August Daphnia share had no clear link to a cool, oxygenated depth index in seven Wisconsin lakes, 2002–2025

We tested whether August Daphnia density share was higher when more measured depth was cool and oxygenated. We combined open zooplankton and physical records for seven fixed northern Wisconsin lakes in 2002–2025. The main index was the fraction of a fixed depth band with temperature at most 20 degrees C and oxygen at least 3 mg/L. It is a depth index, not water volume or measured animal habitat. The panel had 176 matched dates and 164 lake-years. A linear model with lake and calendar-year terms gave a share increase of 0.42 percentage points per 0.10 increase in the index. Its nominal 95% three-year block range was -1.01 to +1.27 points. The result did not establish a common positive link or no effect. Same-day matches, other thresholds, lake trends and annual differences also gave ranges that included zero. Omitting Crystal Bog gave -1.52 points, with a nominal range of -2.95 to -0.50. That lake supplied 56% of squared predictor residuals. A stricter two-date rule did not identify the slope. Most lake-years had one August visit. Other limits include one region, unmeasured predators, depth versus volume weighting, and a threshold index that can miss low-oxygen refuges. This adds a matched environmental test to prior SCI work on Daphnia shares. The package contains unchanged open inputs, source checks, code, all results and draws, figures, and independent arithmetic and clean-repeat checks.

Daphnia · lake oxygen · zooplankton · temperature · long-term monitoring · reproducibility