Throughout the world, large areas of our coastal waters

are becoming so polluted that they lack sufficient oxygen,

one of the basic building blocks of life. This condition is

called “hypoxia.” Although this is sometimes a natural

condition, the increased area of water affected, extended

length of each episode, and higher frequency in recent

decades are due to human activities. Overenrichment of

estuaries and coastal waters with nutrients, especially

nitrogen, stimulates outbreaks or “blooms” of algae that

consume vital oxygen from the water when they

decompose. The effects of hypoxia include fish kills and

shellfish bed losses. These losses can have significant

detrimental effects on the ecological and economic health

and stability of coastal regions.

HYPOXIA

Ecological Society of America

WHAT IS HYPOXIA?

Hypoxia means “low oxygen.” In

aquatic ecosystems, low oxygen

usually means a concentration of

less than 2-3 milligrams of oxygen

per liter of water (mg/l). A complete

lack of oxygen (0 mg/L) is called

anoxia. Since organisms that can

live without oxygen (such as some

microbes) are the only residents

in these areas, they are

sometimes called “dead zones”.

Hypoxia is primarily a problem in

estuaries and coastal waters,

although it can also be a problem

in freshwater lakes.

WHAT ARE THE EFFECTS OF HYPOXIA?

The direct effects of hypoxia include fish kills, which not only deplete valuable fish stocks and damage the

ecosystem, but are unpleasant for local residents and can harm local tourism. Oxygenated water is

necessary for aquatic animals to breathe. Mobile animals, such as adult fish, can often survive hypoxia by

moving into oxygenated waters. When they cannot, such as when young fish need to spend time in the

habitat that has become hypoxic area, the result is a fish kill. Nonmobile animals, such as clams, cannot

move into healthier waters and are often killed by hypoxic episodes. This causes a severe reduction of

the amount, or in extreme cases the complete loss, of animal life in hypoxic zones.

Because hypoxia often occurs in estuaries or near shore areas where the water is poorly mixed, nursery

habitat for fish and shellfish is often affected. Without nursery

grounds the young animals cannot find the food or habitat they

need to reach adulthood. This causes years of weak recruitment

to adult populations and can result in an overall reduction or

destabilization of important stocks.

Fish-eating birds and mammals, such as herons or otters,

cannot live where there are no fish. Hypoxic areas may also be

more susceptible to overfishing, pest outbreaks, storm damage,

or other stresses.

Chemical reactions between hypoxic

water and bottom sediments can

release pollutants stored in the

sediments, further fueling the hypoxic

conditions or otherwise polluting the

ecosystem.

What is mixed water?

In poorly mixed water,

oxygen or pollutants get

trapped in a single water

layer (usually the top or the

bottom). In well-mixed

water, they get dispersed

throughout.

HYPOXIA KILLS FISH

• 15 of the 21 fish kills with identified

causes that occurred in the Barataria

and Terrebonne Estuarine Complex in

1998, were due to low dissolved oxygen

concentrations.

• In North Carolina, two separate hypoxic

events in the Neuse River Estuary in

1995 killed millions of fish. Additional

events in 1998 and 1999 killed tens of

thousands of fish. These fish kills were

closely linked to hypoxia in the estuary.

• In the Chesapeake Bay, low oxygen

levels have killed many blue crabs, one

of the most popular local menu items.
The most serious effects of hypoxia on fisheries are probably:

• longterm weakening of species also stressed by overfishing

• habitat loss

• longterm changes in ecology (e.g. a shift in the dominant

species of algae, which can change the dominant fish

species)

• economic losses

In the northern Gulf of Mexico, small brown shrimp populations occur

in the same years that large hypoxic zones do. In many areas hard

hit by hypoxia, fish and shellfish populations are highly variable, an

indication they are unstable. Recurrent hypoxic conditions prevent

fish and shellfish from recovering from other stresses such

as disease, storms, and overfishing and may cause longterm

changes in the ecology of the area.

This situation is detrimental not only for ecosystems but

for fishermen who rely on these resources for their

livelihood and for consumers who look forward to bountiful

fish and shellfish harvests.

Dead fish. Photo courtesy of USEPA

WHAT CAUSES HYPOXIA?

Hypoxia is a complex ecological phenomenon that arises from the convergence of several factors, some

of which are altered by human activities.

• Physical environment. The shape of the water body and the strength and direction of water flow

determine how well-mixed the water is and how long it spends in the lake, bay, or estuary. Wind

strength and direction and the influence of major storms also affect the rate of mixing. Human activities,

such as changing the amount of freshwater entering an estuary, can reduce the amount of mixing.

Poorly-mixed water increases the chance hypoxia will develop.

• Nutrient enrichment. Nutrients (primarily nitrogen and

phosphorus, but sometimes also including silica, iron, zinc,

and magnesium) are necessary for plant growth. An

overabundance of these nutrients, combined with enough

light and warm, slow-moving, and poorly-mixed water, can

result in an algae bloom. This is the first step in the chain of

reactions that can lead to hypoxia. These overabundant

nutrients may come from point sources such as waste water

treatment plant discharges and nonpoint sources such as

agricultural activities, urban runoff, groundwater, and atmospheric deposition. The amount of pollutants generated and

the rate at which they reach rivers, estuaries and coastal

waters are increased by human activities such as the

destruction of wetlands, grasslands, and forests in favor of

urban or suburban landscapes and many agricultural

practices.

REDUCING NITROGEN LOADS

TO ENCOURAGE HEALTHY

ECOSYSTEMS

Tampa Bay has a long history of high

nitrogen loadings and hypoxic

conditions. Through the Tampa Bay

National Estuary Program and other

efforts, the community agreed to

meet a reduction goal of 17 tons of

nitrogen per year for at least five

years. This goal will allow for

economic growth in the Tampa

region without causing additional

nutrient enrichment problems.

In general, water that is slow-moving, poorly-mixed, and has abundant nutrients is

more likely to become hypoxic. It also tends to stay that way for longer periods of

time.
HOW DOES HYPOXIA DEVELOP?

Hypoxia develops when a series of conditions occur together.

The diagram below explains how they interact. One of the most

important conditions is that there must be a large amount of algae (box 2). These algae have to sink or otherwise end up on the

bottom. This happens when the algae die without being eaten, or

when they are eaten by small animals (zooplankton) whose fecal pellets sink to the bottom (box 3). On the bottom, the algae or

fecal pellets decompose. This process of decomposition consumes oxygen. If the water is not wellmixed, there is no way to

replace the oxygen consumed by decomposition (box 4), and

hypoxia is likely to occur.

A hypoxic zone is created when there is a large

demand for oxygen in bottom waters (which

consumes the available oxygen) and few ways for

new oxygen to get into those waters.

PROTECTING LONG ISLAND SOUND

AND ITS LIVING RESOURCES

Long Island Sound has traditionally suffered

from severe hypoxic events, resulting in

massive fish kills and other ecological

damage. Because of this, a coalition of

management agencies and community

groups has adopted a target of 58.5%

reduction in nitrogen loading to the Sound.

Based on extensive modeling of Long Island

Sound, it is estimated that this reduction in

nitrogen loading will reduce the size of the

area that becomes hypoxic by 75%, reduce

the length of time the remaining portions of

the sound are hypoxic by 85%, and reduce

mortality of sensitive larvae by 90%.

HYPOXIA ON A LARGE SCALE: THE GULF OF MEXICO

The largest hypoxic zone in the United States forms every spring and summer in the Gulf of Mexico

just west of the mouth of the Mississippi River. This hypoxic zone has ranged in size from 16,000-

20,000 km2 since 1993. In 1999, the hypoxic zone was the biggest ever measured at 20,000 km2, or

roughly the size of the state of New Jersey. For several years before the 1993 Mississippi River Floods

it was approximately half of this size. Researchers now agree that the hypoxic zone is the result of the

huge loads of nutrients that pour out of the Mississippi and Atchafalaya Rivers every year combined

with mixing patterns in the Gulf that separate surface waters from bottom waters. These conditions

are optimal for the development of a hypoxic zone. Together, the cities, suburbs, and farms in the

Mississippi River watershed contribute an estimated 90% of the nutrient flow into the Gulf of Mexico.

FOUR QUESTIONS THAT CAN BE USED TO PREDICT HYPOXIA:

2. Is there an algae

bloom?

1. Are nutrients overabundant ?

3. Are there enough zooplankton--

small animals that eat algae--to

consume the bloom?

Hypoxia

unlikely to

occur

4. Is the water well-mixed?

(Even if there are enough zooplankton to eat the

bloom, their fecal pellets can sink to the bottom

in poorly-mixed water and consume oxygen.)

HYPOXIA LIKELY

 no

 yes

no

no

yes

yes

Algae began to die

and sink to bottom.

Algae decompose.

Decomposition

consumes oxygen.

yes

no
PROTECTING WATERS FROM HYPOXIA

The federal government and many state governments have initiated programs to reduce the amount of

nutrients entering our nation’s waterbodies. Many other coalitions of citizens, industry, and federal,

state, and local governments have formed to develop and implement plans for reducing the amount of

nutrients entering coastal waters.

Millions of small actions upstream in places such as Pennsylvania, Minnesota, Vermont, and Colorado,

as well as those along the coasts, have large effects—both good and bad—on the health of our nation’s

coastal waters. Some of the actions that will help keep are coasts healthy are:

• Minimize use of fertilizers and manage them properly. Excess fertilizer washes off lawns and

farms into streams and eventually rivers, where it travels to the ocean. It can also volatilize from

fields or manure lagoons into the atmosphere and be redeposited where it can wash into waterways.

• Remove nutrients from wastewater and urban runoff before they are discharged. These

sources contribute large amounts of nutrients to areas at risk for hypoxia.

• Reduce the amount of electricity use and the number of miles driven. Power generation,

either in an electrical power plant or in a car, generates nitrogen oxides (NOx). NOx are

transformed into nitrates in the air and deposited throughout the country, especially east of the

Mississippi River.

• Practice clean boating. This includes choosing boat

engines that produce the smallest amount of NOx and

using pumpout stations at marinas rather than

discharging raw sewage into the water.

• Preserve land adjoining rivers and streams. This

land, often called a riparian buffer, can play a vital

role in preventing nutrients that wash off fields and

streets from reaching the rivers and coasts. It is also

valuable protection against floods, provides habitat for

wildlife and migrating birds, and improves the overall

health and beauty of the stream or river. Preserving

wetlands also helps keep nutrients out of rivers,

estuaries and coastal waters.

For more information, contact:

Ecological Society of America, 1707 H Street, NW, Suite 400, Washington, DC 20006. 202-833-8773.

esahq@esa.org; http://www.esa.org.

National Oceanic and Atmospheric Administration (NOAA), National Ocean Service, Office of Ocean and

Coastal Management. http://www.ocrm.nos.noaa.gov/.

United States Environmental Protection Agency, Wetlands, Oceans, & Watersheds. http://www.epa.gov/

owow/

Prepared by the Ecological Society of America

1707 H Street, NW, Suite 400, Washington, DC 20006

phone 202-833-8773 · fax 202-833-8775

http://www.esa.org · esahq@esa.org

Restored wetland. Photo from USFWS.