Winter nitrification rates and associated data for this study (under ice cover) and for Lake St. George (near surface at 2 m depth, under ice cover; [21]); for surface nitrification rates in Lake Croche (ice-covered; [38]); and for surface estimates of nitrate accumulation in Wisconsin lakes part of the North Temperate Lakes Long-Term Ecological Research (NTL- LTER) study (ice-covered, 30 years of accumulated data; [19]).

Values below limits of quantitation (LOQ) for nitrification rates are reported, including negative values (following [40]) and sample-specific LOQ (as described in Methods and calculated as per [43,44]) are reported.