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Trophic Status Index (TSI) Monitoring

Background

The Carlson Trophic Status Index (TSI) is a tool used to summarize several measurements of water quality into one index value which can be used to compare a lake to other lakes, or to historic/future data as a measure of degradation or improvement.  In many ways, the index can also be viewed as a measure of the potential for algal productivity.  Since most people value lakes with low algae productivity, the lower the TSI value, the healthier the lake is.

 

Specifically:

In general, if one is planning on sampling from only one location, then it is standard practice to select the deepest spot in the lake.  Since consistency in sampling is very important, it would be helpful if there are geographical reference points that you can use in order to sample the same spot every time.  Some samplers like to use a depth finder to help identify the correct location.  Us of an anchor to stay in the same spot while sampling is recommended.

 

A sampling device [usually made from a section of PVC pipe] is used to get a stratified sample from the top two meters of a lake (the “epilimnion”) since this is the zone where the vast majority of biological activity occurs.  The water samples are analyzed for total phosphorus, chlorophyll “a”, and secchi disc depth.  A brief summary of the significance of each of the parameters in included below.

 

Total Phosphorus (TP)

Phosphorus is a major plant nutrient.  When phosphorus is present in high concentrations it stimulates plant (both algae and other plant) growth.  Although other nutrients can also influence water quality, phosphorus is most often the “limiting factor”, and is also the most easily controlled.

 

When collecting water samples for TP analysis the sampler should rinse out the sample bottles once or twice with lake water.  Fill the bottle to the top, taking care to record the sample collection date, time, location, and sampler's name.  Immediately after collecting the phosphorus sample, add 2 full eyedroppers of sulfuric acid (H₂SO₄) to serve as a preservative.  Alternatively, samples can be stored in a dark cooler, and transported to the laboratory within a few hours of collection so that acid preservative can be added there.

 

Chlorophyll a

All green plants contain chlorophyll a, which allows limnologists to use the concentration of chlorophyll a as an estimate of total algal biomass.

 

As with the phosphorus samples, the sampler should rinse out the sample bottles once or twice with lake water.  Fill the bottle to the top, taking care to record the sample collection date, time, location, and sampler's name.  Samples should be stored in a dark cooler, and transported to the laboratory as quickly as possible (within a few hours).

 

Secchi Disc

A secchi disk (SĕCK·ē) is a black and white plastic disc which is lowered into a lake to measure clarity or transparency of the water.

Ideally, secchi disk readings should be collected between 10 am and 2 pm, and observed off the shady side of the boat.

 

Affordable Option

The most inexpensive way to conduct this analysis is if the sponsoring organization collects the samples themselves.  A.W.  Research Laboratories does offer services in which we collect the samples for the organization on a time and material basis.

 

Report

At the end of the sampling season a final report including analysis, conclusions and recommendations is provided.  The following shows an example of the graphical analysis included with the report and monthly laboratory results.

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