Recorded Daphnia lengths within taxa offset the size decrease from species replacement in sampled Mendota and Monona springs, 1995–2024

We tested whether species replacement reduced the mean recorded spring length of Daphnia in Lakes Mendota and Monona, Wisconsin. We used open NTL-LTER data from 13 shared May–June survey years in 1995–2008 and nine in 2010–2024. We split the change in pooled mean length into two exact parts. The species-mix parts were −0.142 mm in Mendota and −0.113 mm in Monona. The parts from recorded length change within taxa were +0.405 and +0.361 mm. Nominal 95% three-calendar-year block intervals excluded zero for these four parts. The two dominant species explain most of the positive within-taxon parts. These opposite signs remained with complete length data, standard tow depth, only the two main species, and removal of 2024. Mean annual length changes were +0.211 and +0.174 mm, but their intervals included zero. The lake difference in those changes was also uncertain. Thus a shift toward a smaller species did not describe overall recorded size change in these samples. The partition is an account of survey means, not a cause of change. The lakes share water, later years are missing, and lengths came from a subset of counted animals. Changes in age, sampling or measurement can affect the result. Mean-based size indices do not measure biomass or grazing. The package has unchanged source data, provenance, all results, figures, code, and independent checks.

Daphnia · body length · species replacement · lake ecology · North Temperate Lakes · reproducible analysis · ecological change