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Learning More about Estuarine Eutrophication

What is Eutrophication?: Estuarine Water Quality as a Coastal Stress Indicator

Estuarine water quality degraded by excessive nutrient inputs — what experts call eutrophication –is a key barometer of coastal environmental stress.

In coming decades, the nation’s coasts are expected to experience increased environmental stress as a result of further development and population growth. Scientific committees and consensus reports consistently rank nutrient enrichment and eutrophication as leading problems – perhaps the leading problem – for U.S. estuaries.

The National Research Council, for instance, has pointed to “the severity of nutrient-related problems and the importance of the coastal areas at risk” as justifying a national management strategy to combat nutrient over-enrichment.

Some locations of observed eutrophic symptoms
Eutrophication symptoms System or country of observation
Toxic Algal Blooms
  • Pamlico and Neuse River Estuaries (Burkholder et al., 1995, 1999)
  • NE Coast of UK (Joint et al. 1997)
Nuisance Algal Blooms
  • Lower Laguna Madre (Whitledge and Pulich, 1991)
  • Southern North Sea (Gillbricht, 1988)
  • Baltic Sea (Bonsdorff et al, 1997)
  • Mediterranean Sea (e.g. Lac de Tunis: Kelly and Naguib, 1984),
  • Northern Adriatic (Chiaudani et al, 1980),
    Peel Harvey Estuary, Australia (Hodgkin and Birch 1982; Hodgkin and Hamilton 1993)
  • Japan (Okaichi, 1989; Okaichi, 1997)
Depleted Dissolved Oxygen
  • Mississippi River Plume (Rabalais et al., 1996)
  • Chesapeake Bay (Cooper and Brush, 1991)
Loss of SAV
  • Chesapeake Bay (Orth, 1983)
  • Tampa Bay (Greening et al., 1997)
  • Laguna Madre (Onuf, 1995)

Eutrophication is a process in which the addition of nutrients to water bodies, primarily nitrogen and phosphorus, stimulates algal growth. The process occurs naturally, but human activities greatly accelerate it.

Estuaries have always received nutrients from natural sources in the watershed and from the ocean. But in recent decades, population growth and related activities, such as various agricultural practices, wastewater treatment plants, urban runoff, and the burning of fossil fuels, have increased nutrient inputs well beyond levels that occur naturally.

Those increased nutrient inputs promote a complex array of symptoms, beginning with the excessive growth of algae, which, in turn, may lead to other, more serious symptoms such as depleted dissolved oxygen (e.g. hypoxia and anoxia which cause fish kills) and losses of submerged grasses that act as habitat for nursery fisheries as a result of poor water clarity.

Furthermore, nutrient enrichment is a common thread linking an array of problems along the Nation’s coastlines – from harmful algal blooms that can result in massive fish kills and endanger human health, to destruction of coral reefs, seabird and mammal deaths, and development and expansion of “dead zones” where oxygen levels fall to levels unable to support fish and other animal life.

Concerns over just such issues prompted the U.S. Congress to pass the Harmful Algal Bloom and Hypoxia Research and Control Act of 1998. Comparable symptoms have also been reported in Europe (and worldwide, Table 1), leading to passage of the European Commission Water Framework Directive to assess and manage nutrient related problems in European waters.

A Comprehensive National Assessment of Eutrophic Conditions

The 1999 National Estuarine Eutrophication Assessment, by Special Projects Office (SPO) and the National Centers for Coastal Ocean Science (NCCOS), was the first comprehensive assessment of estuarine eutrophication conditions across the U.S.

The assessment was based on nutrient related loading and water quality data and information acquired from scientists and resource managers for 138 US estuaries and coastal waterbodies. The data and information for individual systems was summarized to show conditions on a regional and national basis, painting a picture of the conditions, causes and future outlook of eutrophic symptoms in the nation’s coastal waterbodies (Figs. 1 and 2).

That assessment provides a first cut at understanding the extent of hypoxia and eutrophication nationwide and understanding how and why conditions differ across estuaries. It provides a basis for determining a sound nutrient management strategy for the nation’s estuaries, and also provides a basis for a monitoring and research program to provide data for understudied systems. (Fig. 3).

Figure 1: Expression of Eutrophic symptoms.

Figure 1: Expression of Eutrophic symptoms. These maps depict estuaries with moderate to high levels of expression of eutrophic symptoms, indicating areas of possible concern. Note that these symptoms are not necessarily related in whole to human-related nutrient inputs; natural causes and other human disturbances may also play a role, to various degrees, in the expression of symptoms. (from Bricker et al., 1999)

In a current and ongoing update of that 1999 assessment, the eutrophication model used to determine overall eutrophic status scores estuaries based on six symptoms (Fig. 1).

Three primary symptoms – decreased light availability, changes in algae, and increased organic matter production – represent the first possible stage of water quality degradation associated with eutrophication. In many estuaries, these primary symptoms lead to secondary symptoms: loss of submerged aquatic vegetation, increases in nuisance or toxic algal blooms, and low dissolved oxygen.

The National Estuarine Eutrophication Assessment (NEAA) showed that nutrient-related water quality problems occur in systems along all coasts nationwide and that without preventive efforts, those problems will continually worsen. Of the 138 estuaries examined, the study found 44 to be highly eutrophic, with 36 having a high level of human influence on development of eutrophic symptoms.

The assessment showed that 82 estuaries, representing 67 percent of the U.S. estuarine surface area, exhibited high levels of at least one of the following symptoms: depleted dissolved oxygen, loss of submerged vegetation, or nuisance toxic algal blooms (Fig. 1). High conditions, the researchers found, were common in estuaries along all U.S. coasts, but most prevalent along the Gulf and Middle Atlantic coasts (Fig. 2).

The report recommended that management plans focus on controllable sources of nutrients and on strategies tailored to the individual characteristics of each watershed. These plans must also take into account overall eutrophic conditions and distinguishing factors in each estuary, so that efforts can focus on those that could most benefit from nutrient controls (Fig. 3).

For instance where human actions are the primary influencing factor in systems with well developed eutrophic symptoms, appropriate management actions should be undertaken. The report noted that estuaries that rated as “highly susceptible” because of limited flushing capacity are a special case for preventive actions because they are especially vulnerable to any increases in nutrient loads.

Intended as a “first cut” and notwithstanding its extensive findings, the NEEA clearly acknowledged that much remains to be done to better characterize the extent of eutrophication nationwide; how those conditions impair everyday uses important to the American public; and what management actions are appropriate. The report called for filling data gaps: 17 estuaries lacked sufficient data to allow scientists to assess conditions. For another 32 estuaries, assessment reliability was low because of limited or uncertain data

Figure 2: Estuaries with high levels of expression of eutrophic conditions based on a combination of the 6 indicators shown in Figure 1

Figure 2: Estuaries with high levels of expression of eutrophic conditions based on a combination of the 6 indicators shown in Figure 1 (Note: Conditions are not necessarily related in whole to human-related eutrophication; to various degrees natural causes and other human disturbances may also play a role. For instance, some estuaries in Maine are typified by natural occurrences of toxic algae, which drift in from the open ocean. Once in the estuary, however, these blooms may be sustained by human nutrient inputs, from Bricker et al., 1999)

Over the past five years, the study results – reflected in a number of subsequent high-visibility reports nationwide – have informed public policy decision-making domestically and abroad. The report was widely quoted during congressional reauthorization of the Farm Bill. In Europe, there is growing interest in the assessment methodology, and water quality officials in Portugal have used it to meet reporting requirements of the European Commission Water Framework Directive. The European Commission Eutrophication working group is considering adding the methodology to the list of recommended assessment tools.

Update to Build on, Go Beyond, Initial Assessment

NOAA scientists are presently working to build on and update the 1999 landmark national assessment of eutrophication that helped pave the way to better understanding estuary conditions nationwide.

In September 2002, NCCOS convened an expert working group to discuss design of a long-term monitoring and assessment program, The group used as a starting point the 1999 assessment and the methodological improvements proposed by Bricker and her colleagues.

The group brought together resource managers, state, local and federal scientists, and academic scientist from the United States, Europe, and Mexico. Workshop participants recommended classifying estuaries into types likely to respond differently to perturbation and additional procedural steps. The group also strongly recommended that the database, processes (and/or tools) and outcomes be freely and readily available to stakeholders and potential users

The National Eutrophication Assessment Update Program now under way is collecting more recent data, improving assessment methodologies, and developing tools to help managers choose among management options. (These results and tools will be available online at The update will help policy makers determine, for instance, if systems with conditions projected to deteriorate in fact have done so. It will also help them better evaluate various corrective measures.

Where Best to Apply Limited Resources?

An important goal of the program is to help managers decide where to put their resources to have the most impact. Models, Bricker says, “must include the ability to distinguish between situations where management measures can affect change, and those which are naturally occurring and cannot be managed except operationally, for example by early and timely fisheries interdiction in the case of harmful algal blooms.”

The assessment update is proceeding on two simultaneous and related tracks: classifying estuaries and coastal waters based on physical and chemical characteristics into groups that behave similarly; and identifying impacts on society of nutrient-impaired water quality.

Bricker points to promising preliminary results: She says the approach is helping with identification of indicator variables for tracking with remote or field sampling methods. At that stage, type-specific models can monitor specific variables as early warning signs, and show effects if the loadings are changed.

One-web based assessment tool being developed does just that. It calculates the assessment value automatically based on parameters provided online, and it will be able to recalculate that value when thresholds change ( Users will also be able to compare results with those of other systems, and also with previous assessments in the same estuary.

One key aspect of type classification is the degree to which a coastal system either holds water in, or flushes it through.

Figure 3: A framework for developing a national strategy used to help better focus national attention on existing and emerging priority areas for action; i.e., management, monitoring, and researc. (from Bricker et al., 1999)

Figure 3: A framework for developing a national strategy used to help better focus national attention on existing and emerging priority areas for action; i.e., management, monitoring, and researc. (from Bricker et al., 1999)

“Like a bath tub or chemostat,” Bricker explains, “systems that hold water in will also hold nutrients in, giving phytoplankton the time and opportunity to grow.” With an abundance of nutrients, that growth can be excessive. On the other hand, Bricker says, “a system that pushes water through, either by large fresh water inflow or large tides or both, also pushes nutrients and phytoplankton through, reducing the opportunity for excessive algae to grow inside the system.” This characteristic will also direct the level of management (see Fig. 3).

Exploring New Fields: Socioeconomic Impacts

An important component in the ongoing update program is that it will examine socioeconomic impacts of nutrient-related water quality problems on various human uses of estuaries. Scientists conducting the 1999 assessment report could not thoroughly explore those impacts, and they acknowledge that they still are the least developed of all the assessment components.

Those considerations prompted the NCCOS expert working group members to recommend that social scientists and economists with experience in economic impact analysis help develop the new models. As a result, NCCOS now has developed working partnerships with economists at the University of Maryland Department of Agricultural and Resource Economics, and the U.S. Department of Agriculture’s Economic Research Center.

Looking closely at social and economic costs of eutrophication will break important new ground. Existing studies, Bricker says, “look mostly at costs of lost productivity because indirect and non-use costs are difficult to measure”.

“Preliminary analysis of the Long Island Sound data,” she says, “shows that as nitrogen inputs decrease, dissolved oxygen and recreational catch of Striped Bass increase.”

A next step, says Bricker, may involve converting this information into dollars and cents – so that the impacts on human uses and the economic costs of the loss of use can be highlighted to draw more attention to this pervasive national and global problem.

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