of Hypoxia in the Northern Gulf of Mexico of Hypoxia in the Northern Gulf of Mexico
hile information gaps still exist and several factors discussed below may contribute to hypoxia, the overwhelming

scientific evidence indicates that excess nitrogen from the

Mississippi River drainage basin coupled with hydrologic and climatic factors drives the onset and duration of hypoxia in the

northern Gulf of Mexico. The following section summarizes this

evidence, which is described in the detailed topic reports and the

scientific literature upon which this assessment is based. Analysis

of nutrient trends and sources, and the relative importance of specific human activities in contributing to these trends, are described

here, providing the basis for the chapters that follow. 

Coastal Eutrophication

The development of hypoxia in northern Gulf of Mexico bottom waters, as well as in most ocean and freshwater systems, is

described best by the eutrophication process. Eutrophication is

defined as an increase in the rate of supply of organic matter in an

ecosystem (Nixon 1995). This increase in organic matter can be

due to flux from external sources or to production within the system through biological processes stimulated by nutrient addition.

Controlling or reversing the effects of eutrophication necessitates

understanding the relative roles of externally supplied and internally produced organic carbon. Externally supplied organic carbon

most often comes from wetlands or uplands via rivers. Internally

produced organic carbon comes from the growth of algae.

As shown in Figure 2.1, nutrients stimulate production of

phytoplankton algae. Algae that are not incorporated into the

food web, fecal products, and other debris from zooplankton 

feeding on algae, together with externally supplied particulate

organic carbon, sink into bottom waters as “excess” organic material. Decomposition of this organic material by bacteria and other

organisms consumes oxygen, and when the consumption rate is

faster than its replenishment, oxygen concentrations decrease.

Excess organic carbon and subsequent oxygen consumption

alone do not necessarily lead to hypoxia: stratification is also necessary for the development of hypoxia in bottom waters. In most

saltwater systems, the water stratifies (creating horizontal layers),

with cold and/or saltier water at the bottom and warmer and/or

fresher water at the surface. This layering separates bottom waters

from the atmosphere and prevents re-supply of oxygen from the

surface to replenish deficits created by the decomposing organic

Chapter 2: The Causes ■ 11
matter in the deeper waters. Thus, the

extent of hypoxia is determined by the

balance between the rate of delivery and

decomposition of algae and other organic

matter and the rate of oxygen re-supply,

which is inversely related to the strength

of stratification.

Hypoxia, and other symptoms of

eutrophication, such as growth of nuisance or toxic algae and loss of submerged

aquatic vegetation, are major stresses in

many coastal ecosystems. Over half of the

nation’s estuaries experience low oxygen

and other symptoms of eutrophication

(Bricker et al. 1999). Almost all of these

problems are caused or exacerbated by the

increased flow of nutrients from land due

to human activities. There is growing evidence around the world that low oxygen

is having pervasive effects on shallow

coastal and estuarine areas (Diaz and

Rosenberg 1995). While hypoxia can

occur naturally and has existed throughout geologic time, its occurrence in shallow coastal and estuarine areas appears to

be increasing and is most likely accelerated by human activities (Vitousek et al.

1997; Jickells 1998).

In the Gulf, hypoxic waters are most

prevalent from late spring through late

summer. Hypoxia is more widespread and

persistent in some years than

in others, depending on

river flow, winds, and

other environmental

variables. Hypoxic waters

are distributed from shallow depths near shore (4

to 5 meters) to as deep as

60 meters but more typically appear between

5 and 30 meters.

Hypoxia occurs

mostly in the lower

water column but

encompasses as much

as the lower half to

two-thirds of the entire

column. 

Continuous

time-series data

for the bottom

waters in the core of the hypoxia region

show (1) the gradual decline in oxygen in

the spring with interruptions due to windmixing events, (2) persistent hypoxia and

often anoxia for extended parts of the

record from May through September, (3)

occasional summer upwelling of oxygenated water from the deeper shelf

waters, and (4) the seasonal disruption of

low oxygen in the fall by tropical storms

or cold fronts.

Role of Nutrients

Nitrogen is the most significant nutrient controlling algal growth in coastal

waters, while phosphorus is the most significant nutrient in fresh water (NRC

1993). Silicon also plays an important

role in that it regulates the production of

diatoms, which are a type of algae important as food for fish and invertebrates.

Diatoms, unlike other phytoplankton

species, require silicon to build their cell

walls and are the dominant biomass component of many marine and estuarine

phytoplankton communities, particularly

in the spring. If silicon supply is limited

relative to the other nutrients, then nonsiliceous algae, such as dinoflagellates

and cyanobacteria, may become proportionately more important in the phytoplankton community. Some of these

forms are harmful or toxic. The ratios

among dissolved silica, nitrogen, and

phosphorus in the lower Mississippi

River now closely approximate the typical ratio of these nutrients in diatoms.

Thus, while nitrogen is the nutrient that

controls the quantity of coastal algal production, significant changes in the relative composition of nutrient supplies may

also lead to important alterations in the

type of algae present.

Role of Stratification

If Louisiana shelf waters did not stratify, hypoxia would be unlikely. However,

the river and Gulf oceanography create a

strongly stratified system each year, characterized by relatively warm and fresher

water that forms a high-production lens

over the deeper salty, colder waters. The

greater the density difference (related to

temperature and salinity) between layers,

the more stable the stratification. Very

strongly stratified systems require a lot of

wind energy—sometimes a tropical

storm—in order to mix. If storms do not

mix the waters, they will remain in layers,

isolating bottom waters from aeration

until fall brings cooler surface temperatures and the density of the surface water

approaches that of the bottom water.

12 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

The overwhelming

scientific evidence

indicates that excess

nitrogen from the

Mississippi River

drainage basin coupled

with hydrologic and

climatic factors drives

the onset and duration

of hypoxia in the

northern Gulf of Mexico.
Chapter 2: The Causes ■ 13

Role of Other Potentially

Contributing Factors

In addition to high levels of

nutrients, other factors potentially

contribute to hypoxia in the Gulf.

The most significant of these

other factors are described in the

box on the next two pages. Only

increased nitrogen loads from the

Mississippi–Atchafalaya River system can account for the magnitude of the hypoxic zone and its

increase over time. While other

factors may contribute to the

growth, dynamics, and decline of

the hypoxic zone, none of them

alone can explain its overall size

and persistence.

Internally produced organic

carbon, stimulated by nutrients

(from the land, air, or sea), externally supplied organic carbon,

horizontal stratification, ocean

circulation, and river hydrology

are not competing hypotheses.

Rather, they are interacting factors within the eutrophication

process as it takes place in the

Gulf of Mexico. These factors

have been discussed in the six

technical background papers, in

the comments on those papers, at

a science meeting on these issues

held December 3, 1999,2 and in

recent reports on this subject

sponsored by the Fertilizer

Institute (Carey et al. 1999) and

the Council for Agricultural

Science and Technology

(Downing et al. 1999), as well as

the wide range of previous work

summarized in such volumes as

the 1995 Proceedings of the First Gulf

of Mexico Hypoxia Management

Conference (EPA 1997) and the

December 1994 and June 1996

special issues of Estuaries. 

Nutrient Over-Enrichment

The evidence for nutrient overenriched production in the northern Gulf of Mexico and its linkage

with oxygen depletion in the

lower water column is consistent

with the eutrophication process,

with data and experiences worldwide, and with Gulf- and basinspecific information on a variety

of scales.

Scientific investigations over

the last several decades indicate

Eutrophication occurs when organic matter increases in an ecosystem. Eutrophication can lead to hypoxia when decaying

organic matter on the seafloor depletes oxygen, and the replenishment of the oxygen is blocked by stratification. The flux of

organic matter to the bottom is fueled by nutrients carried by riverflow or, possibly, from upwelling that stimulates growth of

phytoplankton algae. This flux consists of dead algal cells together with fecal pellets from grazing zooplankton. Organic

carbon from the Mississippi River can also contribute to the flux of organic matter.

FIG. 2.1 The Eutrophication Process

2 Notes from the December 3, 1999, science meeting can be found at <http://www.nos.noaa.gov/products/pubs_hypox.html>
14 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

overwhelmingly that oxygen

stress in the northern Gulf of

Mexico is caused primarily by

excess nutrients delivered to Gulf

waters from the Mississippi–

Atchafalaya River drainage basin,

in combination with the stratification of Gulf waters.

This section outlines the scientific evidence supporting river

nutrient loads as the primary factor fueling hypoxia in the Gulf of

Mexico through over-enrichment.

It is supported by analysis of the

sources and loads, as described in

the next section.

Trends in Indicators of Production,

Low-Oxygen Stress, and River

Nutrient Flux

Analysis of river discharge data

and sediment cores from the

Louisiana shelf in the Mississippi

River delta bight indicate that

Landscape changes in the drainage

basin. Wetlands and riparian zones

can improve water quality and reduce

nitrogen flux down the Mississippi

River by enhancing denitrification (the

conversion of nitrate to nitrogen gas,

with subsequent loss from the aquatic

system) and by incorporating nitrogen

into vegetation. However, the natural

capacity of the river basin to remove

nutrients has diminished. Many of the

original freshwater wetlands and

riparian zones that were connected to

streams and rivers are gone. Ohio,

Indiana, Illinois, and Iowa have had

over 80 percent of their wetlands

drained. Indiana, Illinois, Iowa,

Minnesota, Missouri, Ohio, and

Wisconsin collectively have lost the

equivalent of 14.1 million ha (35 million

acres) of wetlands over the past 200

years. Louisiana, Mississippi,

Arkansas, and Tennessee have also

experienced wetland losses that collectively exceed 50 percent.

Organic loading from the Mississippi

River. Externally supplied organic carbon from the Mississippi River has

been proposed as a cause of the formation of hypoxia in the Gulf. For riverborne organic carbon to be an

important contributor to the rise in

hypoxia since the 1950s, significantly

increasing quantities would have to be

delivered to the bottom waters of the

hypoxic zone and would have to be

decomposed by bacteria. Only the particulate organic carbon (POC) fraction

of the total organic carbon load could

contribute directly to hypoxia because

only that fraction would sink to the

bottom waters. The dissolved fraction

would remain in the river-freshened

surface waters and would have to be

biologically assimilated and deposited

as newly formed POC, with large

attendant respiratory losses, to reach

hypoxic zones extending 100–200 km

from the river discharges. Suspended

sediment in the river has declined by

about half since the 1950s, so the POC

load that can settle on the Louisiana

shelf has also most likely decreased

since then. Also, nutrient cycling

affords nitrogen the ability to stimulate

production of organic carbon at rates

that far surpass that supplied by the

river. Whereas decomposition of riversupplied organic carbon consumes

oxygen only once, river-supplied nitrogen can be recycled and thus provides

a continuous source of comparatively

easily decomposed organic carbon

(Ryther and Dunstan 1969). Scientists

reviewed evidence about the role of

organic carbon at a December 1999

meeting and agreed that it is a relatively small factor driving hypoxia—

nitrogen-driven carbon production is

approximately an order of magnitude

greater.

Factors Potentially Contributing to Hypoxia in the Gulf
Chapter 2: The Causes ■ 15

eutrophication and hypoxia in the

northern Gulf increased coincidentally with increases in nitrogen

loads from the Mississippi River.

The data exhibit both long-term

trends and short-term variations.

One particularly important factor

is that peak river flows have

increased over the past 150 years

due primarily to land-use changes,

flood-control and navigation projects in the river, and climate (precipitation) changes. 

Long-term Records—Gulf Ecosystem

Changes. Since there are no comprehensive, direct measurements

of oxygen in the northern Gulf of

Mexico prior to 1985, evidence

for long-term changes in hypoxia

is based on indirect analysis from

sediment records. While it is possible to explore temporal changes

in water-column production,

Channelization of the delta and loss of

coastal wetlands. Although in some

years over-bank flow has been significant, flooding through coastal wetlands has been reduced since the 1927

flood, but geochemical and biological

indicators show that hypoxia has

intensified only since the 1950s.

Coastal wetland loss rates exceeded

100 km2 (about 40 sq mi) per year

between 1950 and 1980. Recent wetland protection and restoration programs helped reduce losses in the

1990s to 65–90 km2 (about 25–35 sq mi)

per year. Both the loss of wetland filtering capacity and the direct contribution of organic matter eroded from

wetlands and carried to the Gulf may

contribute to the problems of hypoxia;

however, the total contribution is relatively small compared with the nitrogen-related factors. Further, carbon

isotope analysis shows that the

sources of carbon over the broad area

affected by hypoxia is produced by

marine phytoplankton and is distinct

from carbon close to shore.

Intrusion of deeper offshore waters.

Some have suggested that flow of

low-oxygen water from water layers in

the deeper Gulf on to the continental

shelf was the source of shelf hypoxia.

However, these features are always

physically distinct, with the shelf

hypoxic zone at depths of less than 60

meters, and the deep low-oxygen layer

at depths of 400–700 meters. Further,

oxygen concentrations and consumption rates, salinity, and temperature of

the deep low-oxygen layer differ considerably from the waters of the

hypoxic zone. Flow of nitrate from

deeper waters may be important at the

shelf edge (at depths of approximately

100 meters); however, all data indicate

that the Mississippi and Atchafalaya

Rivers contribute substantially more

nutrients to the inner shelf and

hypoxic region.

Short- or long-term climate changes.

River discharge and nitrate concentrations, and sediment core data, provide

almost 100 years of record for this system. On that time scale, there is no

indication that climate factors override

the impacts of human activities in the

basin. Average annual flow in the

Mississippi River increased 30 percent

between 1955–70 and 1980–96, compared to the 300 percent increase in

nitrate flux over this period. Episodic

events, such as the 1993 flood, can

nearly double the nitrate flux to the

Gulf in a given year as a result of both

higher-volume flow and increased

loading of nitrate from the drainage

basin. There are indications that the

future climate for this basin may be

wetter and may include more extreme

events, leading potentially to

increased water and nitrate fluxes

(Kunkel et al. 1999).
16 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

Profiles from cores taken in the Mississippi River bight in an area of persistent seasonal bottom-water hypoxic conditions

revealed the following trends related to the growth of bottom-water hypoxia in the area: (1) a steady decrease in the number of

species of ostracods, a family of minute shelled organisms; (2) an increase in the abundance of the mineral glauconite, which

forms only under low-oxygen conditions; (3) an increase in the average concentration of biologically bound silica, indicating

changes in algal community composition; and (4) an increase in the organic carbon accumulation rates, indicating growth in

algal production.

FIG. 2.2 Long-term Records of Changes in the Gulf Ecosystem: 1900–90

1.6

1.2

0.8

0.4

0

1900 1915 1930 1945 1960 1975 1990

Shannon-Wiener

Diversity Index

25

20

15

10

5

0

1900 1915 1930 1945 1960 1975 1990

% of Coarse Grains

1.50

1.25

1.00

0.75

0.50

0.25

0

1900 1915 1930 1945 1960 1975 1990

% Biologically Bound Silica

10

8

6

4

2

0

1900 1915 1930 1945 1960 1975 1990

Accumulation Rate

(mgC/cm2/yr)

Sources: Figures 7.6, 7.7, and 7.9 in the Topic 1 Report for the Integrated Assessment.

ostracod

glauconite

biogenic silica

organic carbon
Chapter 2: The Causes ■ 17

oxygen-relevant geochemistry,

and biology because such properties are preserved sequentially

down a sediment core, it is important to recognize that bottom

mixing and sediment reworking

limits the resolution of temporal

change to scales approaching

decades, as opposed to years. 

The data shown in Figure 2.2

are from cores collected within one

area of the current hypoxic zone.

Therefore they do not necessarily

represent the timing of the transitions for the entire contemporary

hypoxic zone. They do, however,

clearly show that algal production

and deposition, as well as oxygen

stress, were much lower earlier in

the twentieth century and that significant changes have occurred in

the latter half of the century.

The sediment record clearly

shows significant increase in algal

production beginning around the

1950s. Accumulation of organic

carbon and biogenic silica (a

measure of diatom abundance)

both increased significantly after

that time period. Because there

have been no significant increasing trends in either the organic

carbon or the silica river loads, it

is reasonable to infer that these

increases in the sediment record

since the 1950s are due to production of marine algae.

The sediment geochemistry

record indicates that while there

was likely some oxygen stress on

the Louisiana shelf as early as the

beginning of the twentieth century, there was a striking increase

in that stress starting roughly during the 1940s and 1950s. Because

the mineral glauconite forms only

under reducing conditions, its

presence in the sediment is an

indication of low oxygen conditions. Sediment cores show the

history of glauconite formation

from sediments as they were

deposited on the seafloor. As

shown in the core profile in

Figure 2.3, glauconite abundance

was relatively low in the early

1900s, but increased dramatically

in the 1940s and 1950s, indicating

increased low oxygen stress.

Another indirect indicator of a

significant change in the relative

magnitude of hypoxia during this

same transition period is the biodiversity of benthic foraminifera

and ostracods preserved in the

sediments. These single-celled

The amount of nitrogen fertilizer used in the Basin increased dramatically between the 1950s and 1980s (enabling a large

increase in crop production). The area of cropland fitted with artificial subsurface drains has also increased, reducing the

capacity of the landscape to remove nitrogen.

Long-term Records of Changes in the

Mississippi–Atchafalaya River Basin: 1900–99

FIG. 2.3

Sources: Figure 5.7 of the Topic 3 Report for the Integrated Assessemnt; and Figure 1.2 of the Topic 5 Report.

8

6

4

2

0

1900 1920 1940 1960 1980 2000

Estimated Nitrogen Fertilizer 

Use in the MARB

(millions of metric tons/year)

80

60

40

20

0

1900 1920 1940 1960 1980 2000

Estimated Land Drainage

in the MARB

(millions of acres)
organisms and metazoans, respectively,

produce carbonate shells which remain

intact in buried sediments. After a transition period in the 1940s through 1950s,

the diversity of these organisms decreased

significantly, which is typical of stressed

environments. Also, the types of organisms that remained after the transition

period were those that are more tolerant

of low oxygen conditions. 

Long-Term Records—Drainage Basin Changes.

Three key historical trends are important

to consider in relationship to the above

long-term trends in hypoxia. First, most of

the significant flood-control and navigation channelization occurred prior to the

1950s. Second, significant alterations in

the landscape (e.g., deforestation and

expansion of artificial agricultural

drainage) removed much of the “buffer”

for runoff into the Mississippi tributaries

and main stem. The greatest rates of

change occurred in the 50-year period

straddling the beginning of the twentieth

century, with another burst in drainage

development during 1945–60. Third,

there was a dramatic increase in nitrogen

input into the Mississippi River drainage

basin, primarily from fertilizer applications, between the 1950s and 1980s. This,

in turn, has led to significant increases in

the flux of nitrogen from the Mississippi

River system to the Gulf. 

Shorter-Term Records. Extensive monitoring

of the hypoxic zone did not begin until

1985, and areal estimates (see Figure 1.2)

since then have been primarily from a single shelf-wide survey taken during the

mid-summer of each year. Thus, for any

given year, the maximum extent of

hypoxia could have been significantly

greater. Despite this limitation, three significant extreme events illustrate the

nature of the variation in this record:

■ First, during the summer of 1988, following a severe regional drought and

subsequent low nitrogen load (see

Figure 2.4), the hypoxic zone covered

only 40 km2 (15 sq mi), compared to

5,000–20,000 km2 (2,000–8,000 sq mi)

in other years. The hypoxic zone had

developed in the spring but was not

maintained into the summer because of

reduced flow of fresh water and resulting lack of stratification.

■ Second, the size of the hypoxic zone

nearly doubled during the summer following the 100-year flood of 1993.

18 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

The annual flux of

nitrate nitrogen from

the Basin to the Gulf of

Mexico has almost

tripled between the

periods 1955-70 and

1980-96. Organic nitrogen measurements

were not regularly

made before 1973 but

show no trend. Both

streamflow and nitrate

flux have become

much more variable in

the last 25 years.

2.5

2.0

1.5

1.0

0.5

0

30

25

20

15

10

5

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Flux of Nitrogen

(millions of metric tons)

Mississippi River Streamflo

w

(thousands of cubic meters/second)

Annual Nitrogen Loads and

Streamflow to the Gulf: 1955–98

FIG. 2.4

Source: Figure 4.2 of the Topic 3 Report for the Integrated Assessment.

streamflow

organic 

nitrogen nitrat e
Chapter 2: The Causes ■ 19

While it is not clear why this larger

area has persisted in years following

1993, nitrogen recycling and resuspension of recently deposited, algalproduced organic material are likely

important factors.

■ Third, the year 1999 was somewhat

unusual in that both streamflow and

nitrate concentrations during the spring

and summer were above normal in the

upper Mississippi. The streamflow of

the Mississippi River at Thebes, above

the Ohio River confluence for

January–June 1999 averaged 9,300 m3/s,

as compared with a January–June

1980–98 mean of 8,054 m3/s. The mean

nitrate concentrations for January–June

1999 were 3.4 mg/L, versus a 1980–98

mean for these months of 2.7 mg/L.

However, drought conditions were

developing in the upper Ohio Basin,

which produced below-normal streamflow in 1999 of 10,650 m3/s, versus a

mean of 12,070 m3/s for January–June

1980–96. Nitrate concentrations (1.2

mg/L for January–June) were near normal for this period. The combination of

above-normal flows and nitrate concentrations from the upper Mississippi and

below-normal flows from the Ohio

River produced higher-than-normal

nitrate concentrations and flux in the

lower Mississippi River, and may have

contributed to the record large extent

of the hypoxic zone measured in July

1999.

Model Simulations

Model analyses provide further evidence connecting the extent of the

hypoxic zone to river nutrient flux.

Computer models have been constructed

to simulate algal dynamics, nutrient

cycles, sedimentation, organic carbon production and decay, and oxygen dynamics

as functions of river nutrient loads and

other environmental factors, such as variations in sunlight and ocean currents

(Bierman et al. 1994; Justi ́c et al. 1996,

1997). These models were designed to

examine the relative effects of various factors on hypoxia, such as ocean circulation,

vertical stratification, light variation,

nutrient loads, and climate change. These

analyses show that river nutrient load is a

highly significant factor in controlling

hypoxia and that rates of algal production

and oxygen depletion change significantly

when modeled river loads were changed.

Sources, Loads, and Trends of

River Nutrients

Analyses were conducted to determine the flux and sources of nutrients

transported from the Mississippi–

Atchafalaya River Basin (MARB, or the

Basin) to the Gulf of Mexico. These

analyses aimed to identify where in the

Basin the nutrients came from and to estimate the relative importance of specific

human activities—such as agriculture,

point-source discharges, and

atmospheric emissions—

in contributing nutrient flux

to the Mississippi River and

the Gulf.

Nutrient Trends 

River-borne nutrients and

water-column stratification

are the major factors controlling hypoxia in the northern

Gulf of Mexico. The key

nutrients in this process are

nitrogen, phosphorus, and

silica. Of these, nitrogen is

the most important nutrient

leading to the production of

excess algae and subsequent

hypoxia in the Gulf and

other marine areas (NRC

1993). Nitrogen is also the

only nutrient that has

increased significantly in

concentration and loads in

the Mississippi River in

recent decades. Phosphorus

loads have not changed significantly since the early

1970s when records began,

and silica loads decreased

between the 1950s and

1970s and have not changed

significantly since then. 

Nitrogen is present primarily in three

forms in the Mississippi River and its tributaries: dissolved inorganic nitrogen

(nitrate and ammonium), dissolved

organic nitrogen, and particulate organic

nitrogen. Total nitrogen is the sum of

these three forms. For 1980–96 the average total nitrogen flux from the MARB to

the Gulf was estimated to be 1,567,900

metric tons per year. Of this amount,

Scientific

investigations

indicate that

oxygen stress 

in the northern

Gulf of Mexico

is caused

primarily by

excess nutrients

delivered to

Gulf waters from

the Mississippi–

Atchafalaya

River drainage

basin, in combination with the

stratification of

Gulf waters.
20 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

about 63 percent was dissolved

inorganic nitrogen (61 percent

nitrate and 2 percent ammonium),

24 percent was dissolved organic

nitrogen, and 13 percent was particulate organic nitrogen. Most of

the analysis of nitrogen changes

(trends) discussed in the Committee on Environment and Natural

Resources hypoxia topic reports

focused on nitrate, which comprises most of the dissolved inorganic nitrate. The principal reason

for the focus on nitrate, rather

than total nitrogen, is that nitrate

is the most significant bioavailable

form of nitrogen transported into

the Gulf. Dissolved inorganic

nitrogen concentration and flux

have changed more than any

other form of nitrogen and, therefore, potentially have a much

larger effect on algal production

and hypoxia than do dissolved

and particulate organic nitrogen.

Published data (Palmer ca.

1903; Leighton 1907; and Dole

1909) from the period 1897–1906

for four locations in the MARB

show that both nitrate and total

nitrogen concentrations have

increased significantly in the last

100 years (Table 2.1 and Figure

2.5). Mean nitrogen concentrations at these four sites for

1980–98 are also shown in the

table for comparison. Concentrations of total nitrogen increased

significantly at three of the four

sites during the past 100 years,

and essentially all of the increase

can be attributed to dissolved

inorganic nitrogen or nitrate. The

total nitrogen concentration in the

lower Mississippi is estimated to

Number of Organic N Inorganic N

Location Samples DON PON Total Nitrate-N NH4

–N DIN Total Na

(milligrams per liter as N)

Lower Illinois River

1897–1902 weekly 0.59 0.42 1.01 1.25 0.38 1.63 2.64

1980–98 189 0.45 0.60 1.22b 4.09 0.14 4.23 5.46

Upper Mississippi River near Grafton

(below Illinois & above Missouri Rivers)

1899–1900 70 0.48 0.62 1.10 0.59 0.13 0.72 1.82

1980–98 120 0.81 0.63 1.27 2.63 0.11 2.74 4.01

Lower Missouri River

1899–1900 63 0.30 1.53 1.83 0.51 0.06 0.57 2.40

1980–98 186 0.51 0.69 1.03 1.23 0.05 1.28 2.24

Lower Mississippi River

1905–06 52 0.40c 0.76d 1.16 0.56 0.10c 0.66 1.82

1955–65 308 0.52f 0.69g 1.21 0.65e 0.10f 0.75 1.96

1980–98e 104 0.52 0.38 0.92 1.45 0.06 1.51 2.40

Table 2.1 Historical and Recent Data on Nitrogen (N) 

Concentrations in the Mississippi River Basin

FIG. 2.5 Nitrogen Concentrations 

in the Mississippi River Basin

Lower

Illinois

River

ca. 1990

Upper

Mississippi

River

Lower

Missouri

River

Lower

Mississippi

River

0

1

2

3

4

5

6

1980–98

Total N Concentrations a

(mg/L)

a Total nitrogen calculated as the sum of total

organic N + dissolved inorganic nitrogen (DIN).

b Dissolved organic nitrogen (DON) and particulate organic nitrogen (PON) were not analyzed

on all samples in 1980–98. Thus, DON + PON

does not equal total organic nitrogen (N) for

this time period.

c Estimated from concentrations in the upper

Mississippi River and lower Missouri River.

d Estimated as two times the average 1980–98

PON concentration.

e From Table 3.4 of the Topic 3 report.

f Estimated.

g Calculated from 1955–65 daily sediment concentration at Tarbert Landing (4,018 samples;

mean = 460 mg/L) and estimated sediment

nitrogen content of 0.15%; PON = 0.69 +/- 0.41

mg/L.
Chapter 2: The Causes ■ 21

have increased by a factor of 1.3

since 1905–06, and nitrate has

increased by a factor of about 2.5.

Mean annual total nitrogen concentrations in the lower Illinois

River and the Mississippi River at

Grafton, Illinois, doubled in the

last 100 years, while nitrate concentrations increased by factors of

three to more than four. The

exception is the lower Missouri

River, where total nitrogen concentrations decreased slightly due

to a large decrease in particulate

organic nitrogen concentrations

associated with construction of

reservoirs on the Missouri River in

the 1950s and 1960s. Trapping of

sediment in the reservoirs has reduced the discharge of suspended

sediment by more than 50 percent

(Meade 1995), resulting in a similar reduction in particulate organic

nitrogen. However, nitrate concentrations in the lower Missouri

have more than doubled and make

up for most of the decrease in particulate organic nitrogen.

The most significant trend in

nutrient loads has been an increase

in nitrate load, which has almost

tripled from 0.33 million metric

tons per year during 1955–70 to

0.95 million metric tons per year

during 1980–96 (see Figure 2.4).

Figure 2.6 shows trends in

annual nitrogen inputs to and outputs from the MARB. The most

significant changes in both inputs

and outputs occurred between the

1950s and 1980s, when fertilizer

input more than tripled, and crop

harvest and pasture production

increased by about 2.4 times.

Total nitrogen inputs increased by

about 9 million metric tons per

year, from about 13 million to

nearly 22 million metric tons per

year. Outputs increased by about

11 million metric tons per year.

The difference between these

inputs and outputs (the residual)

represents nitrogen potentially

available as loadings to the

aquatic environment (see Figure

2.7). The rapid decline in residual

corresponds to the period of rapid

change in fertilizer application

and crop harvests and may represent increased efficiency in the

use of nitrogen in crop production. Residuals since around 1980

have exhibited no trend, although

they have become highly variable

from year to year. The leveling off

of nitrogen fertilizer, crop outputs

of nitrogen, and the nitrogen

residuals may indicate that a new

steady-state condition was established about 1980. 

Seasonal Variability 

Nitrogen concentrations and

loads vary seasonally. Loads and

freshwater discharge are usually

highest during the late winter,

spring, and early summer when

runoff is highest. The seasonal

and annual cycles of nitrate transport are illustrated in Figure 2.8.

Precipitation leaches the

highly soluble nitrate from the

soil into streams via ground-water

discharge, agricultural drains, and,

under some conditions, overland

runoff. Nitrate is subsequently

transported into the Mississippi

River and eventually discharges to

the Gulf of Mexico.

Nitrogen Sources 

The principal sources of nitrate

in the MARB are river basins that

The largest changes in nitrogen inputs to and outputs from the Mississippi Basin occurred between the 1950s and 1980s.

Fertilizer nitrogen inputs increased from less than 1 million to more than 6 million metric tons per year, and crop harvest and

pasture production more than doubled.

1 0

9

8

7

6

5

4

3

2

1

0

1950 1960 1970 1980 1990 2000

Annual nitrogen ou tpu ts from the MARB

(millions of metric tons)

crop harvest & pastur e

plant senescence

manure volatilization

denitrification

immobilizationfertilizer volatilization

FIG. 2.6 Annual Nitrogen Inpu ts to and Ou tpu ts from the Basin: 1951–96

Sources: Figures 5.7 and 5.9 of the Topic 3 Report for the Integrated Assessment.

8

7

6

5

4

3

2

1

0

1950 1960 1970 1980 1990 2000

Annual Nitrogen Inpu ts to the MARB

(millions of metric tons)

soil mineralization

fertilizer

legume & pastur e

all manur e

atmospheric nitrat e

atmospheric ammonia

municipal & industrial
22 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

drain agricultural land in southern

Minnesota, Iowa, Illinois, Indiana,

and Ohio (see Figure 2.9). This is

an area of intensive corn and soybean production, where large

amounts of nitrogen from fertilizer and manure are applied to

soils every year. Legumes and

atmospheric deposition add

nitrogen to the soils in this

region, which also contain

large amounts of organic nitrogen, some of which is converted to soluble nitrate each

year. The nitrate accumulated

from all sources and not used

by crops or removed by biogeochemical processes is subject to being leached to

streams and ground water by

precipitation. Extensive use of

tile drains in this region can

intercept water with high levels of nitrate and accelerate its

transport directly to ditches

and streams. In addition, in

some basins sewage treatment

plants and industrial sources add

nitrogen directly to streams. 

All of these sources contribute

to the nitrogen load transported

by the Mississippi River to the

Gulf of Mexico. About

56 percent of the nitrate

transported to the Gulf

enters the Mississippi

River above the Ohio

River. The Ohio basin

subsequently adds

another 34 percent of

the nitrate load. 

Since 1980, the

Mississippi and Atchafalaya

Rivers have discharged, on average, about 1.6 million metric tons

of total nitrogen to the Gulf each

year; 0.95 million metric tons of

that is in the form of nitrate.

FIG. 2.8

Sources: Figure 4,1 of the Topic 3 Report for the Integrated Assessment, with an 

update from Donald Goolsby, U.S. Geological Survey.

Daily Flux of Nitrate from the 

Basin to the Gulf: August 1980–1999

10,000

8,000

6,000

4,000

2,000

0

1980 1985 1990 1995 2000

Daily Nitrate Flux

(metric tons)

The daily flux of nitrate varies

from a low of several hundred

metric tons per day during low

streamflow in the fall to several

thousand metric tons per day

during high streamflow in the

spring.

Both inputs to and

outputs from the Basin

have increased dramatically since about 1950.

The difference between

inputs and outputs—the

residual—declined

rapidly between 1969 and

1978, possibly indicating

increased efficiency in

the use of nitrogen in

crop production. Essentially no change in residuals has occurred since

1980, although they have

been highly variable from

year to year.

Nitrogen Mass Balance for the Basin: 1951–96

25

20

15

10

4

3

2

1

0

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Annual Nitrogen Inpu ts and Ou tpu t s

(millions of metric tons)

(millions o

Annual Residual Nitrogen

f metric tons)

FIG. 2.7

Sources: Figure 6.4 of the Topic 3 Report for the Integrated Assessment.

inpu t s

ou tpu t s

residual
Chapter 2: The Causes ■ 23

About 90 percent of the nitrogen

comes from nonpoint sources,

with the remainder coming from

point sources. Table 2.2 presents

the estimated contributions of

nitrate and total nitrogen from

the predominant sources in the

Basin. 

Estimated Contributions of Nitrogen Input Sources to the Total

Nitrogen and Nitrate Nitrogen Yield of the Mississippi–

Atchafalaya River Basin and Flux to the Gulf of Mexico

Table 2.2

Source of Nitrogen Percent of Percent of 

Transported to the Gulf Nitrate Total Nitrogen

Agricultural nonpoint sources 74 65

Other nonpoint sources 16 24

Municipal and industrial point sources 9 11

The principal sources of nitrate are river basins that drain agricultural land in southern Minnesota, Iowa, Illinois, Indiana, and

Ohio, where large amounts of nitrogen are applied to corn and soybean fields.

Nitrate yields

in kg/km2/yr

2 to 100

101 to 500

501 to 1,000

1,001 to 1,500

1,501 to 2,601

USGS gauging station

Basin ID number

FIG. 2.9

Source: Figure 4.8A of the Topic 3 Report for the Integrated Assessment.

Average Annual Nitrate Yields for the 

42 Sub-basins within the Basin: 1980-96