HGAM estimates of mean bottom-water DO as a function of (scaled) day-of-year, year, lake surface area ( A surf ), maximum depth ( z max ) and relative sampling depth ( z sample / z max ) ( Methods ). Predictions are shown for years 1970 and 2020 at a depth of 0.85 x z max in order-of-magnitude lake surface area classes and class-median z max (that is, for A surf = 10 ha, the median z max of lakes 5–50 ha). Day-of-year was scaled to lake-mean ice-on and ice-off derived from the ERA5-Land ‘lake ice depth’ product 61 to account for geographic differences in ice-free season length. Vertical dashed lines mark local oxygen maxima and approximate the timing of vernal and autumnal lake mixing. Shaded areas cover 95% Bayesian CIs.
