Little Rock Lake Total Maximum Daily Load (TMDL) Study

Documents and Data

Little Rock Lake Newsletter (Winter 2009 Edition)

Secchi Disc Information:
The following graph represents Secchi transparency data collected by volunteers in the Citizen Lake-Monitoring Program (CLMP). Values expressed in the graph below are in units of feet.

CLMP volunteers collect water transparency data using an 8-inch, circular, all-white metal plate attached to a calibrated rope. This tool is called a Secchi disk. About once a week during the summer, volunteers boat to a designated spot on their lakes to collect transparency readings. The volunteer lowers the disk into the water until it is no longer visible and notes that depth from the markings on the rope. The disk is then lowered a little further and then raised back up until it is just visible. This second depth reading is averaged with the first, and the final number is recorded on a data sheet. At the end of summer, volunteers send their data sheets to the MPCA to be compiled with other water-quality data.

Water transparency is a quick and easy measurement that tells scientists a lot about a lake's water quality. First it indicates the amount of light penetration into a lake. Second, Secchi transparency provides an indirect measure of the amount of suspended material in the water, which in many cases is an indication of the amount of algae in the water.

Long-term transparency monitoring by CLMP volunteers helps scientists detect signs of degradation to a lake. Generally, the sooner water-quality problems are detected, the easier and less expensive it is to restore the lake to its previous state.

Example of an Oligotrophic lake.

Example of an Hypereutrophic lake.

Lake Status Information:
The following graph is a summary of available summer (June through September) data in STORET (STORET is the national water quality data repository developed by the United States Environmental Protection Agency. All water quality data collected by MPCA or received from external groups is placed in STORET.) collected between 1997 and 2006.

Carlson Trophic Status

Carlson’s Trophic State Index (TSI) is a common means for characterizing a lake’s trophic state (overall health) and associating Secchi, chlorophyll-a, and phosphorus measurements. The term “trophic status” refers to the level of productivity in a lake as measured by phosphorous content, algae abundance, and depth of light penetration. Carlson’s Trophic State Index is one means available to examine the relationship between total phosphorus, chlorophyll-a, and Secchi disk readings in a lake and its overall productivity. Individual TSI values can be calculated from the following equations:

Total phosphorus TSI (TSIP) = 14.42*[ln(TP average)] + 4.15

Chlorophyll-a TSI (TSIC) = 9.81*[ln(Chlorophyll-a average)] + 30.6

Secchi disk TSI (TSIS) = 60 - (14.41*[ln(Secchi average)])

Total phosphorus and chlorophyll-a are measured in micrograms per liter and Secchi disk transparency is measured in meters (3.281 feet per meter). The ln function in these equations is the "natural log" which is different than the "log" function. The TSI scale ranges from 0 (ultra-oligotrophic) to 100 (hypereutrophic). High and/or increasing trophic status values indicate eutrophic conditions (higher productivity).

Carlson’s Trophic State Index

R.E. Carlson

TSI < 30 Classical Oligotrophy: Clear water, oxygen throughout the year in the hypolimnion, salmonid fisheries in deep lakes.

TSI 30 - 40 Deeper lakes still exhibit classical oligotrophy, but some shallower lakes will become anoxic in the hypolimnion during the summer.

TSI 40 - 50 Water moderately clear, but increasing probability of anoxia in hypolimnion during summer.

TSI 50 - 60 Lower boundary of classical eutrophy: Decreased transparency, anoxic hypolimnia during the summer, macrophyte problems evident, warm-water fisheries only.

TSI 60 - 70 Dominance of blue-green algae, algal scums probable, extensive macrophyte problems.

TSI 70 - 80 Heavy algal blooms possible throughout the summer, dense macrophyte beds, but extent limited by light penetration. Often would be classified as hypereutrophic.

TSI > 80 Algal scums, summer fish kills, few macrophytes, dominance of rough fish.