New methods for using diatoms and chrysophytes to infer past pH of low-alkalinity lakes

Distributions of both diatoms and chrysophytes (Mallomonadaceae) are clearly correlated with pH in low‐alkalinity lakes. Their siliceous remains are well preserved in lake sediments and have been used to infer long‐ and short‐term acidification trends. Until now, however, only diatoms have been used quantitatively. We describe new methods for inferring past pH in lake water using diatoms alone, chrysophytes alone, and the two together. We developed the equations with multiple regression analysis of diatom and chrysophyte assemblage data and pH measurements for 47 Adirondack lakes (pH range, 4.7–7.8). Correlations of measured vs. calculated values of pH yield r 2 values of 0.70–0.94 and standard errors of 0.13–0.35 pH unit. The best predictive equations, especially for acidic lakes (pH < 5.5) incorporate both diatoms and chrysophytes. Major advantages of using chrysophytes in addition to diatoms in paleoecological studies are that chrysophytes are more sensitive at lower pH values (<5.0) than diatoms, are nearly all planktonic, represent a different phycological group, can be counted on the same slides as diatoms, and can be counted faster because there are fewer taxa. Stratigraphic diatom and chrysophyte data for two lake sediment cores are used to evaluate the equations. These methods are widely applicable in studies of cultural and natural acidification of low‐alkalinity lakes in many regions of the world.