The term "nutrients" refers broadly to those chemical elements essential to life on earth, but more specifically to nitrogen and phosphorus in a water pollution context. Plants and animals are made up mostly of compounds of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and phosphorus (P), and lesser amounts of sulfur (S), potassium (K), magnesium (Mg), and calcium (Ca). These are the macronutrients. Many other elements are necessary for growth, yet because these are needed in very small amounts they are classified as micronutrients. Animals get nutrients (that is, the chemicals they need to grow and reproduce) from eating plants or other animals; the air (or water in aquatic systems) provides animals only with oxygen. Plants, however, obtain carbon, hydrogen, and oxygen from the air and water, where all three elements are very abundant (as water and carbon dioxide). So the term "nutrients" in a water quality sense really deals with those elements that are necessary for plant growth, but are likely to be limiting -- that is, where used up or absent, plant growth stops. Of the nine macronutrients (C, H, O, N, P, S, K, Mg, and Ca), nitrogen and phosphorus are most likely to become limiting in aquatic environments.

Farmers apply fertilizer nutrients in the form of nitrogen, phosphorus, and potassium (N, P, & K with perhaps micronutrients) to prevent these elements from becoming limiting in the soil. These elements are concentrated in wastewaters from animal pens and septic systems. And these elements (especially N & P) in runoff or wastewater discharges reaching streams, lakes, or seas will promote aquatic plant growth. Abundant plant growth itself is a concern in assessing water quality. The most abundant "plants" in most aquatic environments are algae. When essential nutrients are plentiful, algae multiply. If these algae are microscopic phytoplankton, their growth increases the turbidity of the water. The water then becomes cloudy and colored a shade of green, yellow, or brown (sometimes red). An abundance in an aquatic system of any algae, or of higher plants, can signal excessive inputs of nutrients.

Nitrogen is the most abundant element in air, but occurs in a form (N₂) unusable to most life forms. Since nitrogen is an abundant component of biological tissue, any organic matter in water will contain nitrogen. However, until these larger organic molecules are broken down by bacteria, they are of little or no use to algae or other aquatic plants. Nitrogen is readily utilized by aquatic plants (such as algae) if it is dissolved in the water in an inorganic form: chemicals that are combinations of nitrogen and oxygen (nitrates and nitrites) or nitrogen and hydrogen (ammonia).

The element phosphorus can occur in nature in many forms, but the most abundant dissolved inorganic form in aquatic environments is as orthophosphate (PO₄).

Hawaii (US)
     Fresh water stream -- 
           Geometric mean of wet season* values not to exceed
		250.0 total N   70.0 nitrate + nitrite   50.0 total P 
           Geometric mean of dry season** values not to exceed
	   	100.0 total N    30.0 nitrate + nitrite   30.0 total P 
           Wet season* values not to exceed 
		520.0 total N   180.0 nitrate + nitrite  100.0 total P   
		   	ten percent (10%) of the time
           Dry season** values not to exceed 5.5 ntu ten percent
                  (10 %) of the time
           Wet season* values not to exceed 25.0 ntu two percent
                  (2%) of the time
           Dry season** values not to exceed 10.0 ntu two percent
                  (2%) of the time
     Estuaries (note separate criteria applicable to Pearl Harbor estuary)
           Geometric mean values not to exceed 1.50 ntu (4.00 ntu in PH)
           Values not to exceed 3.00 ntu (8.00 ntu in PH)  ten percent
                  (10%) of the time
           Values not to exceed 5.00 ntu (15.00 in PH) two percent
                  (2%) of the time
     Embayments --
           Geometric mean of wet embayment* values not to exceed 1.50 ntu
           Geometric mean of dry embayment** values not to exceed 0.40 ntu
           Wet embayment* values not to exceed 3.00 ntu ten percent
                  (10%) of the time
           Dry embayment** values not to exceed 1.00 ntu ten percent
                  (10 %) of the time
           Wet embayment* values not to exceed 5.00 ntu two percent
                  (2%) of the time
           Dry embayment** values not to exceed 1.50 ntu two percent
                  (2%) of the time
     Open Coastal waters --
           Geometric mean of wet coast* values not to exceed 0.50 ntu
           Geometric mean of dry coast** values not to exceed 0.20 ntu
           Wet coast* values not to exceed 1.25 ntu ten percent
                  (10%) of the time
           Dry coast** values not to exceed 0.50 ntu ten percent
                  (10 %) of the time
           Wet coast* values not to exceed 2.00 ntu two percent
                  (2%) of the time
           Dry coast** values not to exceed 1.00 ntu two percent
                  (2%) of the time

     *  Measurements made between November 1 and April 30 are wet season values; Wet
        embayments receive an average fresh water inflow from land equal to or greater than
        1% of embayment volume per day; Wet coast values apply where more than 3 mgd
        of freshwater discharge occurs per shoreline mile.
     **Measurements made between May 1 and October 31 are dry season values; Dry
        embayments receive an average fresh water inflow from land of less than 1% of
        embayment volume per day; Dry coast values apply where coastal waters receive
        less than 3 mgd of freshwater discharge per shoreline mile.   

• Web reading resources are arranged from least to most technical.

EPA releases nutrient strategy for waterways

Nutrients and Nitrogen: How the World Stays Green - Texas Watch Volunteer Environmental Monitoring Program

Hypoxia -- Mississippi River Basin Issues (EPA).