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

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Derrames de Petroleo en Cuerpor de Agua