DIATOM SUCCESSION IN AN URBAN RESERVOIR SYSTEM

A 43 cm by 5 cm diameter sediment core sample was obtained from Ford L ake reservoir in Washtenaw County, Michigan, and sectioned at 1 cm int ervals. The purpose of this study was to determine whether diatom comm unities in this reservoir have undergone quantifiable changes in abund ance and composition since its creation. Thirty-one cm of this core ap peared to represent material deposited since the creation of the reser voir based on changes in diatom abundance, the physical composition of the sediment and the change in biogenic SiO2 concentration. Forty-sev en species of diatoms were identified total concentrations of diatom r emains varied from 1 x 10(4) g(-1) to 1 x 10(7) g(-1). Prior to the es tablishment of the reservoir, the diatom flora was dominated by benthi c taxa. Benthic diatoms were numerous throughout the entire core, but eutrophic taxa (e.g., Aulacoseira italica, Aulacoseira granulata, Step hanodiscus niagarae, Fragilaria crotonensis) dominated much of the cor e after the reservoir's creation. Total diatom density increased about tenfold in the about the first 10-15 years after the reservoir's crea tion before declining markedly.