18 



! . 



•! technique that was used to develop models for assessing lake 



X , acidification in Maine and Norway (Davis and Berge 1980, Davis and 



' '■ *. 



Anderson 1985). Davis and Anderson (1985) found that principal 

 ;v% : V component models were less sensitive than multiple regression 



models using individual taxa to variation in the frequencies of taxa 

 caused by environmental factors other than pH. Van dam et al. 

 (1980) also used a principal components procedure to calibrate 

 diatom models and assess the effects of acid precipitation on Dutch 

 moorland pools. 



Reciprocal averaging (RA) is another indirect ordination 

 technique that has been used in diatom-based models. Charles 

 (1985) used reciprocal averaging to ordinate diatom data and he 

 correlated RA axes with environmental variables in a set of 

 Adirondack lakes. pH was a primary determinant of diatom 

 assemblage composition in that set of lakes and a regression equation 

 predicting pH from Charles' first RA axis explained 90% of the 

 variance in pH. In other environmental applications, Servant- 

 Vildary and Roux (1990) have used reciprocal averaging to 

 determine the effects of ionic elements on diatom species 

 composition in saline lakes of the Bolivian Altiplano. 



Canonical correspondence analysis (CANOCO) is a direct 

 ordination technique that has been used in pH reconstructions to 

 define axes that are combinations of taxa responding directly to pH. 

 The axes have then been regressed with pH and the resulting models 

 used to document historical trends in lake acidification. Battarbee et 

 al. (1988), for example, used CANOCO to assess the acidification of 



