20 



the supposed eutrophic preference of Centrales, a notion refuted by 

 the wide range of trophic preferences actually observed for centric 

 taxa (Battarbee 1979). Stockner and Benson (1967) studied historic 

 trends in Lake Washington and proposed the A:C index, a ratio of the 

 number of valves in the tribe Araphidiniae to the number of valves 

 in the order Centrales. Centrales were assumed to be oligotrophic 

 rather than eutrophic indicators in this scheme. Stockner (1971) 

 later qualified the conditions under which this index would 

 accurately indicate trophic state, but subsequent studies (e.g. Brugam 

 1979, Battarbee 1979, Carney 1982, Charles 1985, Whitmore 1985) 

 have shown that the A:C index is not a useful indicator of lake 

 trophic status. The essential problem with these indices is that they 

 assume ecological uniformity of diatom species over broad taxonomic 

 groupings, whereas the individual species actually have ecologically 

 diverse requirements (C. Reimer pers. comm.). 



Schelske et al. (1983) examined concentrations of biogenic silica 

 in sediment cores from the Great Lakes. Increases in biogenic silica 

 were shown over time in the sediments of all of the Great Lakes 

 because eutrophication led to a more rapid production and 

 sequestering to sediments of diatom valves. The peak in 

 sedimentary storage of biogenic silica in Lakes Ontario and Erie 

 occurred in the 1800's and was followed by a decline that resulted 

 from silica limitation (Kilham 1971) as these lakes continued to 

 eutrophicate. Sedimentary biogenic silica increased after 1940 in 

 Lake Michigan and reached a maximum abundance in 1964, after 

 which it declined. Stoermer et al. (1990) used cluster analysis to 

 delineate diatom zonation in a sediment core from Lake Michigan 



