Secchi depth, a convenient variable that reflected phytoplankton 

 standing crop in many lakes. Carlson scaled his TSI with the 

 intention that a ten-unit increase in TSI would be equivalent to a 

 doubling of algal biomass, but his index failed to relate in a uniform 

 way to Chi a. Carlson calibrated his TSI to limnetic Chi a and total P 

 for a set of north-temperate lakes and developed subindices to 

 permit expression of TSI from these latter two variables. 



Kratzer and Brezonik (1981) later modified Carlson's approach 

 by constructing a subindex expressing TSI from water-column total N 

 concentrations after observing N limitation in several Florida lakes 

 located on phosphatic limestones. They proposed an averaged 

 subindex that used mean TSI values based on Chi a and Secchi depth, 

 and the lesser of the two TSI values based on total P and total N. 



Baker et al. (1981) observed different relationships between 

 Secchi depth, total P and Chi a in Florida lakes than Carlson (1977) 

 observed in north-temperate lakes. Huber et al. (1982), therefore, 

 constructed a new TSI for Florida lakes using the Florida Lakes Data 

 Base, a large data set that is maintained at the Water Resources 

 Research Center at the University of Florida. 



Huber et al. based their TSI on Chi a, a more direct measure of 

 phytoplankton biomass than Secchi depth, and they retained Kratzer 

 and Brezonik's total N and averaged subindex approach because of N 

 limitation in some Florida lakes. Huber et al. described lakes with 

 total N/total P values >30 as P-limited, and calculated averaged TSI 

 (TSI(AVG) for these lakes as the mean of TSIs based on Secchi depth 

 (TSI(SD)), Chi a (TSI(Chl a) and total P (TSI(TP)). Lakes with total 

 N/total P values <10 were described as N-limited, and TSI(AVG) was 



