Morphological variation of Stephanodiscus niagarae Ehrenb. (Bacillariophyta) in a Lake Ontario sediment core

Stephanodiscus niagarae populations in a core from Lake Ontario show systematic variation in radial puncta number and mean valve diameter. Puncta number is stable in specimens from samples deposited between 1720 and 1844. Puncta number gradually increases in samples deposited between 1860 and 1947, then increases rapidly to high numbers in samples deposited after 1964. Mean valve diameter is relatively high in specimens from samples deposited between 1720 and 1959. Diameter of specimens deposited during this period fluctuates around a mean of 63.2 #m. Mean diameter of specimens deposited from 1959 to 1980 is only 42.5 #m. We conclude that the trend in puncta number reflects the time course of eutrophication in Lake Ontario. We interpret the precipitous decline in mean diameter as an indication that conditions .in the lake after the late 1950's did not permit sexual reproduction in this species.