Extended Data Figure 2: Principal components analysis of the relationship between mean annual surface water pH, annual meteorological conditions and mean summer lake parameters during 1995–2010.

a, Ordination of mean summer pH in Qu’Appelle lakes (n = 6) during 1 May to 31 August in relation to mean annual meteorological conditions revealed that pH was correlated positively with mean annual and spring (not shown) temperatures, correlated negatively with the date of ice melt, and was unrelated to mean annual levels of precipitation or irradiance. Variables include log₁₀-transformed mean annual temperature (temperature), total annual rainfall (rain), total annual precipitation (precipitation), total snowfall (snow), untransformed daily hours of bright sunlight (irradiance) and the calendar day of the year when ice was completely melted from the lake surface (ice melt date). b, Ordination of mean summer pH in relation to coeval chemical, hydrological and physical conditions in Qu’Appelle lakes, as well as indices of relevant global climate systems. Abbreviations include water temperature (T°C_H20), total inorganic carbon (TIC), dissolved organic carbon (DOC), total dissolved nitrogen (TDN), log₁₀-transformed chlorophyll a (Chl), conductivity (Cond), log₁₀-transformed soluble reactive phosphorus (SRP), log₁₀-transformed total dissolved phosphorus (TDP), log₁₀-transformed dissolved ammonia/ammonium (NH₄), turbidity (Secchi depth), log₁₀-transformed volume of river inflow (inflow), dissolved oxygen (O₂), log₁₀-transformed dissolved nitrite + nitrate (NO₃) and climate indices representing the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO) and the winter (SOI_winter) or annual (SOI_mean) Southern Oscillation Index. This analysis reveals that mean summer pH is correlated positively with the PDO and negatively with the SOI, consistent with the interpretation that warm winters and reduced ice cover result in higher summer pH in Qu’Appelle lakes.

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