Observed spring clarity peaks fell in Lake Mendota relative to Lake Monona from 1996–2008 to 2015–2019

Could an earlier spring clear-water phase explain the recent loss of May–June clarity in Lake Mendota, Wisconsin? We compared open NTL-LTER Secchi records with nearby Lake Monona. The main analysis has 13 paired years in 1996–2008 and five in 2015–2019. Mean observed April–July maximum depth fell from 8.52 to 5.70 m in Mendota and rose from 6.89 to 7.62 m in Monona. The change in the lake difference was −3.55 m. Its 95% year-bootstrap interval was −6.44 to −0.98 m, but a Welch t interval was −7.43 to +0.32 m. Five later years therefore give uncertain precision for a lasting change. Mendota peaks occurred 22.7 days later on average, compared with 4.8 days later in Monona. The lake difference in mean clarity 30–90 days after ice loss fell by 1.70 m in 12 before and four later paired years. Four comparable sampling visits also gave lower Mendota peaks. The observed seasons show lower and later peaks, which weakens a simple earlier-phase explanation. Brief peaks between visits remain possible. The connected-lake comparison cannot identify a cause. This independent analysis builds on prior clear-water studies and an SCI seasonal-mean analysis. It preserves source data, provenance, code, figures, results and coverage limits.

lake ecology · water clarity · Secchi depth · seasonal timing · Lake Mendota · Lake Monona · reproducibility · sampling coverage