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Other Measurements

Chlorophyll a

Chlorophyll a concentration is an estimate of the phytoplankton biomass. All green plants contain chlorophyll a, which constitutes approximately 1-2% of the dry weight of phytoplankton.

Other pigments that occur in phytoplankton include chlorophylls b and c, xanthophylls, phycobilins, and carotenes.

The important chlorophyll degradation products found in the aquatic environment are the chlorophyllides, pheophorbides, and pheophytins.

The presence or absence of the various photosynthetic pigments is used, among other features, to separate the major algal groups.

Dissolved Oxygen

Dissolved oxygen (DO) provides valuable information about the biological and biochemical reactions occurring in the aquatic environment. Oxygen gas dissolves freely in fresh water, and it may also be added to the water from the atmosphere or as a by-product of photosynthesis from aquatic plants.

A dissolved oxygen content of 3 mg/L is generally stressful to aquatic vertebrates and most other aquatic life.

Biological Oxygen Demand

Biological Oxygen Demand (BOD) is a test used to determine the relative oxygen requirements of waters. The test measures the oxygen utilized during a specified incubation period for the biochemical degradation of organic material and the oxygen used to oxidize inorganic materials.


Silicon ranks next to oxygen in abundance in the earth’s crust. It does not occur freely in nature, but it appears as an oxide (silica) in quartz and sand and is combined with metals in the form of many complex silicate minerals, particularly igneous rocks.

Degradation of silica-containing rocks results in the presence of silica in natural waters. Volcanic and geothermally heated waters often contain an abundance of silica.

The silica (SiO 2) content of natural water most commonly is in the 1 to 30 mg/L range, although concentrations as high as 100 mg/L are not unusual and concentrations exceeding 1000 mg/L are found in some brackish waters and brines.


Solids refer to the matter suspended or dissolved in water or wastewater. Solids may affect water adversely in a number of ways. A limit of 500 mg dissolved solids/L is desirable for drinking water.