United States
Office of Water EPA - 820-S-15-001
oEPA Environmental Protection
Agency	MC 4304T	February 2015
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 |' |. 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 f8,9,10,11,12]-
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
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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 [2U, 21, ~, 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 [4U]. 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.
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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 so 5152j Toxm 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 withN 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
H.
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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