The term "nutrients" refers to a wide variety of substances that are necessary for the growth of every living organism on earth, including humans.
In the aquatic environment, nitrogen and phosphorus are two nutrients that are particularly important to the growth of plants and algae. Plants, algae and photosynthetic bacteria contain countless microscopic molecules known as pigments that capture sunlight energy. Chlorophylls belong to one of the most important pigment groups and impart the characteristic green color to so many plants, algae and photosynthetic bacteria.
Plants, algae and photosynthetic bacteria use the energy captured by chlorophylls (and other pigments) along with carbon dioxide and water to assemble other molecules needed for growth, cellular repair and disease prevention. Nutrients such as nitrogen and phosphorus are essential to the assembly of these molecules. During daylight hours, photosynthetic processes also release oxygen into the water.
While nutrients are essential to the survival of all living things, when nutrient concentrations are too high they can have undesirable affects, such as explosive growth of bacteria, phytoplankton and macrophytes, leading to a loss of diversity and ecosystem balance. An over-abundance of either phosphorus or nitrogen is commonly associated with excessive plant or algal growth. An abundance of nitrogen, in particular in the forms of ammonia (NH3) and nitrite (NO2), can be problematic because under certain environmental conditions these can be deadly to fish and other aquatic wildlife.
Because excessive nutrients can be a detriment to water quality, their concentrations are regulated. The legal concentration limit for a particular water body depends on its type, its geographic location, and its use classification, among other things. Regulatory limits for nutrients are set forth in Chapter 62-302 of the Florida Administrative Code (FAC).
Pheophytin, a natural degradation product of chlorophyll, has an absorption peak in the same spectral region as chlorophyll a. It may be necessary to make a correction when pheophytin concentration becomes significantly high.
"Corrected" chlorophyll a refers to the method with the pheophytin correction.
"Uncorrected" chlorophyll a refers to the method without the pheophytin correction.
The Florida Department of Environmental Protection recognizes four methods as appropriate for the measurement of chlorophyll a. They are discussed in the document Applicability of Chlorophyll a Methods DEP-SAS-002/10
(October 24, 2011).