The effect of the Mt. Mazama ashfall on the planktonic diatom community of Lake Washington1

Abstract

Preserved diatom assemblages are found in deep water sediments in Lake Washington above and below an ash layer originating from an eruption of Mt. Mazama 6800 b . p . Total abundance of frustules did not change after the ash event, but a significant shift in community structure occurred. Melosira italica var. subarctica increased after the ashfall, and Fragilaria crotonensis decreased. The changes most likely indicate an increase in the Si: P ratio and decreased light penetration in the euphotic zone during winter and spring.