22 



1973). Predictive approaches that utilize groups of taxa are generally 

 more reliable than those based on a limited number of taxa 

 (Battarbee 1979). 



Diatom indices have been proposed that used trophic 

 autecological classifications of taxa for lakes in Florida (Whitmore 

 1985, 1989) and in Canada (Agbeti and Dickman 1989). In both of 

 these studies, diatoms were classified into 5 autecological categories, 

 and their percentages were structured into an index similar to 

 indices used for pH reconstructions (Nygaard 1956, Renberg and 

 Hellberg 1982). Log-transformed values of the indices were 

 regressed with log-transformed total P and Chi a in the Canadian 

 lakes and with TSI(TP) and TSI(Chl a) in Florida lakes. Log- 

 transformed values of the diatom inferred trophic index (D. I.T.I.) 

 (Agbeti and Dickman 1985) explained 71% of the variance in log- 

 transformed total P in the Canadian lakes, and the TROPH 1 index 

 (Whitmore 1989) explained 83% of the variance in TSI(TP) in Florida 

 lakes. Agbeti and Dickman concluded that the D.I.T.I. diatom index 

 was influenced by unspecified environmental factors. Whitmore 

 showed that pH was an important covariable affecting diatom 

 assemblages in the Florida lakes, though partial correlations 

 demonstrated that the predictive model using the TROPH 1 diatom 

 index was not statistically confounded by pH. Despite the fact that 

 silica limitation or cyanobacterial inhibition may affect 

 paleoproductivity inferences based on diatom accumulation rates 

 (Anderson 1990c), the TROPH 1 index still seems useful because 

 diatom assemblages are qualitatively distinct at the high nutrient 

 conditions where their populations are limited (Whitmore in press). 



