fc " ■* 1 ■ 



n 



- 2 8 



represents an "average" lakewide diatom assemblage (Earle et al. 

 1988). Variance due to site seems to increase when habitat 

 specificity of taxa is considered, i.e. when planktonic and periphytic 

 taxa are separated (Dixit and Evans 1986, Anderson 1990a). 



^%? Planktonic taxa typically show greater representation in deeper 



water, whereas periphytic taxa show relatively little redistribution 

 and are more abundant in the littoral zone. With respect to spatial 



, ;. variation within a lake basin, periphyton contribute a substantial 



■ . amount of the variance observed in diatom assemblages because 



their substrate preferences lead to patchy distributions (Earle et al. 

 1988). In a sediment core from any particular site in a lake, 

 however, periphytic diatoms tend to demonstrate more even 

 accumulation rates than do planktonic taxa (Anderson 1990a, 

 1990b). 



Spatial variation in subfossil diatom assemblages does not seem 

 to invalidate construction of calibration data sets using single 



'*: samples from each lake when lakes are sampled over a limnological 



range. Earle et al. (1988) have shown that single-sample, between- 

 lake differences are high enough to indicate that the comparison of 

 diatom assemblages between lakes is valid if samples are retrieved 

 from deeper areas with gentle slopes rather than from steep-sloped 



; ; areas. 



\ - Spatial variation does not preclude meaningful application of 



predictive models to historical samples provided that the effects of 



^0'- ■■■■ sample variance on inferences are understood. Anderson (1990a) 

 showed that sediment cores retrieved from deeper water sites 

 consistently demonstrated greater resolution of historical changes in 



