FACTSHEET ON WATER QUALITY PARAMETERS
Nutrients
Nutrients play a critical role in the biodiversity and healthy functioning of aquatic ecosystems by supporting the growth
of aquatic plants and aigae that provide food and habitat for fish, shellfish, and smaiier organisms. Nitrogen and
phosphorus are two of the most important nutrients.
Why do we measure nutrients?
Excess (or elevated) nutrients is among the most
common water pollution problems affecting waterbodies
in the U.S. High nutrient concentrations resulting from
human activities can diminish ecosystem health.
Excess nutrients in a waterbody can iead to excess
biological growth (eutroph'car'on) and harmful algal
biooms (HABs). In freshwater, HABs are often caused by
cyanobacteria (blue-green algae) and may have negative
effects on humans and ecosystems (Figure 1).
Certain types of cyanobacteria can produce toxins that
are harmful to humans and animals through recreational
exposure or through consumption of drinking water.
Toxins can also work their way into the aquatic and
terrestrial food webs and potentially harm animals
and humans. Additionally, algal blooms can lead to
hypoxia, or iow dissolved oxygen (DO) in the water, either
through algal respiration or consumption of oxygen, by
decomposers when the algae die off. Persistently low DO
can harm sensitive aquatic animals, resulting in chronic
stress or even mortality.
Nutrients affect other water quality parameters, such as:
pH - A basic parameter controlling water chemistry
and aquatic health. Daily variation in pH is amplified
when nutrients promote increased growth of aquatic
primary producers (plants, algae and cyanobacteria).
Organic carbon - The amount of organic matter in the
water; it may increase if nutrients promote growth of
aquatic plants and aigae.
Total suspended solids - The amount of particulate
matter in the water; it may increase if nutrients
promote algal growth.
Turbidity or water clarity- An indicator of water
transparency. Water may be more turbid (cioudy) if
algae and aquatic plant growth increase in response to
higher nutrient concentrations.
Chlorophyll a - An alga! and aquatic plant pigment
that is useful as an indicator of algal biomass and
that may increase in response to higher nutrient
concentrations.
DO - The amount of oxygen in the water available for
fish and other aquatic organisms. DO may decrease if
an algal bioom proceeds to the point where bacterial
decay of dead algae consumes oxygen.
There can be a lag time between an increase in nutrients
and changes in these parameters. For example, excess
nutrients promote increased algal growth, which can then
lead to subsequent changes in other parameters.
Figure 7. Harmful algal blooms cause thick, green scums that
impact water, recreation, businesses and property values. Credit:
Photo courtesy of USGS, photographer J. Nelson
For factsheets on other water quality parameters, visit:
epa.gov/awma/factsheets-water-quality-parameters.
For more information about the CWA Section 106 Grants Program, visit:
epa.qov/water-pollution-control-section-106-q rants.

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Nutrients
What affects nutrients?
Nutrients in surface water come from both point sources
(specific discharge points) and nonpoint sources
(pollution dispersed over general land area).
Point sources
Discharges from wastewater treatment facilities
and industrial discharges may be rich in both
nitrogen and phosphorus.
Nonpoint sources
Fertilizers - Runoff from cropland contributes
nitrogen and phosphorus from fertilizers to surface
waters (Figure 2).
Manure - Livestock operations produce large
amounts of manure, which contains nitrogen and
phosphorus, if not managed properly, manure
can be carried in stormwater runoff to streams or
contaminate streams when animals have direct
contact with the waterbody.
Urban runoff - Nitrogen and phosphorus sources
in urban areas that can be carried in runoff include
lawn fertilizer, pet waste, and wildlife droppings.
Atmospheric deposition - Atmospheric deposition
occurs naturally but can be contaminated by human
activities, it can be a significant source of nutrients,
especially nitrogen, in certain regions (for example,
downwind of urban and agricultural areas).
Nutrient concentrations in rivers can vary considerably
from year to year, as shown by EPA's Report on the
Environment for Nitrogen and Phosphorus Loads in
Large Rivers in the U.S. (2019) (Mississippi, Columbia, St.
Lawrence, and Susquehanna).
Nutrient concentrations can also fluctuate throughout
the year. Several seasonal trends affect nutrient
concentrations:
Spring - Snowmelt and rain deliver nutrients from
agricultural land into streams and groundwater. Some
studies, as summarized by EPAs report Monitoring and
Evaluating Nonpoint Source Watershed Projects (2016),
have shown that in northern areas, the majority of the
annual nutrient pollutant load can be delivered within a
few weeks. Nutrients in groundwater can mobilize and
be discharged into surface waters. Nutrient availability
in lakes may increase during spring turnover due to
release of nutrients from the sediment at the bottom
of the lake.
Summer - Warmer water temperatures and lower
flows create preferred conditions for increased algal
growth, especially during the summer growing season.
Fall - Nutrient availability in lakes may increase during
fall turnover due to release of nutrients from the
sediment at the bottom of the lake.
Winter - As plants die and decompose in the late fall
and winter, nutrients are released into water.
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Nutrients
What are EPA's recommended criteria for nutrients?
Nutrient concentrations are highly variable in waterbodies
across the U.S. To interpret monitoring data, it is helpful to
be familiar with typical nutrient concentrations for healthy
and disturbed waterbodies in your area. When nutrient
concentrations are higher than an aquatic system's natural
background concentrations, the water has most likely been
affected by urban or agricultural activities, including both
point and nonpoint source contributions.
Unlike many water quality pollutants, there is no single
national criterion for either phosphorus or nitrogen
because the thresholds for impairment vary regionally
and by waterbody type (river, lake, estuary, etc.) and
characteristics (stream/river size, lake depth, etc.). Instead,
EPA has published technical support documents with
recommended water quality criteria for fourteen specific
areas of the U.S. (ecoregions). Criteria for your ecoregion
can be found on EPA's Ecoregional Criteria website (2020a).
As an example, Table 1 shows the ecoregional criteria for
Aggregate Ecoregion III (Xeric West). EPA is encouraging
states and tribes to develop and adopt numeric water
quality criteria for total nitrogen and total phosphorus or
to develop a process to interpret the narrative criteria for
nutrients. For information on state adopted nutrient criteria
please visit EPA's website on State Progress Toward
Developing Numeric Nutrient Water Quality Criteria for
Nitrogen and Phosphorus (2020b).
Table 1. Ecoregional criteria for Aggregate Ecoregion III (Xeric
West).

Rivers and Streams
Total Phosphorus (pg/L)
21.88
Total Nitrogen (mg/L)
0.38
Chlorophyll a (pg/L)
1.78
Turbidity (NTU)
2.34
1 All or parts of the States of: Washington, Oregon, California,
Nevada, Idaho, Wyoming, Montana, Utah, Colorado, New Mexico,
Arizona, and Texas, and the authorized Tribes within the Ecoregion.
Source: USEPA (2014)
How do we measure nutrients?
Nutrients are measured by taking water samples in the field
and are commonly reported as milligrams per liter (mg/L)
or micrograms per liter (|j/L) of water. When deciding
where and when to take samples to measure nutrient
concentrations, consider:
Sources of pollution in your watershed and where they
may enter streams or lakes to identify areas that may
have high concentrations. These may include both
point and nonpoint sources. Identify areas that have
historically had problems with nutrient pollution.
Flow characteristics that may affect how quickly
nutrient pollution moves through the system.
Sampling at different depths to identify vertical
variability in nutrient concentrations through the water
column, especially in the summer months.
Whether it has been dry or has recently rained or
snowed.
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Nutrients
Sample throughout the year (if feasible)
to capture seasonal variability in nutrient
concentrations, sources, and changes in
vertical variability (seasonal turnover in
lakes may increase nutrient availability).
A monitoring program must establish
which forms of nitrogen and
phosphorus to analyze. Both nitrogen
and phosphorus have natural cycles in
which they are converted into different
forms (also called "species") by plants
and microorganisms. Figure 3 shows an
example of a general nitrogen cycle.
Nitrogen and phosphorus exist in
both inorganic and organic forms, and
those forms can be either dissolved
in the water or bound to particulates.
Particulates may remain suspended in
water, or they can settle to the bottom
of a waterbody and potentially release
nutrients back into the water.
Table 2 lists typical nitrogen and
phosphorus forms for which samples
can be analyzed. Total nitrogen and
total phosphorus provide an overall
indication of the amounts of nutrients
in a system. Additional analyses can
be done to determine the different
forms of nitrogen and phosphorus.
If the samples are filtered to remove
particulates, then the dissolved fractions
can be measured (the particles retained
on the filters could also be analyzed).
General Nitrogen Cycle
Denitrification
by Bacteria
Atmospheric
Nitrogen

Fertilizers, animal and
human waste, wet and dry
atmospheric deposition

Nitrogen
Fixation by
Cyanobacteria
Nitrification
by Bacteria

Ammonia and
Ammonium

~ |
Nitrite/Nitrate
Aquatic Plants
and Algae

Fish and
Macroinvertebrates
r
Dead Organic Matter
Figure 3. Example of a general nitrogen cycle.
Taken together, a suite of analyses can help in tracking sources of inputs and
the amounts of the most bioavailable forms of nitrogen and phosphorus.
Table 2. Typical forms of nitrogen and phosphorus for which samples may be analyzed in the laboratory.
Form
Units
Common
Names
Description
Why Sample?
Total nitrogen,
mixed forms
mg/L
TN
Captures all forms of nitrogen.
(Commonly TKN + Nitrate + Nitrite)
Important overall parameter to understand
nutrient levels in a waterbody.
Nitrate + nitrite
mg/L
Nitrite-
nitrogen
(NO2-N),
Nitrate-
nitrogen
(NO3--N)
Inorganic forms of nitrogen,
bioavailable for aquatic plant and
algae growth. Main form in surface
waters with high N concentrations.
Nitrate is the more common form -
nitrite is unstable in natural waters.
Valuable for monitoring the impacts of
inputs such as agricultural and urban runoff,
wastewater treatment plants, leaking sewage
systems, industrial point sources, and other
sources.
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Nutrients
fable 2 continued. Typical forms of nitrogen and phosphorus for which samples may be analyzed in the laboratory.
Form
Units
Common
Names
Description
Why Sample?
Ammonium/
ammonia
mq/l
Ammonia
nitrogen,
N:k N, NHa
Inorganic forms of nitrogen,
bioavailable for aquatic plant and
algae growth.
Valuable to monitor because of ammonia
toxicity to aquatic life and for monitoring the
impacts of wastewater treatment plants,
leaking sewage systems, industrial point
sources, agricultural and urban runoff, and
other sources.
Total Kjeldahl
Nitrogen (TKN)
mg/L
Dissolved
ammonia
plus organic
nitrogen
A measure of organic nitrogen plus
dissolved ammonia.
Useful to evaluate contributions of organic
nitrogen from wastewater treatment plants,
manure, and other potential sources. Useful
for in-depth evaluations of N bioavailability in
surface water.
Inorganic
nitrogen
mg/L
NO2 + NO3
+ NH3 + NH4*
Nitrogen as nitrite, nitrate, ammonia,
and ammonium.
Useful for understanding nitrogen sources and
how readily the nitrogen will be taken up by
plants.
Total phosphorus,
mixed forms
mq/l
TP
Captures all forms of phosphorus.
Important overall parameter to understand
nutrient levels in a waterbody.
Orthophosphate
Mg/L
PO43"
Inorganic form of phosphorus that
is most readily taken up by aquatic
plants and algae. Equivalent to
Phosphorus as PCU3.
Valuable for monitoring inputs such as
fertilizers in agricultural and urban runoff,
sewage, and industrial effluents.
What are the challenges of using nutrients as a water quality parameter?
Monitoring for nutrients can be challenging because it
involves measuring very low concentrations, down to 0.01
mg/L or even lower. Nutrients are also difficult to monitor
because of seasonal and episodic trends. In some cases,
such as agricultural areas, year-round sampling may be
warranted to capture seasonal trends in nonpoint source
pollution. However, sampling is generally conducted in the
summer because of weather and safety considerations.
Another challenge is the lack of specific numeric
thresholds for assessing water quality when the criteria
are narrative.
EPA 841F21007G | July 2021
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