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CC.3. Nutrients

Nutrients are elements that are essential for plant growth and include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), and silicon (Si). N, P, and K are considered primary nutrients, and N and P are the major limiting nutrients in most aquatic environments.

When considering candidate causes, N and P are evaluated to determine the trophic status (or relative nutrient condition) of freshwater systems. In streams, nutrient enrichment can change the types and abundances of algae and plants. In lakes, streams large rivers, and estuaries, trophic status may be expressed in terms of oligotrophy (low nutrients, minimally productive), mesotrophy (moderate nutrients, moderately productive), or eutrophy (high nutrients, highly productive).

In most cases, nutrients are not the proximate stressors for aquatic communities. Although certain forms of N [i.e., unionized ammonia (NH₄), nitrite (NO₂) and, in some cases, nitrate (NO₃)] may be toxic, toxic chemicals are dealt with in another module. Nutrients have indirect adverse effects on aquatic communities through their effect on primary production, the growth and accumulation of plant and algal biomass, and the species composition of plant and algal (i.e., phytoplankton in lakes or periphyton in streams) communities (Images CC.3-1 and CC.3-2; Dodds and Welch, 2000).

Increasing primary production and changes in plant and algal species composition can be proximate causes of effects on consumers by:

• Altering food resources – including the amount of food resources, their type (e.g., living plant and algal biomass versus detritus),or their palatability (e.g., changes in cell size in algae for filter-feeding animals),
• Altering habitat structure – including changes in benthic interstitial space, ease of movement across benthic surfaces or through the water column and availability of macrophytes as habitat for some species and life stages, and
• Algal toxins – some algae that are characteristic of eutrophic conditions can be toxic to fish, invertebrates, and even humans.

These increases in primary production and plant and algal biomass can affect other physical and chemical characteristics of the water body, such as pH and dissolved oxygen that can be proximate stressors for the aquatic community.

Algal growth and biomass accumulation associated with excess nutrients also hinder recreation, fishing, hunting, and aesthetic enjoyment of waterbodies and may interfere with drinking water treatment and use of water by industrial facilities.

CC.3.1. What to Consider When Determining if Candidate Causes Associated with Excess Nutrients Should be Included

Candidate causes associated with excess nutrients should be considered when observations support portions of the source-to-impairment pathways in the conceptual model for nutrients (Figure CC.3-1). The conceptual model and some of the other information are also useful for Step 3, Evaluate Data from the Case.

CC.3.1.1. Checklist of Sources, Site Evidence and Biological Effects

The checklist below identifies observations that can help you to choose whether to include candidate causes associated with excess nutrients among your candidate causes. The checklist may also prompt you to observe and collect additional evidence to support or weaken the case for these candidate causes to be the proximate cause of the impairment. The title for each column is linked to a text providing greater detail.

You may be aware of other situations when candidate causes associated with excess nutrients should be considered or might be screened from the analysis. Please send us your comments.

Choose to list candidate causes associated with excess nutrients based on the presence of sources, site evidence, and biological effects:

When listing candidate causes associated with nutrients, also consider these commonly associated candidate causes:

• Dissolved oxygen
• Temperature
• Suspended and bedded sediments
• pH
• Ammonia toxicity
• Pathogens
• Co-migrating contaminants

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CC.3.1.2. Sources and Activities that Suggest Listing a Nutrient-related Candidate Cause

Discharges of nutrients from point sources (pipes or canals) enter water bodies from discrete locations and may be continuous, making them easier to identify and monitor. They may be municipal or industrial effluents, confined animal feeding operation discharges, collected runoff from construction and development sites, and collected leachates from municipal landfills or waste disposal sites. In general, point sources, which are not stormwater-related, are relatively constant with respect to loadings (National Research Council, 2000).

Nutrients in runoff and ground water enter waterbodies from their terrestrial watersheds. They may enter waterbodies diffusely from overland flow or groundwater discharge or at discrete locations, such as agricultural drainage tiles or stormwater outfalls. Moreover, accounting for and controlling these inputs is more difficult because they typically result from diffuse human activities across larger land areas than point sources. In addition, groundwater flow is typically unobserved and contributing source areas may be far from the actual receiving waters. Agriculture and urbanization in the upstream watershed are common alterations that are sources of nutrients. Silviculture, grazing, lawns and golf courses, may also produce runoff containing elevated nutrients. Both climate and physical characteristics of a watershed can increase the potential for nutrients to enter waterbodies via nonpoint source pathways. Land alterations generally increase nutrient delivery to streams, because they often enhance water flow across the landscape and mobilize nutrients that would otherwise be sequestered, thus increasing inputs to receiving waters. In addition, activities that disturb soils and thereby increase erosion also increase nutrient input to surface waters. The effect of such land alterations is further enhanced if soil concentrations of nutrients are increased by use of fertilizers, land application of manure or sludge, and leaking septic systems. High rainfall, steep slopes, and clay soils promote the transport of nitrogen and phosphorus via soil erosion and runoff, while low soil organic matter content and underlying sand, gravel, karst, or bedrock promote the transport of nitrogen to streams and rivers via groundwater (USGS, 1999). More direct inputs of nutrients may be evidenced by steeply sloping banks or unfenced pastures.

Atmospheric deposition, either directly on the water body or on the watershed, can be an important source of nitrogen. The primary source of this deposited gaseous nitrogen is combustion of fossil fuels in power plants and motor vehicles. Deposition of nutrients in dust can be an important source in some regions and ecosystem types (e.g., Caribbean coral reefs).

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CC.3.1.3. Site Evidence that Suggests Listing a Nutrient-related Candidate Cause

Observations of aquatic plants may suggest that the excess nutrients are present. These observations could include prolific rooted emergent or floating macrophytes, or algae in the water column (i.e., phytoplankton) or attached to submerged surfaces (i.e., periphyton). The algae may be dominated by filamentous forms or may form algal mats (Image CC.3-6) or blooms.

CC.3.1.4. Biological Effects that Suggest Listing a Nutrient-related Candidate Cause

If the biological impairment is defined by changes in the aquatic plant community, nutrients should be considered as a candidate cause. Key biological parameters indicative of elevated nutrient concentrations include increased concentrations of chlorophyll a, changes to the structure of periphyton or algal assemblages, and a decrease in the abundance of submerged rooted macrophytes such as eel grass.

For the animal assemblages, nutrients are generally not the proximate stressor, but often these assemblages change as a result of the effects of food or habitat. Increased phytoplankton may increase the abundance of filter feeders while increased periphyton may increase the abundance of scrapers. Increased macrophytes may favor fish that are ambush predators over those that are pursuit predators. Heavy growth of periphyton may decrease the habitat quality of gravel substrates for many species.

Algal blooms may include species that produce algal toxins. Fish kills or kills of macroinvertebrates suggest that algal toxins may be the cause of impairment.

Because nutrient enrichment often leads to decreases in dissolved oxygen concentrations, fish kills and other biological effects suggesting decreases in dissolved oxygen also may suggest high nutrient concentrations (see dissolved oxygen).

Specific biological measures that may be related to nutrients are discussed in the following subsections.

Chlorophyll a — Chlorophyll a (Chl a), a plant pigment produced by algae, is a common measure of algal biomass, and increases often indicate excess nutrients. There are several recognized methods for measuring Chl a, including spectrophotometric or fluorometric techniques. Sampling, filtration, and preservation techniques are important in the accuracy of these analyses (Carlson and Simpson, 1996).

Algal and plant community structure — Periphyton includes all organisms (algae, bacteria, protozoans, and fungi) attached to submerged substrates. Periphytic algal communities have been used as indicators of water quality in freshwater ecosystems and indices are being developed for estuarine habitats. Periphyton can be collected by colonization of artificial substrates, such as periphytometers, or by direct sampling of natural substrates. Methods for characterizing periphytic assemblages employ characterizations of taxonomic structure or standing crop, including species identification and counts; relative abundance and dominance; community diversity, evenness, and similarity; biomass (i.e., ash-free dry mass, adenosine triphosphate content); pigment content (usually chlorophyll a); and biovolume (ASTM, 1999; APHA et al., 1998) (Table CC.3-1). Functional characteristics include measurements of photosynthesis (i.e., oxygen production, carbon assimilation), respiration (i.e., oxygen consumption), or enzymatic activity (e.g., alkaline phosphatase activity). The calculation and application of diversity and similarity indices to biological data, as well as other statistical techniques (e.g., principal components analysis or multi-dimensional scaling) have been summarized by Clarke and Warwick (1994), Legendre and Legendre (1998), and McCune et al. (2002). A review by Wetzel (1979) summarized problems associated with the collection and identification of periphyton that need to be considered during data interpretation, including substrate selectivity; poor differentiation between alive and dead or moribund individuals, particularly of diatoms; and sample replication and reproducibility.

Table CC.3-1. Commonly-used metrics for periphyton assemblages that may respond to excess nutrients

Periphyton metric	Expected response to increased nutrients	Description
Ash-free dry mass (mg/m²)	Increase	Ash-free dry mass of periphyton per unit area is a measure of the organic component of the periphyton.
Achnanthes minutissima (RA*)	Decrease	This cosmopolitan diatom species is relatively intolerant of high nutrients.
Eutrophic diatoms (RA or richness)	Increase	These diatom taxa are identified as tolerant of eutrophic (i.e., high nutrient) conditions [see van Dam et al. (1994) for classifications].
Ratio, ACC to CMN diatoms	Decrease	The diatom genera Achnanthes, Cocconeis, and Cymbella are relatively intolerant of eutrophic conditions, while the genera Cyclotella, Melosira, and Nitzschia are relatively tolerant of eutrophic conditions.
Nitrogen-heterotrophic diatoms (RA)	Increase	These diatom taxa can use or require organic nitrogen for their metabolism [see van Dam et al. (1994) for classifications].
Diatom tolerance value	Increase	If diatoms are scored using a descending scale, with hypereutrophic species (van Dam et al., 1994) scoring high (i.e., 3) and oligotrophic species scoring low (i.e., 1), the tolerance value is the average score for all diatoms identified in a sample.
Dominant diatom species (RA)	Increase	Under high nutrient conditions, one or a few diatom species often dominate the assemblage.

*RA = relative abundance

In streams and rivers, periphytic algae live attached to submerged and benthic substrates. In extreme cases, a mat of periphyton may cover all benthic surfaces of a stream. In lakes, deleterious “algal blooms” may form at different times of the year. These blooms represent a significant alteration in the structure of the phytoplankton community, where one or a few species reproduce rapidly becoming both numerous and dominant (millions of cells per ml). In some cases, the dominating alga can produce toxins capable of killing fish or animals such as cattle or hogs that happen to drink the water. Examples of freshwater algae known to produce toxins include species of Anabaena and Cylindrospermopsis; some freshwater dinoflagellates also produce toxins. Blooms of toxin-producing algae are referred to as hazardous algal blooms.

In addition to increased algal or macrophyte standing crops, nutrient enrichment often changes the taxonomic structure of plant assemblages. If only phosphorus increases, nitrogen-fixing cyanobacteria (blue-green algae) may dominate phytoplankton assemblages in lakes, and in streams, chlorophytes (Chlorophyta or green algae) and cyanobacteria (Cyanophyta or blue-green algae) may dominate over diatoms. These changes in taxonomic structure also may result in changes in cell size or palatability, which can affect planktonic-feeding zooplankton in lakes or algal-feeding grazers in streams. In addition, increases in phytoplankton can raise turbidity and reduce penetration of light, potentially leading to decreases in benthic flora, such as submerged aquatic vegetation.

Macroinvertebrates and Fish — Benthic macroinvertebrate communities consist of bottom-dwelling aquatic insects and other invertebrates that occur in streams, rivers or lakes. These communities can be useful indicators of anthropogenic stress, as the composition and density of the macroinvertebrate taxa are a function of water quality during the recent past, including random discharges of pollutants that may not be detected by water quality monitoring. In addition, these organisms survive and thrive under a wide range of environmental conditions, are easily collected, and are associated with well-documented and readily availably assessment methodologies. Some community metrics may suggest the type of stressor(s) impacting an aquatic system, or serve as more general indicators of stress (Table CC.3-2).

An increase in algal growth resulting from increased nutrient loadings can affect the food webs of aquatic ecosystems, including the macrobenthic component. For example, increased productivity of periphyton in small streams will shift the macrobenthic community to one that is dominated by species, such as gastropod snails and other algal grazers and scrapers, that use algal mats for food and habitat. Increased algal growth also means increased algal decomposition and increased production of fine organic particles, which, in turn, may greatly increase the densities of filter-feeding macrobenthos, such as the larvae of black flies.

Table CC.3-2. Commonly used metrics for stream macroinvertebrate and fish assemblages that may respond to excess nutrients

Macroinvertebrate metric	Expected response to increased nutrients	Description
EPT taxa (RA or richness)	Decrease	Ephemeroptera (mayfly), Plecoptera (stonefly), and Trichoptera (caddisfly) larvae represent taxa that often decrease with increases in many types of stress, including nutrient enrichment (Klemm et al., 2003).
Mollusca and Crustacea (RA or richness)	Increase	Mollusks, such as gastropod snails and fingernail clams, and crustaceans, such as amphipods or isopods are tolerant of nutrient enrichment (Griffith et al., 2005).
Most dominant taxa (RA)	Increase	Reduced diversity and uneven distribution of individuals among taxa is characteristic of increased stress, including nutrient enrichment (Barbour et al., 1996).
Tanytarsini (RA or richness)	Decrease	This tribe of Chironomidae is generally considered intermediate in tolerance to stressors, such as nutrients (Deshon, 1995).
Grazers and Scrapers (RA or richness)	Increase	Grazers and Scrapers feed on the increased algal production associated with nutrient enrichment (Barbour et al., 1999).
Sensitive species (RA or richness)	Decrease	Sensitive species decrease in response to increased stress, including nutrient enrichment (Miltner and Rankin, 1998).
Tolerant species (RA)	Increase	More tolerant species may increase in abundance in response to decreases in sensitive species as result of nutrient enrichment (Miltner and Rankin, 1998).

Excess macrophytes or algae can alter habitats for both fish and macroinvertebrates by taking up space and altering substrate surfaces. Excess algae can contribute to turbidity, which interferes with sight-feeding fish (See Sediments).

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CC.3.1.5. Site Evidence that Supports Excluding Nutrients as a Candidate Cause

There are no site observations that specifically provide evidence of the absence of nutrients. General reasons for excluding a candidate from the list are described in Step 2.2 of the Step-by-Step guide and in Tips for Listing Candidate Causes.

We strongly caution against using benchmarks of effects (e.g., water quality criteria) as evidence for excluding nutrients from your initial list of candidate causes, because different species have different nutrient requirements and different sites have different naturally occurring levels of nutrients.

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CC.3.2. Ways to Measure Nutrients

Nitrogen — Common forms of N in aquatic ecosystems include gaseous nitrogen (N₂), ammonia (from nitrogen fixation, anhydrous fertilizers, animal wastes and decomposition of organic matter), nitrite and nitrate (NO₂^- and NO₃^-, respectively, from nitrification or fertilizers), and organic nitrogen compounds [see Wetzel (2001), Chapter 12, for a discussion of the nitrogen cycle]. Of these forms, the most important in terms of nutrient impairments is nitrate, due to its abundance and mobility in the surface and subsurface environment. As with many environmental pollutants, various methods are used to analyze nutrients, and these methods are not always interchangeable (Table CC.3-3).

Table CC.3-3. Measures of nitrogen

Nitrogen form	Units	Common names	Measures (see also ASTM, 1999 and online)
Ammonia	μg/L (as N)	Ammonia nitrogen, NH4-N, NH3	Dissolved ammonium + unionized ammonia are measured via phenate colorimetric method [Standard Methods 4500-NH3 F, G (APHA et al., 1998)].
Nitrite	μg/L (as N)	Nitrite nitrogen, NO2-N	Dissolved nitrite is measured via a colorimetric method [Standard Methods 4500-NO2- B (APHA et al., 1998)].
Nitrate	mg/L (as N)	Nitrate nitrogen, NO3-N	Dissolved nitrate is reduced to nitrite in the presence of Cd, followed by colorimetric method [Standard Methods 4500-NO3- E, F (APHA et al., 1998)]. Nitrate and nitrite may also be analyzed via ion chromatography [Standard Methods 4110 B (APHA et al., 1998)].
Dissolved inorganic nitrogen	mg/L (as N)	DIN	Nitrate, nitrite and ammonia are commonly reported together as DIN.
Dissolved organic nitrogen	mg/L (as N)	DON, dissolved Kjeldahl nitrogen, dissolved ammonia + organic nitrogen	Measure ammonium, then convert organic nitrogen to ammonium by digestion and re-measure for total ammonium using a colorimetric method [Standard Methods 4500-Norg B, C (APHA et al., 1998).
Total nitrogen	mg/L (as N)	TN	Method needs to specify whether it is done via persulfate digestion, which converts all forms of N to nitrate, followed by colorimetric method [Standard Methods 4500-Norg D (APHA et al., 1998)] or via analysis of total Kjeldahl nitrogen (TKN, see previous) with addition of nitrite and nitrate.

Phosphorus — Phosphorus is present as dissolved orthophosphate (PO₄^3-), various organic phosphorus compounds, and sediment-associated (particulate) phosphorus [see Wetzel (2001), Chapter 13 for a discussion of the phosphorus cycle]. Of the total phosphorus in aquatic systems, about 25% is considered to be biologically available. Dissolved orthophosphate is the most biologically available form of phosphorus, but is only a small percentage (generally less than 10%) of the total P in aquatic ecosystems. Because the organic and inorganic forms of phosphorus vary significantly in their reactivity and availability for biological productivity, determining phosphorus levels can be complicated. The most commonly used measurement for phosphorus is total phosphorus (Table CC.3-4).

Table CC.3-4. Measures of phosphorus

Phosphorus form	Units	Common names	Measures (see also ASTM, 1999 and online)
Ortho-phosphate	μg/L (as P)	PO4-P, SRP or soluble reactive phosphorous	Dissolved inorganic phosphorus and labile organic phosphorus are measured via a colorimetric method [Standard Methods 4500-P C or E, F (APHA et al., 1998)].
Total dissolved phosphorous	mg/L (as P)	TDP	Dissolved phosphorus compounds are converted to orthophosphate via digestion, followed by measurement via a colorimetric method; dissolved organic phosphorus (DOP) can be calculated by subtracting PO4-P.
Total phosphorous	mg/L (as P)	TP	Phosphorus compounds are converted to orthophosphate via digestion, followed by colorimetric method. Digestion methods for TDP and TP are outlined in Standard Methods 4500-P B (APHA et al., 1998); persulfate typically is used for the digestion.

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APHA (American Public Health Association); AWWA (American Water Works Association); WEF (Water Environment Federation). (1998) Standard methods for the examination of water and wastewater, 20th edition. Washington, DC: American Public Health Association.

ASTM (American Society for Testing Materials). (1999) Annual book of ASTM standards. Philadelphia, PA: American Society for Testing and Materials.

Barbour, MT; Gerritsen, J; Griffith, GE; et al. (1996) A framework for biological criteria for Florida streams using benthic macroinvertebrates. J North Amer Benthol Soc 15:185-211.

Barbour, MT; Gerritsen, J; Snyder BD; et al. (1999) Rapid bioassessment protocols for use in wadeable streams and rivers: periphyton, benthic macroinvertebrates, and fish, 2nd edition. Prepared by U.S. Environmental Protection Agency, Office of Water, Washington, DC; EPA 841-B-99-002.

Carlson, RE; Simpson, J. (1996) A coordinator's guide to volunteer lake monitoring methods. North American Lake Management Society. 96 pp.

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Deshon, JE. (1995) Development and application of the Invertebrate Community Index (ICI). In: David, WS; Simon, TP, eds. Biological criteria and assessment: tools for risk-based planning and decision making. Boca Raton, FL: Lewis Publishers; pp. 217-243.

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Klemm, DJ; Blocksom, KA; Fulk, FA; et al. (2003) Development and evaluation of a macroinvertebrate biotic integrity index (MBII) for regionally assessing Mid-Atlantic Highlands streams. Environ Manag 31:656-669.

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Miltner, RJ; Rankin ET. (1998) Primary nutrients and the biotic integrity of rivers and streams. Freshwater Biol 40:145-158.

National Research Council. (2000) Clean coastal waters: understanding and reducing the effects of nutrient pollution. Washington, DC: National Academy Press.

USGS (US Geological Survey). (1999) The quality of our nation's waters: nutrients and pesticides. U.S. Geological Survey Circular 1225, 82 p. Available at http://pubs.usgs.gov/circ/circ1225.

van Dam, H; Mertens, A; Sindeldam J. (1994) A coded checklist and ecological indicator values of freshwater diatoms from the Netherlands. Neth Aqua Ecol 28: 117-133.

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Wetzel, RG. (2001) Limnology, 3rd edition. San Diego, California: Academic Press.

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