Lake Production, Succession, and Eutrophication - Hydro-Environmental Analysis: Freshwater Environments - page 462

halving of algal biomass (chlorophyll-a). The simpliied forms of the equations derived by Carlson
(1977) are given as follows (from Carlson and Simpson 1996):

( ) =− ( )

TSISD 60 14 41 . n SD

( ) = ( ) +

TSICHL 981 . n CHL 306 .

( ) = ( ) +

TSITP 14 .42 ln TP 415 .

where
SD is the Secchi depth (meters)
CHL is chlorophyll-a concentrations (micrograms per liter)
TP is total phosphorus (micrograms per liter)

Carlson created the interpretation scheme in Table 16.5 for the computed TSI (Carlson 1977).
Note that the values in Table 16.5 are interrelated, based on Carlson's data analyses, so that the
TSI can be based on any one of the metrics (SD, TP, or CHL) and is not additive and should not be
averaged (Carlson and Simpson 1996) since it is predicated on the idea that it is predicting algal
biomass. Carlson and Simpson (1996) suggested using chlorophyll as the primary index for tro-
phic state classiication where data for chlorophyll and phosphorus are available. Deviations of the
Secchi depth and TP indices from the chlorophyll index could then be used to infer additional infor-
mation about the functioning of the lake. Carlson (1977) also emphasizes that these relationships
are indications of trophic status, not water quality. Carlson points out a common misconception that
eutrophic is often taken as synonymous with poor water quality. Water quality is a subjective deter-
mination while the trophic status is a “neutral” determination; basically, “it is what it is.”
A limitation to Carlson's method is that it was developed for northern lakes, so its applicability
to southern lakes is limited. The following section describes a modiication of the TSI method that

TABLE 16.5
Carlson's Trophic State Index

TSI CHL ( μ g L -1 ) SD (m) TP ( μ g L -1 ) Attributes

<30 <0.95 >8 <6 Classical oligotrophy : Clear water, oxygen throughout the year in

the hypolimnion, salmonid isheries in deep lakes.

30-40 0.95-2.6 8-4 6-12 Deeper lakes still exhibit classical oligotrophy, but some shallower

lakes will become anoxic in the hypolimnion during the summer.

40-50 2.6-7.3 4-2 12-24 Water moderately clear, but increasing probability of anoxia in

hypolimnion during summer.

50-60 7.3-20 2-1 24-48 Lower boundary of classical eutrophy : Potential for decreased

transparency, anoxic hypolimnia during the summer and
macrophyte growth, warmwater isheries only.

60-70 20-56 0.5-1 48-96 Dominance of blue-green algae, algal scums probable, extensive

macrophyte problems.

70-80 56-155 0.25-0.5 96-192 Heavy algal blooms possible throughout the summer, dense

macrophyte beds, but extent limited by light penetration. Often
would be classiied as hypereutrophic .

>80 >155 <0.25 192-384 Algal scums, summer ish kills, few macrophytes, dominance of

rough ish.

Source: Carlson, R.E. and Simpson, J., A Coordinator's Guide to Volunteer Lake Monitoring Methods , North American
Lake Management Society, Madison, WI, 1996.