Responses of Diatom and Chrysophyte Assemblages in Lake 227 Sediments to Experimental Eutrophication

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Abstract
We examined the diatom and chrysophyte assemblages preserved in Lake 227 sediments prior to, during, and following the annual additions of phosphorus (P) and nitrogen (N) (from 1969–1989). Premanipulation (1957–1968) diatom and chrysophyte assemblages were dominated by taxa characteristic of acidic, unproductive lake conditions. In 1969, the assemblages immediately shifted in response to increased nutrient availability and/or increased pH values in the epilimnion. Large increases in the 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 assemblages. 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 characterized by decreases in M. crassisquama and cysts 1, 15, 29, 233, and 120, and corresponding increases in Synura sphagnicola Korshikov scales and cysts 62 and 33. The stratigraphical analyses and numerical rate-of-change analyses clearly showed that diatom and chrysophyte species composition, particularly chrysophyte cysts, closely tracked short-term changes in lakewater chemistry. The amount of variance in the stratigraphic assemblages explained by lake-water chemical changes was high (68–90%), indicating the close algae–chemistry relationships in Lake 227 from 1969–1989.

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