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| Enlarge ImageRIVER PLUMES ON THE GULF COAST—On
Nov. 7, 2004, a satellite captured the outflow of river
sediments and dissolved nutrients into the Gulf of Mexico,
as well as the abundance of algae and phytoplankton in
the water. Dark green or black patches near the shore
indicate blooms of marine plants. White spots are clouds.
Each summer, a stagnant, oxygen-depleted “dead
zone” forms in the middle of the Gulf, likely caused
by a surplus of nutrients from inland sources. (SeaWiFS Project, NASA Goddard, and ORBIMAGE.) |
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| Enlarge ImageOverabundance of nutrients can cause
certain marine plants to grow like weeds, choking off
food sources for fish and shellfish, or literally asphyxiating
the creatures from lack of oxygen. ( ©DigitalVision. ) |
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| Enlarge ImageNEXT STOP, COASTAL WATERS—Pesticides
and fertilizers sprayed far from the coast still find
their way to the ocean, running off farmlands into rivers,
streams, and groundwater. Minor changes in agricultural
practices could save money for farmers and reduce nutrient
pollution in the sea. ( ©DigitalVision.) |
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Related Links |
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Andrew R. Solow, Senior Scientist and
Director, Marine Policy Center
Woods Hole Oceanographic Institution The most widespread, chronic environmental problem in the coastal ocean
is caused by an excess of chemical nutrients. Over the past century, a
wide range of human activities—the intensification of agriculture,
waste disposal, coastal development, and fossil fuel use—has
substantially increased the discharge of nitrogen, phosphorus, and
other nutrients into the environment. These nutrients are moved around
by streams, rivers, groundwater, sewage outfalls, and the atmosphere
and eventually end up in the ocean.
Once they reach the ocean,
nutrients stimulate the growth of tiny marine plants called
phytoplankton or algae. When the concentration of nutrients is too
high, this growth becomes excessive, leading to a condition called
eutrophication.
There is a clear connection between
eutrophication and two significant environmental problems: harmful
algal blooms (HABs) and the depletion of oxygen dissolved in bottom
waters (hypoxia). The effects of both HABs and oxygen depletion are
felt throughout the coastal ecosystem, with direct and indirect effects
on human health, food supplies, and recreation.
For
scientists seeking to understand it, eutrophication is a challenge
because the physical and biological processes linking nutrients and
their impacts are complex. For policymakers seeking to manage these
impacts, the challenge is weighing the economic benefits of the
activities that generate nutrients with the environmental costs of
eutrophication.
Too much of a good thing
In the ocean, as on
the land, photosynthesis combines energy from the Sun, carbon dioxide,
and nutrients such as nitrogen and phosphorus to produce carbon-rich
plant material. This natural process is called primary production and
forms the base of the marine food chain. It also provides most of the
oxygen in the atmosphere. Without primary production, the world would
be a much different (and a good deal less pleasant) place.
But every silver lining has a cloud. Of the thousands of species of
algae, perhaps only a hundred are toxic. When these species occur in
high concentrations, they can color the water and produce what are
popularly referred to as “red tides” or “brown tides.” Scientists
prefer to call these outbreaks harmful algal blooms or HABs. (See The Growing Problem of Harmful Algae.)
Toxic algae enter the marine food chain when they are consumed by small
marine animals called zooplankton and by fish or shellfish. The toxins
that accumulate in these consumers are then passed up the food chain to
marine mammals, seabirds, and even humans, where they can cause illness
or even death.
Blooms of some non-toxic species of algae can
also cause problems. For example, the North Atlantic right whale is in
grave risk of extinction. This species feeds seasonally off Cape Cod on
concentrated patches of zooplankton called copepods. In some years, an
algal species called Phaeocystisblooms in Cape Cod Bay. Although Phaeocystis is not toxic, large blooms essentially clog surface waters and right whales cannot find the copepod patches they need to eat.
Non-toxic HABs include large blooms of seaweed or macroalgae that can
coat beaches, interfering with recreational activities. Other HABs clog
seagrass beds and coral reefs, which provide nurseries for commercially
important fish and support high levels of biological diversity
necessary for a healthy environment.
Harmful algal blooms occur
in every part of the world. In the U.S. and other developed countries,
monitoring efforts and fishery closures have reduced the incidence of
human illness caused by toxic algae. However, both monitoring and
closures have economic costs that can be substantial. Perhaps the most
striking example of this is the complete loss of the wild shellfish
resource in Alaska—which once produced 5 million pounds annually—to
persistent paralytic shellfish poisoning.
It is difficult to
assess the precise way in which human activities influence the
occurrence and severity of HABs. The physical and biological processes
involved are not well understood, and long-term observations are sorely
lacking. To complicate matters, HABs can and do occur in relatively
pristine conditions. But there is a clear connection between nutrient
levels and primary production, and there is general agreement among
scientists that, other factors being equal, the conditions that favor
high levels of primary production also favor HABs.
Is it getting stuffy in here?
Phytoplankton can
cause problems even when they are dead. After they die, phytoplankton
sink to the bottom where they decay through bacterial action. The
bacteria that cause decay use oxygen dissolved in bottom waters. As
long as the bottom waters are well mixed with oxygen-rich surface
waters, the oxygen is renewed. However, under certain conditions, ocean
waters become stratified so that there is little vertical mixing, and
depleted oxygen is not replaced.
Stratification tends to occur
in the summer because the configuration of warm surface waters over
colder bottom waters is stable. Stratification also tends to be
stronger near the mouths of rivers where the lighter fresh water
overlays denser salt water in a stable configuration. Finally, stratification is strong in enclosed and semi-enclosed parts
of the ocean—such as bays and lagoons—that are cut off from the
large-scale circulation patterns that promote mixing.
When
vertical mixing is weak or absent, oxygen-depleted bottom waters are
not refreshed, resulting in a condition called hypoxia. Hypoxic
areas—popularly known as “dead zones”—can have a dramatic effect on
marine life. In some cases, oxygen depletion occurs so quickly that it
cuts off escape routes and results in fish kills.
Even when
animals simply avoid low-oxygen areas, as they usually do, the indirect
effects of hypoxia can be substantial. Suitable habitat is lost,
putting pressure on populations. Hypoxic zones also can interfere with
the migratory behavior of shrimp, lobsters, and other species. More
generally, by altering the environment in which marine species thrive,
hypoxia can lead to a decline in biological diversity.
A widespread problem
Hypoxia occurs throughout
the world. Two of the best-known hypoxic areas are in the Black Sea and
the Baltic Sea. In the U.S., dead zones occur regularly in Long Island
Sound, the Chesapeake Bay, and the northern Gulf of Mexico. In the
Baltic Sea, hypoxia has contributed to the collapse of the Norwegian
lobster fishery. There is evidence that the hypoxia off the coast of
Louisiana has harmed the valuable shrimp fishery and possibly
contributed to the replacement of bottom-dwelling species such as
snapper with less valuable mid-water species such as menhaden.
Hypoxia can occur naturally. For example, the bottom waters of the
Black Sea have been depleted of oxygen for thousands of years. Hypoxia
has also occurred naturally in the Chesapeake and the Gulf of Mexico.
But there is little doubt that, by increasing the level of nutrients in
the ocean, human activities have increased the frequency, extent, and
severity in these areas and throughout the coastal ocean.
Decisions we can live with
Determining the
appropriate public policy response to eutrophication is difficult.
Because so many economic activities contribute nutrients to the marine
environment, nutrient regulations potentially touch a large part of the
economy. Also, the effects of eutrophication are complicated and
difficult to measure, especially in economic terms. Comparing the costs
and benefits of alternative policies is difficult. While eutrophication
is ubiquitous, different regions differ in both cause and effect. For
this reason, policy must be customized to local situations: One size
does not fit all.
The good news is that, because so many
human activities produce nutrients, there are many opportunities to
make small, low-cost changes with a large cumulative impact.
One
promising approach to nutrient reduction involves the adoption of
so-called “best management practices” in agriculture. A major
contributor to nutrient pollution is the loss of fertilizer from
agricultural fields. Not only does this ultimately lead to
eutrophication, but it is also costly to farmers. By using improved
agricultural methods such as no-till planting, farmers can put more
fertilizer in the soil and less in the ocean.
Another area
for possible action is improving the treatment and disposal of human
and animal waste. Although this is likely to be costly, it has benefits
beyond the reduction of eutrophication.
Finally, policies could
also be aimed at curtailing the conversion—and even promoting the
restoration—of wetlands and other natural buffers that intercept and
sequester nutrients before they reach the ocean.
Aquaculture for remediation?
There are some
novel ideas as well. A project is currently underway at Woods Hole
Oceanographic Institution to examine the feasibility of using shellfish
aquaculture to reduce nutrients in the coastal ocean. The experimental
shore-based aquaculture system at the National Center for Mariculture
in Eilat, Israel, uses shellfish to absorb excess nutrients excreted by
fish. Researchers at WHOI are trying to determine whether the same idea
is feasible in the ocean. As the shellfish produced by such an
enterprise have economic value, this is an example of a win-win
situation. (See Down on the Farm Raising Fish.)
As environmental problems go, coastal eutrophication is not
particularly glamorous. It is difficult to justify costly measures to
eliminate this problem, particularly in the short term. But low-cost
options do exist and would be a step in the right direction. Not only
would this make economic sense today, but it would set us on a course
to lighten the tread of society on natural systems.
—
This article is adapted from “Red Tides and Dead Zones: Eutrophication
in the Marine Environment,” which first appeared in U.S. Policy and the
Global Environment.
Posted: December 22, 2004 [top] |
