The clearest 28-day May–July Secchi depth fell in Mendota relative to Monona from 1996–2008 to 2015–2019

We tested whether the spring water clarity loss in Lake Mendota relative to Lake Monona remained when each year's clearest period could move within May–July. We used open North Temperate Lakes LTER Secchi data. We estimated daily depths between surveys and found the largest 28-day mean for each eligible season. We required measured values on both sides of the season and limited survey gaps. The main comparison used 13 shared years in 1996–2008 and five in 2015–2019. The Mendota-minus-Monona peak mean changed by −2.45 m, with a 95% year bootstrap interval of −4.47 to −0.83 m. Two- and three-year blocks and 14- and 42-day windows gave the same direction. The selected midpoint moved later by 14.74 days relative to Monona. The Mendota-only change was −1.68 m, with an interval of −3.33 to +0.32 m. Thus, the clearest broad period had a lower mean relative to Monona even when its date could move within May–July. This is a follow-up to earlier work on these same lakes. Eight of 13 early Mendota windows began at the May boundary. Only two later shared years supported a wider April–August search. The test cannot exclude peaks outside May–July or distinguish a lower peak from a shorter clear period. Monona is connected to Mendota, prior trends differ, and the study does not identify a cause. The package preserves source data, credit, hashes, the local plan, code, results, figures, and offline reproduction steps.

lake clarity · Secchi depth · seasonal timing · Lake Mendota · Lake Monona · open data · reproducible research