27 



sediment cores, and it interferes with assessment of the importance 

 of species on samples. 



Effects of Spatial Variation on Diatom Assemblages 

 In paleolimnological work, there is frequently an implicit 

 assumption that diatom assemblages have been homogenized by 

 resuspension prior to deposition so that a single surface-sediment 

 sample or a sediment core reflects lakewide mean limnological 

 conditions. If spatial variability exists in species composition of 

 diatom assemblages in surficial sediments, variance is introduced 

 into the calibration data sets used for models describing 

 diatom/limnological relationships. Spatial variability in diatom 

 assemblages from sediment cores may also affect the precision of 

 historical inferences. 



Anderson (1990a) studied variability in diatom concentrations 

 and accumulation rates in 10 sediment cores from Lough Augher and 

 found that diatom accumulation rates and concentrations were not 

 spatially uniform. Differences resulted partly from variance in bulk 

 sediment accumulation rates that was not related in a predictable 

 way to water depth (Anderson 1990b). Factors including localized 

 resuspension, stream inputs, slumping and the effects of 

 macrophytes on wind circulation patterns were responsible for the 

 spatial differences in sedimentation rates. Anderson concluded that 

 no single sediment core reflected the mean accumulation rate of the 

 whole basin. 



Studies on the spatial heterogeneity of species composition in 

 surficial sediment samples have shown that no single sample 



