Equal monthly sample counts preserve the observed Mendota–Monona peak clarity deficit in 2015–2019 and 2022

We tested whether sample counts explain lower observed spring clarity peaks in Lake Mendota relative to Lake Monona. We used open NTL-LTER Secchi records. For each lake-year, we selected one recorded date from each of May, June and July. We averaged the two highest depths over all possible selections. An exact empirical-distribution formula removes simulation error. The Mendota-minus-Monona index changed from +0.81 m in 1995–2008 to -0.85 m in 2015–2019 and 2022. The change was -1.66 m (95% paired-year bootstrap interval -2.69 to -0.75 m; Welch interval -2.92 to -0.40 m). Wider windows gave negative changes of -1.82 to -1.85 m. An independent CSV reconstruction passed. A clean repeat reproduced all 14 numerical result files. A closely related SCI study appeared during this investigation. We reproduce its April–July top-two contrast of -2.63 m with independent code. The added calculation checks monthly sample counts. Both studies use the same source observations, so they are not independent field evidence. Only six later years pass the May–July coverage rule. Equal counts do not equalize visit days or recover brief peaks. The result supports an observed seasonal deficit, not its cause or a permanent change. The package preserves source data, rights, provenance, the local plan, later changes, code, results and figures.

lake clarity · Secchi depth · seasonal peaks · sampling effort · exact expectation · independent reproduction · Mendota · Monona