Autecological studies of diatoms as pH indicators have opened the way to estimating a lake's past pH on the basis of its diatom species compos it ion and relative abundance. Estimating the rate of lake acidification from its sediment subfossil diatoms is possible when these subfossils can be identified and accurately enumerated down the length of the lake sediment core. This technique holds considerable promise in assessing the temporal impact of acid precipitation for acid-sensitive lakes.
When diatom inferred pH was regressed against observed pH for 23 lakes located north of Lake Superior a significant (P<0.01) correlation (r=0.71) resulted. Diatom downcore stratigraphy for two of these lakes (lakes Cs & B) indicated that their pH had dropped from 6.3 to 5.3 (Lake Cs) and from 5.6 to 5.3 (Lake B) over the last 20 years. However, when the "B" index was used instead of log alpha, no significant change in downcore diatom inferred pH was apparent for Lake B. We concluded that log alpha was overly sensitive to small changes in the relative abundance of alkaliphilic diatoms. Thus, in lakes where alkaliphilic diatoms are rare, index "B" rather than log alpha should be employed. In lakes of pH 6 or above where the relative abundance of alkaliphilic diatoms is higher (e.g. Cs & Fenton Lakes) index "B" and log alpha provide essentially identical downcore pH profiles.