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How the Clean Lakes Program Monitors Water Quality

In order to assess lake water quality the Indiana Clean Lakes Program samples a number of different parameters.


Ammonia (NH3): a form of dissolved nitrogen that is readily used by algae. It is found in water where dissolved oxygen is lacking, such as in an eutrophic hypolimnion.

Nitrate (NO3-): an oxidized form of dissolved nitrogen that is converted to ammonia by algae under anoxic (low or no oxygen) conditions. It is found in streams and runoff when dissolved oxygen is present, usually in the surface waters.

Organic Nitrogen: Organic nitrogen includes nitrogen found in plant and animal materials and may be in dissolved or particulate form.

Soluable Reactive Phosphorus (SRP): dissolved phosphorus readily usable by algae. SRP is often found in very low concentrations in phosphorus-limited systems where the phosphorus is tied up in the algae and cycled very rapidly.

Total Phosphorus: includes dissolved and particulate forms of phosphorus. TP concentrations greater than 0.03 mg/L (or 30 micro-g/L) can cause algal blooms in lakes and reservoirs.


Chlorophyll-a: plant pigments of algae consist of the chlorophylls (green color) and carotenoids (yellow color). Chlorophyll a is the most dominant chlorophyll pigment. Thus, chlorophyll a is often used as a direct estimate of algal biomass.

Plankton: important members of the aquatic food web. Plankton includes algae (microscopic plants) and zooplankton (tiny shrimp-like animals that eat algae). Plankton are collected by filtering water through a very fine mesh net (63-micron openings = 63/1000 millimeter). The plankton net is towed up through the lake’s water column from the one percent light level to the surface.


Dissolved Oxygen: dissolved gaseous form of oxygen. It is essential for respiration of fish and other aquatic organisms. D.O. enters water by diffusion from the atmosphere and as a by-product of photosynthesis by algae and plants. Epilimnetic waters continually equilibrate with the concentration of atmospheric oxygen. Excessive algae growth can over-saturate (greater than 100% saturation) the water with D.O when rate of photosynthesis production is greater than the rate of oxygen diffusion to the atmosphere. Hypolimnetic D.O. concentration is typically low as there is no mechanism to replace oxygen that is consumed by respiration and decomposition. Fish need at least 3-5 mg/L of D.O. to survive.

Light Penetration: the rate at which light transmission is diminished in the upper portion of the lake’s water column. Another important light transmission measurement is determination of the 1% light level. The 1% light level is the water depth to which one percent of the surface light penetrates. The 1% light level is considered the lower limit of algal growth in lakes and this area and above is referred to as the photic zone.

Secchi Disk Transperency: the depth to which the black and white Secchi disk can be seen in the lake water. Water clarity, as determined by a Secchi disk, is affected by two primary factors: algae and suspended particulate matter. Particulates (soil or dead leaves) may be introduced into the water by either runoff or sediments already on the bottom of the lake. Erosion from construction sites, agricultural lands, and riverbanks all lead to increased runoff. Bottom sediments may be resuspended by bottom-feeding fish such as carp, or by motorboats or strong winds in shallow lakes.



A complete explanation of how to interprate lake data can be found here.

More information on our quality assurance project plan can be found here.