derrames de petroleo en cuerpor de agua
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Of Oil Spills In
Aquatic Environments
1The Behavior and Effects
INTRODUCTION
WHEN WE THINK of oil spills, we u sua lly think of oil
tankers spilling their cargo in oceans or seas. How ever, oilspilled on land often reaches lakes, rivers, and wetlands,
wh ere it can also cause dam age. Oceans and other
saltwater bodies are referred to as marine env ironments.
Lakes, rivers, and other inland bodies of water are called
freshwater environments. The term a quatic refers to both
marine and freshwater environments.
When o il is spilled into an aqu atic env ironment, it can
harm organ isms that live on or around th e water surface
and those that live un der w ater. Spilled oil can also
dam age parts of thefood chain, including hu man food
resources.
The severity of the imp act of an oil spill dep ends on avariety of factors, includ ing cha racteristics of the oil itself.
Natu ral cond itions, such as water temp erature and
weat her, also influence the behavior of oil in aqu atic
environmen ts. Various types of habitats have d iffering
sensitivities to oil spills as w ell.
PHYSICAL PROPERTIES OF OIL
THE TERM OIL describes a broad range ofhydrocarbon-
based substances. Hydrocarbons are chemical comp oun ds
composed of the elements hyd rogen and carbon. This
includ es substances that are commonly thou ght of as oils,such as crude oil and refined petroleum p rodu cts, but it
also includ es animal fats, vegetab le oils, and other n on-
petroleu m oils. Each typ e of oil has distinct ph ysical and
chemical prop erties. These p roperties affect the w ay oil
will spread and break down , the hazard it may pose to
aquatic and hu man life, and th e likelihood that it will pose
a threat to natural and m an-mad e resources.
The rate at w hich an oil spill spreads will determine its
effect on the environm ent. Most oils tend to sp read
horizontally into a sm ooth and slipp ery surface, called a
slick, on top of the w ater. Factors wh ich affect the ability ofan oil spill to spread includ e surface tension, specific gravity,
an d viscosity.
Surface tension is the measure of attraction between the
sur face molecules of a liquid . The h igher th e oils
sur face tension, the more likely a sp ill will remain in
place. If the su rface tension of the oil is low, the oil will
spread even without help from wind and water
currents. Because increased temp eratures can red uce a
liquids surface tension, oil is more likely to spread in
warm er waters than in very cold waters.
Specific gravity is the density of a substance comp ared to
the d ensity of water. Since most oils are lighter thanwa ter, they float on top o f it. How ever, the sp ecific
grav ity of an oil spill can increase if the lighter
substances within the oil evaporate. H eavier oils,
vegetable oils, and animal fats may sink an d form tar
balls or may interact with rocks or sediments on the
bottom of the water bod y.
Viscosity is the measure of a liqu ids resistance to flow.
The higher t he viscosity of the oil, the greater th e
tend ency for it to stay in one place. (Honey is an
examp le of a highly viscous liquid.)
THE FATE OF SPILLED OILNATURAL ACTION S are always at work in aqu atic
environ men ts. These can redu ce the severity of an oil spill
and accelerate the recovery of an affected area. Some
natur al actions includ e weathering, evaporation, oxidation,
biodegradation, and emulsification.
Weathering is a series of chem ical and ph ysical chang es
that cause spilled oil to break dow n an d become h eavier
than water. Wave action m ay result in natu ral dispersion ,
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breaking a slick into droplets wh ich are th en d istributed
vertically throu ghou t the w ater colum n. These droplets
can also form a second ary slick or thin film on the
sur face of the water.
Evaporation occurs wh en th e lighter or more volatile
substances within the oil mixture become vap ors and
leave the su rface of the w ater. This process leaves
behind th e heavier comp onents of the oil, which may
un dergo further weathering or may sink to the bottom
of the ocean floor. Spills of lighter refined p roducts, suchas kerosene and gasoline, contain a high p roportion of
flamm able comp onents know n as light ends. These may
evaporate w ithin a few hou rs, causing minimal harm to
the aqu atic environ men t. Heavier oils, vegetable oils,
and animal fats leave a thicker, more viscous residue.
These typ es of oils are less likely to evap orate.
Oxidation occurs w hen oil contacts the w ater and
oxygen combines with the oil hyd rocarbons to produ ce
wa ter-soluble compoun ds. This process affects oil slicks
mostly around their edges. Thick slicks may only
pa rtially oxidize, forming tar balls. These den se, sticky
black sph eres may linger in the environment, w ashingup on shorelines long after a spill.
Biodegradation occurs w hen microorganisms, such as
bacteria, feed on oil hyd rocarbons. A wide range of
microorgan isms is requ ired for a significant red uction of
the oil. To sustain biod egradation, nu trients such as
nitrogen and phosphorus are sometimes add ed to the
water to encourage th e microorganisms to grow and
reprodu ce. Biodegradation tend s to work best in warm-
water environments.
Emulsification is the process that forms emulsions, wh ich
are mixtu res of sma ll droplets of oil and wa ter.
Emulsions are formed by w ave action, and they greatly
ham per w eathering and cleanup processes. Two typ es o
emu lsions exist: water-in-oil and oil-in-water. Water-in-
oil emu lsions are frequently called chocolate mou sse,and they are formed wh en strong wave action causes
water to become trapp ed inside viscous oil. Chocolate
mou sse emulsions may linger in the environment for
month s or even years. Oil and water em ulsions cause oil
to sink and disapp ear from the surface, giving the visual
illusion that it is gone and the threat to the environm ent
has ended.
These natu ral actions occur d ifferently in freshw ater
versus marine environmen ts. Freshwater env ironmental
impacts can be more severe because w ater movem ent is
minimized in these habitats. In standing water bod ies, oil
tends to p ool and can remain in the environment for longperiods of time. In flowing streams and rivers, oil tends to
collect on p lants and grasses growing on th e banks. Oil can
also interact with the sediment at the bottom of the
freshwat er bod ies, affecting organism s that live in or feed
off of sedimen ts.
Fate ofspilledoil.
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EFFECTS OF OIL ON PLANTSAND ANIMALS
SOME TOXIC SUBSTAN CES in an oil spill may
evaporate q uickly. Therefore, plant, animal, and hu man
exposure to the m ost toxic substances are redu ced with
time, and a re usually limited to th e initial spill area.
Although som e organisms may be seriously injured or
killed v ery soon a fter contact with the oil in a spill, non -
lethal toxic effects can be m ore subtle and o ften longerlasting. For example, aqu atic life on reefs and shorelines is
at risk of being smothered by oil that wash es ashore. It can
also be poisoned slowly by long-term exposure to oil
trapp ed in shallow water or on beaches.
Both p etroleum and non-petroleum oil can affect the
environment surroun ding an oil spill. All types of oil share
chemical and ph ysical properties that p rodu ce similar
effects on the env ironment. In some cases, non-petroleu m
oil spills can prod uce more harm ful effects than petroleum
oil spills.
Chap ter five discusses in greater d etail how oil spills
imp act wildlife .
Sensitivity of Aquatic Habitats
Aquatic environm ents are made u p of comp lex
inter re la t ions be tween plant and animal spec ies
and the i r phys ica l envi ronment . Harm to the
phys ica l envi ronment wi l l of ten lead to harm for
one or more spec ies in a food chain , which may
l ead t o damage f o r o t he r spec i e s f u r t he r up t he
chain . Where an organism spends most of i t s
t imein open water , near coas ta l a reas , or on the
shorel inewill determine the effects an oi l spi l l is
l ike ly to have on tha t organism.
In open water, fish and wh ales have the ability to swim
away from a sp ill by going d eeper in the water or furth er
out to sea, redu cing th e likelihood that they will be harmed
by even a m ajor spill. Aqu atic animals that generally live
closer to shore, such as tu rtles, seals, and d olph ins, risk
contamination by oil that washes on to beaches or by
consuming oil-contaminated prey. In shallow waters, oil
may ha rm sea grasses and kelp beds, which are used for
food, shelter, and nesting sites by many d ifferent species.
Spilled oil and cleanup operations can th reaten d ifferent
types of aqu atic habitats, with different resu lts.
Coral reefs are important nu rseries for shrimp, fish, and
other an imals as well as recreational attractions for
divers. Coral reefs and th e aquatic organisms that live
within and around them are at risk from exposu re to the
toxic substances within oil as well as smoth ering.
Exposed sandy, gravel, or cobble beaches are usu ally
cleaned by m anu al techniqu es. Although oil can soak
into sand and gravel, few organ isms live full-time in
this hab itat, so the risk to animal life or the food chain is
less than in other h abitats, such as tidal flats.
Crews work to keep oil from entering a marsh.
Sheltered beaches have very little wave action to
encourage n atural d ispersion. If timely cleanup efforts
are not begun, oil may remain stranded on these
beaches for years.
Tidal flats are broad , low-tide zones, usually containingrich p lant, animal, and bird commu nities. Deposited oil
may seep into the m ud dy bottoms of these flats, creating
pot entially harm ful effects on the ecology of the area.
Salt marshes are found in sheltered w aters in cold and
temperate areas. They host a variety of plant, bird, and
mamm al life. Marsh vegetation, especially root systems,
is easily dam aged by fresh light oils.
Mangrove forests are located in tropical regions and are
home to a d iversity of plant and animal life. Mangrove
trees have long roots, calledprop roots, that stick out well
above the water level and help to hold the man grove
tree in place. A coating of oil on these p rop roo ts can befatal to the man grove tree, and because they grow so
slowly, replacing a mangrove tree can take decades.
Marshes and swamps with little water m ovement are
likely to incur m ore severe imp acts than flowing w ater.
In calm water conditions, the affected habitat may take
years to restore.
Other standing water bodies, such as inland lakes and
pon ds, are home to a v ariety of birds, mamm als, and
fish. The hu man food chain can be affected by spills in
these environments.
River habitats may be less severely affected by sp ills than
standing w ater bodies because of water movemen t.How ever, spills in these water bod ies can affect plan ts,
grasses, and mosses that grow in the env ironment.
When rivers are used as drinking w ater sources, oil
spills on rivers can pose direct threats to hu man health.
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Sensitivity of Birds and Mammals
An oil spill can harm birds and mam mals in several ways:
direct physical cont act, toxic contamination, destruction of food
sources and habitats, and reproductive problems.
Physical contact When fur or feathers come into contact
with oil, they get matted dow n. This matting causes fur
and feathers to lose their insulating p roperties, placing
anim als at risk of freezing to dea th. For birds, the risk of
drow ning increases, as the comp lex structure of theirfeathers that allow s them to float or to fly becomes
damaged.
Toxic contamination Some sp ecies are susceptible to the
toxic effects of inhaled oil vapor s. Oil vap ors can cau se
damage to the animals central nervous system, liver,
and lungs. Anim als are also at risk from ingesting oil,
wh ich can red uce the an imals ability to eat or digest its
food by dam aging cells in the intestinal tract.
Destruction of food resources and habitats Even species
wh ich are n ot directly in contact with oil can be harm ed
by a spill. Predators that consum e contaminated p rey
can be exposed to oil through ingestion. Because oil
contamination gives fish and other animals un pleasant
tastes and smells, predators will sometimes refuse to eat
their prey and will begin to starve. Sometimes a local
pop ulation of prey organisms is destroyed, leaving n o
food resources for predators. Depend ing on the
environmental conditions, the spilled oil may linger in
the environment for long p eriods of time, add ing to the
detrimen tal effects. In calm w ater cond itions, oil that
interacts with rocks or sediments can rema in in the
environment indefinitely.
Reproductive problems Oil can be transferred from birds
plum age to the eggs they are hatching. Oil can sm other
eggs by sealing p ores in the eggs an d p reventing gasexchange. Scientists have a lso observed developm ental
effects in bird embr yos that w ere exposed t o oil. Also,
the nu mber of breeding animals and the of nesting
habitats can be red uced by the spill. Long-term
reprodu ctive problems have also been shown in some
stud ies in an imals that have been exposed to oil.
SUMMARY
SPILLED OIL immed iately begins to move and w eather,
breaking down and changing its physical and chemical
prop erties. As these p rocesses occur, the oil threatenssurface resources and a wid e range of subsu rface aquatic
organisms linked in a comp lex food chain. Many d ifferent
types of aquatic habitats exist, with varied sensitivities to
the ha rmful effects of oil contam ination an d d ifferent
abilities to recup erate from o il spills. In som e areas,
habitats and pop ulations can recover quickly. In other
environments, how ever, recovery from persistent or
stranded oil may take years. These d etrimental effects are
caused by both petroleum and non-petroleum oil.
8 Understanding Oil Spills and Oil Spill Response