Sampled spring Secchi maxima fell in Mendota relative to Monona from 1995–2008 to 2015–2022

Can a shift in spring timing explain the later water-clarity loss in Lake Mendota relative to nearby Lake Monona, Wisconsin? We used fixed North Temperate Lakes LTER Secchi and ice records. A local plan set periods from an earlier SCI study. The planned mean Secchi depth from day 30 to day 120 after each lake's ice-off fell by 1.15 m in Mendota relative to Monona (95% three-year block interval −1.99 to −0.28 m). This test had 12 baseline year pairs and only four later pairs, in 2015, 2016, 2018 and 2019. A narrower window and an ice-date adjustment gave intervals that include zero. We then added exploratory checks of sampled maxima. The mean annual April–July maximum changed from 8.59 to 6.24 m in Mendota and from 6.84 to 7.56 m in Monona. The change in their difference was −3.06 m (95% independent-year interval −5.40 to −1.01), with 14 baseline pairs and seven later pairs. The later set excludes 2020. Matching visits within three calendar days gave −2.94 m (−5.17 to −0.98). Fixing effort at four matched visits per year gave −2.35 m (−4.02 to −0.96). Matching days after ice-off with a coverage check gave −3.27 m (−6.19 to −0.77; five later pairs). The early 2010–2014 period showed no Mendota-specific loss. Thus the observed later loss remains after several timing and effort controls. A short peak between visits can still be missed. The data do not identify a cause, and Monona receives Mendota water. The package preserves source files, visit matches, all estimates, a local plan, analysis notes, figures, code and independent arithmetic checks. This is a follow-up using the same source data as the earlier SCI study.

water clarity · Secchi depth · Lake Mendota · Lake Monona · spring timing · ice-off · sampling effort · seasonal maximum · North Temperate Lakes LTER · reproducibility