2007 National Lake Survey - Wisconsin Results
Analysis Overview
Buckskin Lake, Florence County
DNR Photo
As analysis methods are established for the 2007 National Lake Survey, descriptions will be posted here. Many of the evaluation methods will involve creating summary indices that combine several different types of data. For example, an index of ecological health would likely include the composition and relative abundance of key groups of animals and plants.
Currently established analysis methods include:
- Lake productivity: Wisconsin Trophic State Index
Aquatic plants: Aquatic Macrophyte Community Index
Anticipated analysis methods:
- Ecological health: Phytoplankton Index of Biotic Integrity (IBI), Zooplankton IBI, Benthic Macroinvertebrate and Observed/Expected (O/E) Indices, Diatom assemblage changes, Lake Habitat Quality Score, and the Lake Habitat Modification Assessment
Recreational value:
Aquatic Indicators of Stress: chemical, onshore, biological, and ranking
Wisconsin Trophic State Index
The most commonly used index of lake productivity is the Carlson's Trophic State Index (TSI), which incorporates 3 water quality parameters: Secchi disk depth, chlorophyll a concentration, and total phophorus concentration. TSI values range from low (<30), representing very clear, nutrient-poor lakes, to high (>50) for extremely productive, nurient-rich lakes.
To better tailor the Carlson's TSI to Wisconsin's lakes, the index was modified in the early 1990's by the WDNR to create the Wisconsin Trophic State Index based on the following formulae:
-
TSI(SD) = 60 - 14.4 ln(SD)
TSI(CHL) = 34.8 + 7.56 ln(CHL)
TSI(TP) = 28.2 + 7.73 ln (TP)
-
SD = Secchi depth (meters)
CHL = chlorophyll a (μg/L)
TP = total phosphorus (μg/L)
ln = natural log
TSI = Wisconsin Trophic State Index
While historically there's been a tendency to average the three TSI values, this is generally not a good practice. The index is a prediction of algal biomass, so typically the TSI(CHL) value is a better predictor than either of the other two indices.
However, TSI(TP) and TSI(SD) are still useful because the interrelationships between them and TSI(CHL) can be used to identify other environmental factors that may influence algal biomass. For example, if TSI(CHL) is greater than TSI(SD), large particulate algae may dominate. In contrast if TSI(SD) and TSI(TP) are both greater than TSI(CHL), light attentuation may be due to naturally high color or suspended solids, rather than algae.
RSS