The sampled clarity peak moved later relative to the Daphnia count peak in Mendota than in Monona in matched 1996–2018 surveys

We tested a change in spring clarity timing relative to Daphnia timing at Lakes Mendota and Monona, Wisconsin. We used visits with both measurements on the same day in each lake. We matched visits between lakes within seven days and required spring coverage. The main test has 11 earlier years in 1996–2008 and seven later years in 2010–2018. The later-minus-earlier change in clarity date minus Daphnia count peak date, in Mendota minus Monona, was +18.5 days. Its nominal 95% three-calendar-year block interval was +8.4 to +29.2 days. Size-weighted Daphnia peaks on the same years gave +6.8 days, with an interval from −13.2 to +27.2 days. Mean dates from whole sampled curves gave −0.3 days, with an interval from −3.5 to +2.9 days. Size weights changed 13 of 36 lake-year peak dates. The paired difference between the count and size-weighted change estimates was itself uncertain. Using all same-day visits without a match between lakes also gave an uncertain change. The result supports a change in selected sampled count maxima. It does not establish a general change in grazer timing or a causal effect of an invasive species. This follow-up adds a matched four-series dataset to prior clarity and Daphnia studies. The package has unchanged source data, provenance, code, all selections, results, figures, independent checks, and exact repeat instructions.

Daphnia · water clarity · seasonal timing · Secchi depth · Lake Mendota · Lake Monona · matched surveys · body size · sampling · reproducibility