The Mendota–Monona spring clarity peak gap increased by 15 days in selected 1996–2019 surveys

We tested whether the date of maximum observed spring water clarity changed in Lake Mendota relative to Lake Monona, Wisconsin, USA. We used open NTL-LTER Secchi data from 1995–2022. We matched April–July visits within seven days and required coverage at both ends of the window. The selected sample has 13 years in 1996–2008 and eight years in 2010–2019. We omitted transition year 2009. The mean Mendota-minus-Monona peak-date gap changed from −3.5 to +11.0 days. The change was +14.5 days, with a nominal 95% interval of +4.9 to +23.5 from blocks of three retained years. Calendar-year blocks gave +3.8 to +23.7. All visits and a common fortnightly grid gave changes of +15.6 and +9.8 days. With a three-day matching limit, the change was +6.8 days and its interval included zero. The median-gap change and a period effect conditional on the ice-off gap were also uncertain. No 2020–2022 year passed the main coverage rules. Earlier work already linked food webs to spring phase timing in these same lakes. This study adds a paired date estimate with explicit sampling checks and reproducible files. It describes selected sampled maxima; it does not identify the exact biological phase date or an invasion cause. The package preserves source data, hashes, provenance, code, all results, figures, and independent numerical checks.

lake ecology · Secchi depth · clear-water phase · phenology · sampling effort · Lake Mendota · Lake Monona · reproducibility