ArticlePDF Available

Eutrophication and hypoxia in coastal areas: a global assessment of the state of knowledge

January 2008

Authors:

Suzie Greenhalgh

Manaaki Whenua - Landcare Research

Robert Diaz

Virginia Institute of Marine Science

Zachary Sugg

Lincoln Institute of Land Policy

Content uploaded by Suzie Greenhalgh

Content may be subject to copyright.

www.wri.org10 G Street, NE Washington, DC 20002

Tel: 202-729-7600 Fax: 202-729-7610

MARCH 2008

6WORLD RESOURCES INSTITUTEMarch 2008

WRI POLICY NOTE

WATER QUALITY: EUTROPHICATION AND HYPOXIA No. 1

POLICY NOTE: Eutrophication and Hypoxia in Coastal Areas

5March 2008WORLD RESOURCES INSTITUTE

POLICY NOTE: Eutrophication and Hypoxia in Coastal Areas

EUTROPHICATION AND

HYPOXIA IN COASTAL AREAS:

A GLOBAL ASSESSMENT OF

THE STATE OF KNOWLEDGE

MINDY SELMAN, SUZIE GREENHALGH, ROBERT DIAZ AND ZACHARY SUGG

Within the past 50 years, eutrophication—the overen-

richment of water by nutrients such as nitrogen and

phosphorus—has emerged as one of the leading causes of water

quality impairment. Our research identiﬁ es over 415 areas

worldwide that are experiencing symptoms of eutrophication,

highlighting the global scale of the problem.

Recent coastal surveys of the United States and Europe found

that a staggering 78 percent of the assessed continental U.S.

coastal area and approximately 65 percent of Europe’s Atlantic

coast exhibit symptoms of eutrophication.

1,2

In other regions,

the lack of reliable data hinders the assessment of coastal

eutrophication. Nevertheless, trends in agricultural practices,

energy use, and population growth indicate that coastal eutro-

phication will be an ever-growing problem.

This policy note focuses on what is currently known about the

extent of eutrophication globally, and how to improve the state

of our knowledge to more accurately inform and drive policy

decisions for mitigating eutrophication. Complementary policy

notes will discuss in more detail the sources of nutrients and

actions that can be implemented to mitigate eutrophication.

Key Messages

Eutrophication—the overenrichment of waters by nutrients—threat-

ens and degrades many coastal ecosystems around the world. The two

most acute symptoms of eutrophication are hypoxia (or oxygen deple-

tion) and harmful algal blooms, which among other things can destroy

aquatic life in affected areas.

Of the 415 areas around the world identiﬁ ed as experiencing some

form of eutrophication, 169 are hypoxic and only 13 systems are clas-

siﬁ ed as “systems in recovery.”

Mapping and research into the extent of eutrophication and its threats

to human health and ecosystem services are improving, but there is

still insufﬁ cient information in many regions of the world to establish

the actual extent of eutrophication or identify the sources of nutri-

ents. To develop effective policies to mitigate eutrophication, more

information is required on the extent of eutrophication, the sources of

nutrients, and the impact of eutrophication on ecosystems.

Recommendations

To improve knowledge of where eutrophication is occurring and its

impacts, environmental agencies or coastal authorities worldwide

need to proactively assess and monitor water quality, speciﬁ cally those

variables commonly linked to eutrophication such as nutrient levels

and dissolved oxygen. In addition, internationally accepted methods

and deﬁ nitions for assessing and classifying eutrophic coastal wa-

ters—including proxies for eutrophication—need to be developed.

In Asia, Africa, and Latin America, environmental agencies or coastal

authorities should:

1. Undertake systematic and routine assessments of coastal areas,

particularly those exhibiting symptoms of eutrophication.

2. Develop transparent and public reporting procedures for tracking

the occurrence of eutrophication and hypoxia, as well as monitoring

their impact on ecosystem health.

3. Develop and adopt decision-support tools—such as nutrient bud-

gets and water quality models—that can facilitate the development

of appropriate local and regional responses to eutrophication.

In the United States, Europe, and Australia, environmental agencies

or coastal authorities should:

1. Continue coastal zone assessments.

2. Ensure that eutrophication assessment methodologies are being

consistently applied.

3. Enhance existing decision-support tools and develop tools for those

areas where none currently exist.

Bay, Baltic Sea, Black Sea, Gulf of Mexico (Mississippi-Atcha-

falaya plume), and Tampa Bay. The absence of detailed nutri-

ent budgets for the majority of eutrophic and hypoxic systems

around the world highlights the lack of concerted research

efforts in these areas, compromising the ability to effectively

address and manage nutrient overenrichment.

HOW DO WE IMPROVE THE STATE OF KNOWLEDGE?

To better assess the global, regional, and local extent of eutrophi-

cation, we need more and better data. Improving our knowledge

of where eutrophication is occurring, the sources contributing

to the problem, and the effects of eutrophication on marine

ecosystems and human communities ultimately inﬂ uences the

ability of policy-makers to decide where and how resources can

be used most effectively to address eutrophication.

Closing the knowledge gap requires increased and improved

water quality monitoring and assessment of estuarine and

coastal waters. Without this information, communities cannot

effectively manage coastal ecosystems and address land-based

sources of pollution. Specifically, time series monitoring

data—on nutrient levels, chlorophyll a (as an indicator of the

presence of phytoplankton), and dissolved oxygen—as well

as eutrophication response parameters—such as loss of sub-

aquatic vegetation, ﬁ sh kills, and algal blooms—are needed to

evaluate the extent and degree of eutrophication. Moreover,

widespread use and adoption of methods to classify and deﬁ ne

eutrophic waters (such as the ones developed by NOAA and

OSPAR) are needed to increase the transparency, consistency,

and comparability of water quality information. For example,

currently no common deﬁ nition of “hypoxia” exists and the

interpretation of what is hypoxic can vary widely from place

to place.

In countries or regions where robust and systematic monitor-

ing programs are unlikely to be implemented due to limited

resources, it would be beneﬁ cial to develop credible and con-

sistent proxies for water quality (such as turbidity or sulﬁ de

smell) that could be used to identify likely eutrophic areas.

Finally, decision support tools that enable researchers and

policy-makers to formulate appropriate responses to eutro-

phication need to be continually developed and more widely

adopted by local and regional entities. Such tools include:

• Watershed models to assess nutrient delivery and ecosys-

tem impacts;

• Nutrient balance assessments to determine the sources of

nutrients and relative contribution of each source;

• Regional and international online information portals

for compiling and sharing water quality information and

research;

• Nutrient loss estimation tools; and

• Tools to help assess the effectiveness of alternative sce-

narios for reducing nutrient inputs.

Implementation of these tools is especially needed to estimate

coastal impacts and target appropriate policy responses in coun-

tries and regions where actual monitoring data are scarce.

Eutrophication is an issue that requires greater attention by gov-

ernments and society in general. Left untouched, it may have

dire consequences for many ecosystems, the food webs that they

support, and the livelihoods of the populations that depend on

them. To get a better grasp on the immediate and long-term

consequences of eutrophication, we need more resources and

better information. Improving our knowledge and information

on eutrophication is the ﬁ rst step in developing robust policy

measures to begin reversing or halting its impacts.

ABOUT THE AUTHORS

Mindy Selman is a senior associate in the People and

Ecosystems Program of the World Resources Institute.

Ph: +1-202-729 7644. Email: mselman@wri.org

Suzie Greenhalgh is a senior economist at Landcare

Research New Zealand Ltd. Ph: +64-9-574 4132.

Email: greenhalghs@landcareresearch.co.nz

Robert Diaz is a professor of marine science at the College

of William and Mary, Virginia Institute of Marine Science.

Ph: +1-804-684 7364. Email: diaz@vims.edu

Zachary Sugg is a research analyst with the People and

Ecosystems Program of the World Resources Institute.

Ph: +1-202-729 7605. Email: zsugg@wri.org

ACKNOWLEDGMENTS

The authors are grateful for the contributions provided by

Donald Anderson, James Galloway, Paul Harrison, Laurence

Mee, Qian Yi, and Nancy Rabalais, who served on a WRI

eutrophication advisory panel. The authors would also like to

acknowledge Kersey Sturdivant for his research assistance in

identifying areas of eutrophication. In addition, the authors

thank Craig Hanson, Lauretta Burke, Amy Cassara, Dan

Tunstall, Laurence Mee, and James Galloway for reviewing

earlier drafts of this paper. Finally, we extend our thanks to

the Linden Trust for Conservation and the David and Lucile

Packard Foundation for their support of this research.

NOTES

1. See Bricker, S. B., Longstaff, W. Dennison, A. Jones, K. Boicourt, C.

Wicks, and J. Woerner. 2007. Effects of Nutrient Enrichment in the

Nation’s Estuaries: A Decade of Change. NOAA Coastal Ocean Program

Decision Analysis Series No. 26. Silver Spring, MD: National Centers

for Coastal Ocean Science. Online at: http://ccma.nos.noaa.gov/publica-

tions/eutroupdate/

2. See OSPAR Commission. 2003. OSPAR integrated report 2003 on the

eutrophication status. London, U.K.: OSPAR.

3. See Howarth, R. and K. Ramakrrshna. 2005. “Nutrient Management.”

In K. Chopra, R. Leemans, P. Kumar, and H. Simons, eds. Ecosystems

and Human Wellbeing: Policy Responses. Volume 3 of the Millennium

Ecosystem Assessment (MA). Washington, DC: Island Press.

4. See National Oceanic and Atmospheric Administration (NOAA). “Global

Warming: Frequently asked questions.” Available at: http://lwf.ncdc.

noaa.gov/oa/climate/globalwarming.html. (accessed January 15, 2008)

5. See Galloway, J.N, J.D. Aber,, J.W. Erisman, S.P. Seitzinger, R.W.

Howarth, E.B. Cowling, and B.J. Cosby. 2003. “The Nitrogen Cascade.”

Bioscience 53( 4): 341–356

6. For a discussion on the impacts of eutrophication on coastal ecosystems,

see Mee, L. 2006. “Reviving Dead Zones.” Scientiﬁ c American 295 (5):

79–85.

7. See: UN Chronicle Online Edition. “Risks, Untreated Sewage Threat-

ens Seas, Coastal Population.” http://www.un.org/Pubs/chronicle/2003/

issue1/0103p39.html. (accessed January 15, 2008)

8. Harmful algal blooms refer to several species of algae that produce tox-

ins that are harmful to aquatic life. See Anderson, D. M., P. M. Gilbert,

and J. M. Burkholder. 2002. “Harmful Algal Blooms and Eutrophica-

tion: Nutrient Sources, Composition, and Consequences.” Estuaries 25

(4b): 704–726.

9. See: Terra Daily Online Edition. “Hong Kong red tide spreads,” (June

10, 2007). http://www.terradaily.com/reports/Hong_Kong_Red_Tide_

Spreads_999.html (accessed February 8, 2008).

10. See Lu, S., and I. J. Hodgkiss. 2004. “Harmful algal bloom causative

collected from Hong Kong waters.” Hydrobiologia.512(1-3): 231-238

11. Hypoxia is generally deﬁ ned as having a dissolved oxygen concentration

of 2.0 milligrams per liter or less.

12. See Diaz, R. J., J. Nestlerode, and M. L. Diaz. 2004. “A global perspec-

tive on the effects of eutrophication and hypoxia on aquatic biota.” In

G. L. Rupp and M. D. White, eds. Proceedings of the 7th International

Symposium on Fish Physiology, Toxicology, and Water Quality, Tallinn,

Estonia, May 12-15, 2003. Athens, Georgia: U.S. Environmental Protec-

tion Agency, Ecosystems Research Division (EPA 600/R-04/049).

13. See Rabalais, N. N., and R. E. Turner. 2001. Coastal hypoxia conse-

quences for living resources and ecosystems. Washington, DC: American

Geophysical Union.

14. See Europe Now|Next. “Is the Black Sea recovering?” http://www.eu-

rope.culturebase.net/contribution.php?media=307 (accessed December

30, 2008)

15. See Howarth, R. and K. Ramakrrshna. 2005. “Nutrient Management.”

In K. Chopra, R. Leemans, P. Kumar, and H. Simons, eds. Ecosystems

and Human Wellbeing: Policy Responses. Volume 3 of the Millennium

Ecosystem Assessment (MA). Washington, DC: Island Press.

16. See Spokes, L. J., and T. D. Jickells. 2005. “Is the atmosphere really an

important source of reactive nitrogen to coastal waters?” Continental

Shelf Research 25: 2022–2035.

17. Region-speciﬁ c maps as well as the data used to create these maps is

made available on WRI’s website, www.wri.org/project/water-quality.

18. See Diaz, R. and R. Rosenburg. 1995. “Marine benthic hypoxia: a

review of its ecological effects and the behavioural responses of benthic

macrofauna.” Oceanography and Marine Biology: an Annual Review 33:

245–303.

19. The primary data sources include:

• Diaz, R. J., J. Nestlerode, and M. L. Diaz. 2004. “A global perspec-

tive on the effects of eutrophication and hypoxia on aquatic biota.” In

G. L. Rupp and M. D. White, eds. Proceedings of the 7th International

Symposium on Fish Physiology, Toxicology, and Water Quality, Tallinn,

Estonia, May 12-15, 2003. Athens, Georgia: U.S. Environmental Protec-

tion Agency, Ecosystems Research Division (EPA 600/R-04/049).

• Bricker, S. B., Longstaff, W. Dennison, A. Jones, K. Boicourt, C. Wicks,

and J. Woerner. 2007. Effects of Nutrient Enrichment in the Nation’s

Estuaries: A Decade of Change. NOAA Coastal Ocean Program Decision

Analysis Series No. 26. Silver Spring, MD: National Centers for Coastal

Ocean Science. Online at: http://ccma.nos.noaa.gov/publications/eu-

troupdate/.

• OSPAR Commission. 2003. OSPAR integrated report 2003 on the eutro-

phication status. London, UK: OSPAR.

• OzEstuaries. 2005. “Anoxic and Hypoxic Events.” Online at: http://www.

ozestuaries. org/indicators/anoxic_hypoxic_events.jsp (August, 2005).

• United Nations Environment Program (UNEP). 2006. Challenges to

International Waters – Regional Assessments in a Global Perspective.

Nairobi, Kenya: United Nations.

20. See Diaz, R. J., J. Nestlerode, and M. L. Diaz. 2004. “A global perspec-

tive on the effects of eutrophication and hypoxia on aquatic biota.” In

G. L. Rupp and M. D. White, eds. Proceedings of the 7th International

Symposium on Fish Physiology, Toxicology, and Water Quality, Tallinn,

Estonia, May 12-15, 2003. Athens, Georgia: U.S. Environmental Protec-

tion Agency, Ecosystems Research Division (EPA 600/R-04/049).

21. OSPAR called for the uniform assessment of coastal waters by signatory

countries. Signatory countries to the 1992 Oslo-Paris Convention for

the Protection of the Marine Environment of the North-East Atlantic

(OSPAR Convention) include Belgium, Denmark, France, Germany,

Ireland, The Netherlands, Norway, Portugal, Spain, Sweden, and the

United Kingdom.

ABOUT WRI

The World Resources Institute is an environ-

mental think tank that goes beyond research

to ﬁ nd practical ways to protect the Earth and

improve people’s lives. Our mission is to move

human society to live in ways that protect the

Earth’s environment and its capacity to pro-

vide for the needs and aspirations of current

and future generations.

Forthcoming publications in this series include:

1. Eutrophication: Sources and Drivers of Nutrient Pollution

2. Eutrophication: A Policy Framework for Addressing

Nutrient Pollution

Please visit www.wri.org/policynotes for links to available

Policy Notes.

BAR CODE TO COME

Red algal bloom at Leigh, near Cape Rodney, NZ.

PHOTO BY MIRIAM GODFREY. USED BY PERMISSION OF NIWA SCIENCE.

POLICY NOTE: Eutrophication and Hypoxia in Coastal Areas POLICY NOTE: Eutrophication and Hypoxia in Coastal Areas

2 3

March 2008WORLD RESOURCES INSTITUTE 4WORLD RESOURCES INSTITUTEMarch 2008

WHY IS EUTROPHICATION A PROBLEM?

The rise in eutrophic and hypoxic events has been primar-

ily attributed to the rapid increase in intensive agricultural

practices, industrial activities, and population growth, which

together have increased nitrogen and phosphorus ﬂ ows in

the environment. Human activities have resulted in the near

doubling of nitrogen and tripling of phosphorus ﬂ ows to the en-

vironment when compared to natural values.

By comparison,

human activities have increased atmospheric concentrations

of carbon dioxide, the gas primarily responsible for global

warming, by approximately 32 percent since the onset of the

industrial age.

Before nutrients—nitrogen in particular—are delivered to

coastal ecosystems, they pass through a variety of terrestrial and

freshwater ecosystems, causing other environmental problems

such as freshwater quality impairments, acid rain, the forma-

tion of greenhouse gases, shifts in community food webs, and

a loss of biodiversity.

Once nutrients reach coastal systems, they can trigger a number

of responses within the ecosystem. The initial impacts of nutri-

ent increases are the excessive growth of phytoplankton, micro-

algae (e.g., epiphytes and microphytes), and macroalgae (i.e.,

seaweed). These, in turn, can lead to other impacts such as:

• Loss of subaquatic vegetation as excessive phytoplankton,

microalgae, and macroalgae growth reduce light penetra-

tion.

• Change in species composition and biomass of the ben-

thic (bottom-dwelling) aquatic community, eventually

leading to reduced species diversity and the dominance

of gelatinous organisms such as jellyﬁ sh.

• Coral reef damage as increased nutrient levels favor

algae growth over coral larvae. Coral growth is inhibited

because the algae outcompetes coral larvae for available

surfaces to grow.

• A shift in phytoplankton species composition, creating

favorable conditions for the development of nuisance,

toxic, or otherwise harmful algal blooms.

• Low dissolved oxygen and formation of hypoxic or “dead”

zones (oxygen-depleted waters), which in turn can lead to

ecosystem collapse.

The scientiﬁ c community is increasing its knowledge of how

eutrophication affects coastal ecosystems, yet the long-term

implications of increased nutrient ﬂ uxes in our coastal waters

are currently not entirely known or understood. We do know

that eutrophication diminishes the ability of coastal ecosystems

to provide valuable ecosystem services such as tourism, recre-

ation, the provision of ﬁ sh and shellﬁ sh for local communities,

sportﬁ shing, and commercial ﬁ sheries. In addition, eutrophica-

tion can lead to reductions in local and regional biodiversity.

Today nearly half of the world’s population lives within 60 ki-

lometers of the coast, with many communities relying directly

on coastal ecosystems for their livelihoods.

This means that a

signiﬁ cant portion of the world’s population is vulnerable to the

effects of eutrophication in their local coastal ecosystems.

Harmful Algal Blooms and Hypoxia. Two of the most acute and

commonly recognized symptoms of eutrophication are harmful

algal blooms and hypoxia.

Harmful algal blooms can cause fish kills, human illness

through shellﬁ sh poisoning, and death of marine mammals

and shore birds.

Harmful algal blooms are often referred to

as “red tides” or “brown tides” because of the appearance of

the water when these blooms occur. One red tide event, which

occurred near Hong Kong in 1998, wiped out 90 percent of

the entire stock of Hong Kong’s ﬁ sh farms and resulted in an

estimated economic loss of $40 million USD.

9,10

Hypoxia, considered to be the most severe symptom of eutro-

phication, has escalated dramatically over the past 50 years,

increasing from about 10 documented cases in 1960 to at least

169 in 2007.

11,12

Hypoxia occurs when algae and other organ-

isms die, sink to the bottom, and are decomposed by bacteria,

using the available dissolved oxygen. Salinity and temperature

differences between surface and subsurface waters lead to

stratiﬁ cation, limiting oxygen replenishment from surface

waters and creating conditions that can lead to the formation

of a hypoxic or “dead” zone.

Two of the most well-known hypoxic areas are the Gulf of

Mexico and the Black Sea. The Gulf of Mexico has a seasonal

hypoxic zone that forms every year in late summer. Its size

varies; in 2000, it was less than 5,000 km2, while in 2002 it

was approximately 22,000 km2 (or the size of the U.S. state of

Massachusetts). While the economic consequences of the Gulf

of Mexico dead zone are still unclear, concern over its increas-

ing size led to the formation of the Mississippi River/Gulf of

Mexico Watershed Nutrient Task Force in 1997 to develop a

strategy to reduce the ﬁ ve-year running average areal extent of

the Gulf of Mexico hypoxic zone to less than 5,000 km2.

The Black Sea, which was once the largest dead zone in the

world, had 26 commercially viable ﬁ sh species in the 1960s but

The ﬁ rst comprehensive list of hypoxic zones was compiled by

Diaz and Rosenberg in 1995 and identiﬁ ed 44 documented

hypoxic areas.

Twelve years later, there are 169 documented

hypoxic areas, a nearly four-fold increase. The list of hypoxic

areas assembled by Diaz was compiled from scientiﬁ c litera-

ture and identiﬁ ed the majority of documented hypoxic areas.

However, the list did not include areas with suspected—but

not documented—hypoxic events or systems that suffer from

other impacts of eutrophication such as nuisance or harmful

algal blooms, loss of subaquatic vegetation, and changes in

the structure of the benthic aquatic community (for example,

decline in biomass, changes in species composition, and loss

of diversity).

To supplement this list of hypoxic zones, the authors undertook

an extensive literature review to catalog systems experiencing

any symptoms of eutrophication, including—but not limited

to—hypoxia.

The eutrophic areas identiﬁ ed were categorized

as:

• Documented hypoxic areas. Areas with scientiﬁ c evidence

that hypoxia was caused, at least in part, by nutrient

overenrichment. This category includes the most recent

list of hypoxic areas compiled by Diaz (excluding hypoxia

caused by natural upwelling of nutrients), as well as some

additional systems identiﬁ ed by our research.

• Areas of concern. Systems that exhibit effects of eutrophi-

cation, such as elevated nutrient levels, elevated chloro-

phyll a levels, harmful algal blooms, changes in the benthic

community, damage to coral reefs, and ﬁ sh kills. These

systems are impaired by nutrients and are possibly at risk

of developing hypoxia. Some of the systems classiﬁ ed as

areas of concern may already be experiencing hypoxia, but

lack conclusive scientiﬁ c evidence of the condition.

• Systems in recovery. Areas that once exhibited low dis-

solved oxygen levels and hypoxia, but are now improving.

For example, the Black Sea once experienced annual

hypoxic events, but is now in a state of recovery. Others,

like Boston Harbor in the United States and the Mersey

Estuary in the United Kingdom, also have improved

water quality resulting from better industrial and waste-

water controls.

WHERE ARE THE DATA INCONSISTENCIES AND GAPS?

The actual extent and prevalence of eutrophication in many

regions is only beginning to be studied. As a consequence, data

do not exist or are not publicly available for many areas that

may be suffering from the effects of eutrophication. In addi-

tion, the data that do exist are often inconsistent in terms of

parameters measured, indicators used, and the scale at which

data are reported.

Given the state of global data, the number of eutrophic and

hypoxic areas around the world is expected to be greater than

the 415 listed here. The most underrepresented region is Asia.

Asia has relatively few documented eutrophic and hypoxic

areas despite large increases in intensive farming methods,

industrial development, and population growth over the past

20 years. Africa, Latin America, and the Caribbean also have

few reliable sources of coastal water quality data, making it

difﬁ cult to assess the true level of eutrophication.

The scale at which data for eutrophic areas are reported var-

ies greatly. For example, red tides and other eutrophic events

are frequently recorded in the South China Sea, Yellow Sea,

and Bohai Sea. However, the actual extent of eutrophication

or hypoxia in speciﬁ c bays and estuaries within these systems

is unknown. Therefore, these areas are coarsely recorded in

Figure 1 and most likely represent several affected bays and

estuaries along the Chinese coast. Conversely, within the

well-studied Chesapeake Bay system, there are 12 distinct and

documented eutrophic and hypoxic zones.

The United States, European Union, and Australia—all of which

have each undertaken comprehensive coastal surveys in the past

ﬁ ve years—have the most comprehensive coastal data on eutro-

phication. However, even within the United States and Europe,

the quality, consistency, and availability of water quality data

varies. For example, in Europe, the coastal survey coordinated

through the Commission for the Protection of the Marine En-

vironment of the North-East Atlantic (OSPAR)

highlighted the

variability in quality and availability of monitoring data among

the various European countries.

Several coastal areas lacked

dissolved oxygen measures and many of the secondary effects

of eutrophication, such as ﬁ sh kills and changes in the benthic

community, were not extensively monitored. The U.S. National

Oceanic and Atmospheric Agency’s (NOAA) National Estuarine

Eutrophication Assessment program, which conducted national

eutrophication surveys in 1999 and 2007, experienced data

issues similar to those encountered in the OSPAR survey. Of

the 141 U.S. estuaries evaluated in 2007, 30 percent lacked

adequate data for assessing their eutrophication status.

Most systems identiﬁ ed as hypoxic or eutrophic also lack

detailed data on the sources of nutrient impairments and the

relative contribution of each source. Some systems that have

developed detailed nutrient budgets include the Chesapeake

only ﬁ ve species by the 1980s.

The growth of the Black Sea

hypoxic zone was attributed to the intensiﬁ cation of agriculture

in the former Soviet Union. It has been “in recovery” since

the economic collapse of Eastern Europe in the 1990s, which

resulted in signiﬁ cant reductions in fertilizer use.

WHAT ARE THE SOURCES OF NUTRIENTS?

Agriculture, human sewage, urban runoff, industrial efﬂ uent,

and fossil fuel combustion are the most common sources of

nutrients delivered to coastal systems. Among regions, there are

signiﬁ cant variations in the relative importance of each nutrient

source. For example, in the United States and the European

Union, agricultural sources (particularly animal manure and

commercial fertilizers) are generally the primary contributors

to eutrophication, while sewage and industrial discharges,

which usually receive some treatment prior to discharge, are a

secondary source. However, in Latin America, Asia, and Africa,

wastewater from sewage and industry are often untreated and

may often be the primary contributors to eutrophication. It is

currently estimated that only 35 percent of wastewater in Asia

is treated, 14 percent in Latin America and the Caribbean, and

less than 1 percent in Africa.

Atmospheric sources of nitrogen are also recognized as a sig-

niﬁ cant contributor of nutrients in coastal areas.

Nitrogen

from fossil fuel combustion and volatilization from fertilizers

and manure is released into the atmosphere and redeposited

on land and in water by wind, snow, and rain. In the Chesa-

peake Bay in the mid-Atlantic region of the eastern United

States, atmospheric sources of nitrogen account for a third

of all controllable nitrogen that enters the Bay; similarly, in

the Baltic Sea in Europe, atmospheric nitrogen accounts for

a fourth of all controllable nitrogen.

WHERE ARE THE GLOBAL EUTROPHIC AND HYPOXIC

AREAS?

Our research identiﬁ ed 415 eutrophic and hypoxic coastal

systems worldwide (Figure 1).

Of these, 169 are documented

hypoxic areas, 233 are areas of concern, and 13 are systems

in recovery.

The coastal areas reported as experiencing eutrophication are

steadily growing. This is because of the increasing prevalence

of eutrophication and advances in identifying and reporting

eutrophic conditions.