Low Daphnia years had 2.37 percentage points of excess overlap under a local frequency model in seven Wisconsin lakes, 2002–2025

We tested whether low summer Daphnia density occurred in the same years across seven northern Wisconsin lakes. We used open NTL-LTER records from 2002–2025. The panel has 159 complete June–August lake-years from 534 samples. We defined low years by local density ranks within fixed six-year blocks. We compared 21 lake pairs with a reference that preserves each lake and block low-year frequency and its missing years. Mean joint frequency was 11.48%, against a reference mean of 9.11%. The excess was 2.37 percentage points. A planned one-sided test with 19,999 permutations gave p = 0.02695. The central 95% null range was −1.86 to +2.43 points; it is not a confidence interval. Rotations gave a similar result. Evidence weakened with one sample per month, and the excess fell to 0.90 points for July–August alone (p = 0.19765). A high-density check also showed excess overlap. Thus, the main result gives modest evidence for alignment under one stated season and reference model. It does not establish a shared absolute food shortage, a cause, or a trend in synchrony. The package preserves raw source data, metadata, code, all reference draws, figures, independent arithmetic checks, and offline repeat instructions.

Daphnia · zooplankton · lakes · spatial synchrony · low density · randomization test · missing data · reproducibility