Summer Trophic Status Index (TSI) Monitoring

What is Trophic Status Index (TSI)?

The Carlson Trophic Status Index (TSI) is a tool used to summarize measurements of water quality into one index value. This value can be used to compare lakes in the same region or as a historical comparison of improvement or degradation over time. 
TSI Values are calculated based on 3 measurements:
  1. Total Phosphorus: Phosphorus is a requirement for plant growth. When phosphorus concentrations are high, large amounts of plant (including algae) growth can be supported. Phosphorus is most often the limiting nutrient, and therefore the most important nutrient to control in order to prevent overgrowth of plant and algae biomass.
  2. Chlorophyll a: All green plants contain chlorophyll a; therefore chlorophyll a concentrations are used as an estimate of overall plant/algae biomass.
  3. Secchi Disk: A Secchi Disk is a white metal disk used to measure the clarity or transparency of the water. The depth at which the disk is no longer visible is the Secchi Disk Reading. 

Sampling Procedure

We recommend completing this monitoring on a monthly basis from May through September each summer. Volunteers  typically collect the samples, however A.W. Research staff can train volunteers or to collect the samples for an additional fee. 
  1. Samples are collected at the deepest hole of the lake. If the lake has multiple deep holes or multiple bays, samples may be collected in several locations. 
  2. At the sampling site, water samples are collected using a 2-meter integrated sampler. This sampling device is simply a 2-meter long pipe used to collect the top portion of the water column for the sample. 
  3. Two sample bottles are filled from the sampler at each site. Bottles are placed immediately into a cooler with ice. 
  4. Next a transparency reading is taken using a Secchi disk. The Secchi disk is lowered into the water until it is no longer visible. The depth is recorded as the transparency (Secchi) reading.
  5. Samples should be returned to the lab within 24 hours or preserved in the field. Please call our lab if you are unable to drop off your samples within 24 hours and we can make arrangements for field preservation. 
Interested in starting a monitoring program on your lake?

Contact:
Sarah Fogderud
Client Services Manager
[email protected]
218-829-7974
Picture
A Secchi Disk is used to measure the transparency of the water.

TSI Sampling Forms and Information

Sampling Instructions
File Size: 535 kb
File Type: pdf
Download File
How to Make an Integrated Sampler
File Size: 454 kb
File Type: pdf
Download File
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After several years of sampling data has been collected, we are able to look at the long-term trend of the lake. This graph shows the average TSI for all samples collected for each month during the sampling season. This analysis helps us to understand the nutrient cycle of the lake and identify potential sources of pollution.