes that more closely mimic natural conditions. It
may also point the way to dismantling older dams and channel structures, where those structures are thought to have created more
impacts than benefits for downstream ecosystems.
China's Three Gorges Dam project, six times the length of the Hoover Dam, is thought to be the largest civil-engineering project of
this century, and one of the largest ever. The project is expected to provide extensive flood control and clean electric power
generation. But it will also alter water temperatures and greatly reduce downstream flow speeds and deposition, instead trapping
water and sediment behind the dam.
The Yangzte River, site of the Three Gorges Project, is a major watercourse with diverse habitat. At present, hundreds of fish species
live in the river, many of which are considered worthy of special protection. Proposed operating procedures call for the dam to store
water in October, at a time when saltwater intrusion into the river from the estuary is at its peak. Without the current level of
freshwater flows to dilute the saltwater, saline conditions will prevail over a much wider area, degrading farmland and creating
conditions inhospitable for many fish species.
At present, more than half a billion tons of sediment currently flow to the estuary, where they settle in a rich delta plain, creating
the diverse and productive habitat typical of estuaries. When the dam begins operation (currently scheduled for 2009), more than
70% of these sediments will be trapped behind the dam, starving the delta and allowing eroding ocean currents to attack the
coastline.
The Three Gorges Dam project is expected to have devastating consequences for species like the Chinese sturgeon and Chinese
paddlefish, which are already rare, primarily through habitat destruction. The breeding period for some fish species will be delayed
because of altered water temperatures, and young fish will be unable to migrate downstream to lower reaches. Migratory routes for
other rare species, such as the white flag dolphin, will be blocked. The net result of dam operation may therefore be a reduction in
the number and abundance of top carnivore species, allowing a smaller number of species to dominate. This will in turn alter
pressure on prey species and cause a ripple effect through the food web. In the view of Chinese authorities, these changes, although
regrettable, are justified because of the project's energy and flood-control benefits.
Human activities in a watershed can increase the rate at which sediment is eroded and transported downstream to
estuary systems. Once at the river mouth, excess sediment can impair beneficial water uses and impact fish and
wildlife habitat. Click on Earth from Space to access National Air and Space Administration images of Earth's surface.
Select "Earth's Water Habitats" from the thumbnail images presented on this page. The link will take you to a menu
of options for viewing water images. Select "Estuary" from the drop-down menu and choose "Thumbnail and Text"
from the options at the right of the page. Then press "Start Search." A long menu of small ("thumbnail") photographs
will appear. Scroll down the page until you reach the image of the Ganges River Delta. Click on the image to bring up
an enlarged version. Examine the enlarged image closely, with the following questions in mind. Then return to the
list of thumbnail images (press "Back" on your Web browser) and select the image for the Corpus Christi, Texas.
Repeat the sequence of enlarging and inspecting the image, then return to the menu once more and select the image
for the Irrawaddy River Delta, Burma (Myanmar). (If the images on the menu are not as described in this exercise,
select any two or more infrared images of estuary systems.)


Notice that in these infrared images, red usually indicates regions of plant growth, with darker red showing
dense natural vegetation and lighter reds showing cultivation. Notice also that water is shown in varying
shades of blue. Describe the pattern and extent of light blue (indicating that the water contains a lot of
sediment) in each photograph. Now observe the location of dark red patches along the coast, indicating the
presence of estuarine habitat such as mangroves. Comment on the potential for excess sediment transported
down the rivers to come into contact with estuarine vegetation, including wetlands and mangrove forests.
Now imagine that a dam has been built on the rivers upstream of these estuaries. How might these images
look with a dam in place to trap sediment and slow downstream flows? (Hint: Think about the changes in
image color that would result and what those changes would mean for the estuary ecosystems.)
Click on Marine Species to access notes about typical trophic relationships in the coastal environment of
Everglades National Park, Florida.

Using the information on this page as a general reference, discuss how the removal of certain plant species
(for instance, through burial by increased sedimentation) could affect the food web in the ecosystem
described in this page.
Scroll to the bottom of the page and click on the highlighted words Everglades Hydrology under the
heading Additional Resources. Read the article about the history of the Everglades and the impact that
humans have had on that system. What have been the primary forces in shaping modern Everglades
hydrology? Discuss these forces and the changes that would be necessary to restore the Everglades to its
"natural" condition.

Click on Colorado pikeminnow to access a Fish and Wildlife Service site relating to the protection of the
endangered Colorado pikeminnow, one of the largest minnow species in the world. Read the text at the
beginning of the page, then continue down the page until you get to Recovery Strategies. Although this site was
developed for a particular species, it has relevance to a great many fish species in ecosystems all over the world.

For each recovery strategy, suggest possible associated habitat responses, and describe in general terms
how the activity would be likely to affect (or not affect) primary producers, small animals (primary
consumers), and large animals (higher level consumers).
For at least one strategy, suggest one structural and one nonstructural response to mitigating the impacts
of the activity. A structural response is one that involved building a new structure or device, or using
different construction methods for an existing structure or device. A nonstructural approach is one that
asks people to change the way they behave, for instance, the timing of certain actions, or the ways they
dispose of waste materials.

Thinking Environmentally:




From local, national, and international news, compile a list of the many ways humans are altering abiotic
and biotic factors on a local, regional, and global scale. Analyze ways that local changes may affect
ecosystems on larger levels and ways that global changes may affect local levels.
Write a scenario of what would happen to an ecosystem or to the human system in the event of one of
the following: (a) All producers are killed through a loss of fertility of the soil or through toxic
contamination. (b) All parasites are eliminated. (c) Decomposers and detritus feeders are eliminated.
Support all of your statements with reasons drawn from your understanding of the way ecosystems
function.
Consider the various kinds of relationships humans have with other species, both natural and domestic.
Give examples of relationships that (a) benefit humans, but harm other species, (b) benefit both humans
and other species, and (c) benefit other species, but harm humans. Give examples in which the
relationship may be changing—for instance, from exploitation to protection. Discuss the ethical issues
involved in changing relationships.
Explore how the human system can be modified into a sustainable ecosystem in balance with (i.e.,
preserving) other natural ecosystems without losing the benefits of modern civilization.
Copyright © 1995 - 2010 Pearson Education . All rights reserved. Pearson Prentice Hall is an imprint of
Pearson .