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.
Water samples are collected and analyzed for nutrient concentrations on a regular basis by a number of agencies, researchers, and volunteer programs around the state. Most monitoring programs require the collection of surface water samples from a boat in "open" water (i.e., not from the shoreline or a dock). Samplers collect water by holding a special container just under the surface until it's nearly full. It is then sealed and kept in cool storage until it can be analyzed in a water chemistry laboratory. Some monitoring programs freeze their samples to enable large numbers of samples to be collected and processed over time.
For more information on monitoring programs in your area, check with your local water management district at http://www.dep.state.fl.us/secretary/watman/
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Many phosphorus compounds occur naturally in rock and soil, especially in the limestone found under much of Florida. In some parts the state, phosphorus is so abundant that it is mined out of the soil and used for a variety of agricultural and industrial purposes. The same is true of nitrogen. That is why one must always consider the local geologic conditions when assessing a lake's nutrient levels; if the surrounding soils are high in phosphorus or nitrogen, the water will likely contain high concentrations.
Also, nutrient levels can fluctuate considerably over time. If possible, water chemistry data should be collected on a regular basis, at a minimum of once a month for two years or more. This allows one to see if fluctuations are part of a natural cycle or if they are part of an upward or downward trend. So, when reviewing nutrient data for a specific lake, try comparing current nutrient values with historic values. Do they differ considerably or are they similar? Can you detect a pattern over time?
The water atlas "Public View" provides both corrected and uncorrected results because many laboratories reported uncorrected for several years and then (many about 1998) changed to report corrected chlorophyll a. If you use both for a long-term trend you must be aware that there will a slight difference between the two values because of the correction. The presence of pheophytin may cause a slight overestimation of chlorophyll a when the standard method (trichromatic equation) is used.
Several publications in the "Beginner's Guide to Water Management" circular series from the University of Florida LAKEWATCH program are very useful references in learning about water quality and how it is measured.