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CADDIS Volume 2: Sources, Stressors & Responses


Ways to Measure Nutrients


Common forms of nitrogen 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 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 1).

Table 1. Measures of nitrogen.
Nitrogen form Units Common names Measures [also see ASTM (1999)]
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 measurement of nitrate by a colorimetric method [such as in Standard Methods 4500-N C (APHA et al. 1998)] or via analysis of total Kjeldahl nitrogen [TKN, such as in Standard Methods 4500-Norg B, C (APHA et al. 1998)] with addition of separate measurements of nitrite and nitrate (see above).


Phosphorus is present as dissolved orthophosphate (PO43-), various organic phosphorus compounds, and sediment-associated (particulate) phosphorus [see Wetzel (2001), Chapter 13 for 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 2).

Table 2. 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 phosphorus 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 phosphorus 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 phosphorus 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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