Paired summer surveys did not show a clear change in the Daphnia length difference between Mendota and Monona, 2001–2008 to 2011–2018

We tested whether recorded summer Daphnia length changed in Lake Mendota relative to Lake Monona, Wisconsin, USA. We used fixed open NTL-LTER data and required valid July, August and September records in both lakes. Six earlier years in 2001–2008 and four later years in 2011–2018 met the rules. They had 107 valid visits. Density-weighted visit lengths were averaged with equal month and year weights. Mendota mean length changed from 1.339 to 1.574 mm. Monona changed from 1.261 to 1.388 mm. The change in Mendota minus Monona was +0.108 mm. Its central 95% three-year calendar block resampling range was −0.281 to +0.466 mm. All main checks gave paired ranges that included zero. Thus the selected surveys did not establish a relative length change. They did not establish constant size. A pooled size decomposition had a +0.188 mm paired change, with −0.058 mm from taxon proportions and +0.247 mm from length within categories. Only 4507 of 10000 resamples had defined components, so we report no component intervals or mechanism claim. Monona has no source observations in 2019–2023. The main sample excludes zero catches and poorly measured visits. The lakes are connected and do not provide an experimental invasion test. This result does not test the cause of the known biomass loss. The package has unchanged source files, provenance, code, all results, figures and independent arithmetic checks.

Daphnia · Lake Mendota · Lake Monona · body length · zooplankton · paired surveys · missing data · size decomposition · null result