Autumn Daphnia density fell relative to May–June density in sampled Lake Mendota years, 2010–2024 versus 1995–2008

We tested whether a reported autumn Daphnia loss continued in Lake Mendota, Wisconsin. We used open NTL-LTER data and required complete May–June and October–November samples in each year. In ten complete years from 1995–2008 and eleven from 2010–2024, mean autumn density fell from 97,124 to 12,954 individuals/m². Mean May–June density changed from 148,200 to 139,056 individuals/m². The ratio of arithmetic seasonal means fell to 0.142 of its earlier value (95% three-year block interval 0.040–0.421). A log-ratio measure also showed a loss. Fixed-date, tow-depth and zero-year checks retained the within-lake pattern. In nine before years and five after years with complete samples from both Mendota and Monona, the change in the Mendota geometric seasonal ratio divided by the Monona change was 0.101 (0.017–1.285). This lake difference remains uncertain. The study extends earlier reports of autumn loss; it does not discover that loss or identify its cause. Sampling times changed, years are missing, and the lakes share water and organisms. The package contains the frozen source data, code, coverage records, figures and independent arithmetic checks.

Daphnia · Lake Mendota · Lake Monona · seasonal change · zooplankton · long-term monitoring · reproducibility