1

Preventing Eutrophication: Scientific Support for Dual 

Nutrient Criteria

Summary

Nutrient pollution resulting from excess nitrogen 

(N) and phosphorus (P) is a leading cause of 

degradation of U.S. water quality. The scientific 

literature provides many examples that illustrate 

the effects of both N and P on instream and 

downstream water quality in streams, lakes, 

estuaries, and coastal systems. Development of 

numeric nutrient criteria for both N and P can be 

an effective tool to protect designated uses in the 

nation's waters. The purpose of this fact sheet is 

to describe the scientific basis supporting the 

development of criteria for both N and P to 

prevent eutrophication and the proliferation of

harmful algal blooms.

Background

Nitrogen and phosphorus together support the 

growth of algae and aquatic plants, which 

provide food and habitat for fish, shellfish and 

other organisms that live in water. Excess N and 

P in aquatic systems can stimulate production of 

plant (including algae and vascular plants) and 

microbial biomass, which leads to depletion of 

dissolved oxygen, reduced transparency, and 

changes in biotic community composition -- this 

is called eutrophication [

1

]. In addition to the 

impacts on aquatic life, excess nutrients can also 

degrade aesthetics of recreational waters [

2

,

3

,

4

],

and increase the incidence of harmful algal 

blooms, which may endanger human health

through the production of toxins that can 

contaminate recreational and drinking water 

resources[

5

,

6

]. 

Under the Clean Water Act, states and

authorized tribes are responsible for establishing 

water quality standards that specify appropriate 

designated uses, establish criteria to protect 

those uses, develop anti-degradation policies and 

implementation methods, and provide for the 

protection of downstream waters. Numeric 

nutrient criteria are an important element of 

water quality standards and are an effective tool 

for preventing nutrient pollution, for example, in 

helping to derive numeric limits in discharge 

permits. Development of numeric nutrient 

criteria is one aspect of a coordinated and 

comprehensive approach to nutrient

management [7]. EPA has published several 

guidance documents to assist states and 

authorized tribes in deriving numeric nutrient 

criteria for both N and P to protect aquatic 

systems [

8

,

9

,

10

,

11

,

12]. 

Why develop criteria for both N and P?

Nutrient management efforts have traditionally

focused on controlling a single limiting nutrient 

(i.e., N or P) based on a paradigm that assumes

primary production is N-limited in marine 

waters and P-limited in freshwaters. 

Conceptually, the assumption is that if the key 

limiting nutrient is controlled, primary 

production is limited and the cascading effects 

of eutrophication do not occur. In practice, 

however, there are scientific reasons that make 

this an overly simplistic model for management 

of nutrient pollution as described below. 

Trophic status may vary both spatially and 

temporally.

The scientific literature demonstrates that 

nutrient concentrations vary across a landscape 

as a result of a multitude of factors, including 

climate, flow, geology, soils, biological 

processes, and human activities. This variability 

in concentration means that the relative 

contribution of and limitation by N and P can 

change spatially and temporally - even within 

the same watershed. 

There are numerous examples in the scientific 

literature documenting exceptions to the 

conventional nutrient limitation theory. For 

example, N limitation has been shown to occur 

in lakes with small watershed areas relative to 

size [13], streams have demonstrated temporal 

Office of Water EPA - 820-S-15-001

MC 4304T February 2015
2

and spatial changes in nutrient limitation [

14

,

15], 

many estuaries show seasonal shifts from P 

limitation in spring to N limitation in summer

[

16

,

17], and co-limitation is commonly observed 

across freshwater and marine systems [

18

,

19]. 

Because of the highly variable nature of nutrient 

limitation in aquatic systems, numeric criteria 

for both N and P provide the greatest likelihood 

of protecting aquatic systems. 

Aquatic flora and fauna have a diverse set of 

nutritional needs.

The concept of single nutrient limitation relies 

on the assumption that at any moment in time 

the growth of all organisms will be limited by 

the nutrient in shortest supply. However, the 

scientific literature demonstrates that aquatic 

flora and fauna have different nutritional needs. 

Some species may exhibit N limitation while 

others show P limitation or co-limitation by both 

N and P [

20

, 

21

, 

22

, 

23

,

24]. Because of the diversity 

of nutritional needs amongst organisms, numeric 

criteria for both N and P are more likely to 

protect aquatic systems.

N fixation does not fully offset N deficiency.

Arguments for controlling P only in freshwaters

have relied on the idea that reductions in N are

compensated by cyanobacterial N fixation. It has 

been suggested that this process undermines N 

control and serves to maintain P limitation [

25].

This theory has also been extended to marine 

waters [

26], yet scientific evidence indicates that

N fixation is not able to fully offset N deficiency

in either fresh or marine waters [

27

,

28

,

29

,

30]. 

Because N fixation is highly variable across 

waterbody types, numeric criteria for both N and 

P are likely to be more effective in protecting 

aquatic systems.

Both N and P have a role in protecting 

downstream waters.

Focusing on only the perceived limiting nutrient 

in upstream waters can enhance export of the 

uncontrolled nutrient downstream. For example,

limiting P in streams can reduce phytoplankton 

biomass, which, in turn, can make N more 

available for transport downstream [

31]. Waters 

where N and P concentrations exceed saturation 

thresholds are particularly vulnerable to 

becoming nutrient sources [

32

, 

33

,

34].

Both N and P are important to consider when 

assessing downstream impacts at any scale (e.g.,

10 miles, 100 miles, or 1000 miles from the 

source). For example, nutrient concentrations in 

streams may not trigger an adverse effect until 

some distance downstream where other factors -

light, temperature, substrate, or velocity - no 

longer suppress the response to nutrients [

35

,

36

,

37

, 

38

,

39]. Lakes with a nutrient limitation status 

sufficiently different from that of upstream 

waters may also be impacted by upstream 

nutrient loads [

40]. Estuarine and coastal waters

are especially sensitive to upstream sources 

given that they are physically, chemically, and 

biologically distinct from freshwater systems

[

41

,

42]. 

Research in the Northern Gulf of Mexico

highlights the importance of considering both N 

and P when assessing downstream impacts. 

Increasing N inputs from the Mississippi River 

into the Gulf of Mexico have been observed to 

change the trophic status of the Gulf, over time 

forcing P limitation [

43]. In 2007, EPA's Science 

Advisory Board (SAB) recommended that 

reduction strategies for both N and P be 

implemented to protect downstream waters in 

the Gulf [

44

,

45]. The SAB recommendation has 

been supported by more recent research 

demonstrating that reductions of both N and P to 

the Gulf of Mexico should be implemented to 

protect aquatic habitat and limit further 

expansion of the low dissolved oxygen zone [

46].

Controlling only P may not effectively prevent the 

occurrence of harmful algal blooms in 

freshwaters.

P control has achieved reductions in algal 

biomass over the last several decades in some 

waters. However, as explained above, P 

management may be only half the solution to the 

eutrophication problem. As sources of N and P 

have increased and watersheds have built up an 

ever greater nutrient pool, the role of N control 

in protecting freshwater resources has become 

increasingly clear, especially in preventing the 

occurrence of harmful algal blooms. 

Recent scientific evidence has demonstrated that 

certain harmful algal taxa thrive, and are even 

potentially more toxic, in conditions where N is

disproportionately available relative to P. 
3

Several studies have shown that toxic algae such 

as cyanobacteria possess unique physiological 

characteristics that allow them to out compete 

other species in N-rich/P-poor conditions. They 

may do this by either utilizing novel forms of 

available N and P (e.g., urea, particulate or 

organic P) or substituting other elements for 

physiological P requirements (e.g., using sulfur 

instead of P during lipid synthesis)

[

47

,

48

,

49

,

50

,

51

,

52]. Toxin production has also been 

shown to increase under conditions of nutrient 

imbalance [

53

,

54]. Several field and laboratory 

studies, for example, have found the production 

of microcystin, a common cyanotoxin that is 

linked to human illness, to be strongly 

associated with N concentration [

55

,

56

,

57

,

58]. 

More recent analysis of lakes across the U.S. 

found similarly strong associations between N 

concentrations and high levels of microcystin

[

59].

Conclusion

Nutrient pollution is a major cause of 

degradation in U.S. waters. Given the dynamic 

nature of aquatic systems, the need to protect 

downstream waters, and the threat of harmful 

algal blooms, the weight of the scientific 

evidence supports the development of nutrient 

criteria for both N and P. 

For More Information

Additional information on the development of 

numeric nutrient criteria is available on our 

website:

http://water.epa.gov/scitech/swguidance/standar

ds/criteria/nutrients/guidance_index.cfm

Contact Brannon Walsh at the EPA Office of 

Water at 202-566-1118 or 

walsh.brannon@epa.gov

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