About Lake Data

Secchi Depth
Trophic State Index
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Trophic State Index
Eutrophication is the process by which lakes are enriched with nutrients, increasing the production of rooted aquatic plants and algae. The extent to which this process has occurred is reflected in a lake's trophic classification or state:

oligotrophic - nutrient poor and low productivity; high transparency (deep secchi depth), low chlorophyll-a, low phosphorus

mesotrophic - moderately productive; intermediate clarity, chlorophyll and phosphorus concentration

eutrophic - very productive and fertile; low clarity/shallow secchi; high chlorophyll and phosphorus concentrations.

hypereutrophic - extremely productive with noxious surface scums of algae

CARLSON'S TROPHIC STATE INDEX (TSI)

Trophic State Indices (TSIs) are an attempt to provide a single quantitative index for the purpose of classifying and ranking lakes, most often from the standpoint of assessing water quality. In recent years the Carlson (1977) Index appears to have attained general acceptance in the limnological community as a reasonable approach to this problem. This is a measure of the trophic status of a body of water using several measures of water quality including: transparency or turbidity (using Secchi disk depth recordings), chlorophyll-a concentrations (algal biomass), and total phosphorus levels (usually the nutrient in shortest supply for algal growth).

TSI ranges along a scale from 0-100 that is based upon relationships between secchi depth and surface water concentrations of algal chlorophyll, and total phosphorus for a set of North American lakes. Its major assumptions is that suspended particulate material in the water controls secchi depth and that algal biomass is the major source of particulates; The lowest value of zero would correspond to a secchi depth of 64 meters (greater even than Crater Lake, Oregon and Lake Tahoe, CA/NV at its clearest back in the 1960's)! A value of 100 would correspond to a secchi of only 6.4 cm (less than 3 inches- yuck !). A set of equations were then derived to describe these relationships with higher values corresponding to increased fertility, that is, more eutrophic. An increase in TSI of 10 units corresponds to a halving of secchi depth and a doubling of phosphorus concentration.

The Minnesota Pollution Control Agency (MPCA) classifies lake water quality information according to Ecoregions that divide the country into areas that have similar land use, soils, topography and natural vegetation (sometimes called "potential" vegetation since people have often changed what was once present by cutting forests or draining wetlands for creating farmland). Minnesota includes 7 ecoregions of which 4 contain 98% of the state's lakes (Fig 1).

NLF - Northern Lakes & Forests
NCHF - North Central Hardwood Forests
WCBP - Western Corn Belt Plains
NGP - Northern Glaciated Plains

Figure 1

The graphs below show what the secchi, chlorophyll and total phosphorus (TP) data for Minnesota lakes monitored by the Minnesota Pollution Control Agency look like. They are from the MPCA's Year 2000 Lake Assessment Report.

Figure 3a. Log total phosphorus vs log chlorophyll-a scatterplot. Derived from ecoregion reference lakes summer-mean measurements

3a

Figure 3b. Total Phosphorus vs Secchi scatterplot. Derived from ecoregion reference lakes summer-mean measurements.

3b

Note how sensitive the lower secchi graph (3b) is to increasing TP. As the TP increases (slides down the curve) from about 10 up to 20 ugP/L (ppb), the secchi depth, the water clarity, drops off rapidly from about 4.5 meters to less than 3 meters visibility. You should also be aware that while the drawn line fits the data pretty well, there is still a lotof scatter in the fitted curve- more than 100% in general, particularly at higher secchi (more transparent) lakes. The reasons for this probably are that the algal communities in some of the monitored lakes are not strictly regulated only by phosphorus; nitrogen availablility, light, and zooplankton effects can also distort the simplest view that only phosphorus matters. This effect is also very striking at very high TP levels where the secchi depth is already about as low as it can get short of jumping back into the boat. and the water is now so turbid that algal chlorophyll may actually be limited by light penetration associated with shading from the algae themselves (see those data points to the far right of the line in Figure 3 A ). Another effect often seen in this type of large data set is that the lakes with the extremly high TP values (over 300 ugP/L for those outliers in Figure 3A) are often in agricultural landscapes where stream inflows have a lot of suspended sediment that clouds the water and also light-limits algal growth. So you don't get as much algal chlorophyll as you 'd expect fron just looking at the TP value.

Table 2 shows how these water quality variables and the TSI differ across Minnesota's ecoregions. The top line of the table summarizes how limnologists generally classify lakes according to differences in secchi, chlorophyll and total -P. The ranges are a result of differences in opinion and so there is no exact definition of a lake's trophic status based on just a few parameters. They are very useful guidelines for making management decisions, setting priorities for funding, and tracking short- and long-term trends for discovering problems early and assessing how well lake and watershed restoration programs are working.

.Table 2