Removing the deepest oxygen reading reduced gradient flags from 20 to 4 in 123 Wisconsin lake-years selected for March–June profile temperature ranges of at most 1°C

We tested whether nearly uniform spring temperature also indicated uniform oxygen in Wisconsin lake profiles. Open North Temperate Lakes LTER data supplied 123 selected lake-years from nine lakes in 1984–2025. Each selection was the March–June profile with the smallest temperature range, at most 1°C, under fixed depth and coverage rules. A gradient flag required mean deep oxygen below 80% of mean upper oxygen and a difference above 1 mg/L. There were 20 flags, or 16.3%. A paired check removed the deepest oxygen reading while keeping all dates and layer boundaries fixed. The count fell to 4, or 3.3%. All eight Wingra flags changed; four flags remained in Allequash, Fish and Monona. The depth check was added after the first results were inspected. It measures the effect of a depth choice and does not show that the deepest readings are wrong. Five of the 20 original flagged deep means were below 5 mg/L; a gradient flag is not a test of low oxygen relative to saturation. An independent CSV reconstruction matched every selected date, mean and flag. These data support direct checks of oxygen gradients when a temperature rule is used to describe spring conditions. They do not identify failed mixing, a time trend or a cause of oxygen loss. Irregular visits can miss oxygen renewal. The package preserves unchanged source data, provenance, code, all results, figures and offline repeat instructions.

dissolved oxygen · lake mixing · depth profiles · spring monitoring · measurement sensitivity · reproducibility