Extended Data Figure 3: Elastic net analysis to identify and rank predictors of changes in under-ice pH in Buffalo Pound Lake during winter.
From: Decrease in CO2 efflux from northern hardwater lakes with increasing atmospheric warming
Water quality parameters at 1.5 m above the lake bottom were analysed weekly using uniform methods during 1985–2003 from the date of ice-cover formation to the date of ice melt. Analysis was performed using 125 weekly observations with complete water chemistry. Parameters include concentrations of dissolved oxygen (O2), sodium (Na+), carbonate (CO32−), loge-transformed dissolved aluminium (logeAl), fluoride (F−), potassium (K+), loge-transformed orthophosphate (logePO43−), calcium (Ca2+), dissolved magnesium (Mg+), loge-transformed nitite + nitrate (logeNO3−), loge-transformed dissolved manganese (logeMn), bromide (Br−), total phosphorus (TP), dissolved iron (Fe), chloride (Cl−), loge-transformed ammonium (logeNH4+), bicarbonate (HCO3−), sulphate (SO42−) and temperature (temp). Coloured lines indicate how standardized regression coefficients (y axis, left) develop (right to left) as the initial pool of predictors (y axis, right) is refined by removing collinear and non-significant variables. Evaluation of the standardized coefficients of the most parsimonious model (vertical dashed line; equation under graph) demonstrates that changes in microbial metabolism (O2 decline × respiratory quotient of 1.2 = CO2 production)4,11 was the main factor regulating variation in water-column pH under ice, showing a nearly fourfold greater coefficient (0.14) than did either HCO3− or Ca2+ (0.04). Although dissolved logeAl, logeNH4+ and CO32− were also significant predictors of changes in winter pH (standardized coefficients 0.03–0.07), concentrations of these solutes (means ± s.e.m.; n = 17) were too low (<0.01 M) to regulate lake-water pH relative to the effects of changes in O2 (0.62 M), HCO3− (3.69 M) or Ca2+ (1.22 M). This analysis suggests that metabolically produced CO2 mainly regulates variation in winter pH by the production of carbonic acid (reduces pH), but that pH decline is slightly tempered by CO2-induced dissolution of sedimentary CaCO3, the main form of sedimentary carbon in Buffalo Pound9.
