Misuse of inorganic N and soluble reactive P concentrations to indicate nutrient status of surface waters

TitleMisuse of inorganic N and soluble reactive P concentrations to indicate nutrient status of surface waters
Publication TypeJournal Article
Year of Publication2003
AuthorsDodds, WK
JournalJournal of the North American Benthological Society
Pagination171 -181
Accession NumberKNZ00865
Keywordsammonium, dissolved reactive phosphorus, inorganic nutrients, Nitrate, nutrient limitation, phosphate, water-quality monitoring

Dissolved inorganic N (DIN) and soluble reactive P (SRP) have been used by some to indicate the trophic status of waters, and concentration ratios (DIN:SRP) to indicate nutrient deficiency. The utility of such measurements should be questioned, particularly based on well-known problems associated with determination of the concentration of SRP, which is commonly assumed to represent PO43−. Another potential problem with using inorganic nutrient pools to represent trophic state and nutrient availability ratios arises because concentration values are in units of mass per unit volume, and cannot be used with certainty to estimate supply (i.e., turnover rate of the nutrient pool, expressed either in mass per unit volume per unit time or simply as per unit time) to organisms without information on uptake and remineralization. Two data sets with lotic water-column nutrient values were explored, a large, continental-scale data set with analyses and collections done by many laboratories, and a more limited data set collected and analyzed by the same laboratory. In concert, the data sets indicated that at high total N (TN) (i.e., >5 mg/L) and total P (TP) (i.e., >2 mg/L) concentrations, >60% of the nutrient is usually made up of dissolved inorganic forms, but at low levels the ratio of dissolved inorganic to total nutrients is highly variable. Last, DIN:SRP is a weak surrogate for TN:TP and thus should be used with caution to indicate nutrient limitation.