RESPONSES OF DIATOM AND CHRYSOPHYTE ASSEMBLAGES IN LAKE-227 SEDIMENTSTO EXPERIMENTAL EUTROPHICATION

We examined the diatom and chrysophyte assemblages preserved in Lake 2 27 sediments prior to, during, and following the annual additions of p hosphorus (P) and nitrogen (N) (from 1969-1989). Premanipulation (1957 -1968) diatom and chrysophyte assemblages were dominated by taxa chara cteristic of acidic, unproductive lake conditions. In 1969, the assemb lages immediately shifted in response to increased nutrient availabili ty and/or increased pH values in the epilimnion. Large increases in th e relative abundance of Synedra PIRLA 2 and S. rumpens var. familiaris , Mallomonas crassisquama, M. doignonii var. tenuicostis, and cysts 1, 15, 29, 233, and 120 largely replaced the prefertilization assemblage s. From 1970-1990, the diatom assemblage was dominated by alkaliphilic or pH-indifferent taxa. A second chrysophyte assemblage shift, which coincided with a lowering of the N:P addition ratio in 1975, was chara cterized by decreases in M. crassisquama and cysts 1, 15, 29, 233, and 120, and corresponding increases in Synura sphagnicola Korshikov scal es and cysts 62 and 33. The stratigraphical analyses and numerical rat e-of-change analyses clearly showed that diatom and chrysophyte specie s composition, particularly chrysophyte cysts, closely tracked short-t erm changes in lakewater chemistry. The amount of variance in the stra tigraphic assemblages explained by lake-water chemical changes was hig h (68-30%), indicating the close algae-chemistry relationships in Lake 227 from 1969-1989.