Abstract
<title>Abstract</title> <p>Environmental monitoring programs increasingly require tools that compare biological change across multiple assemblages without treating all taxa as interchangeable. We applied a multi-taxon Community Dynamics Index (CDI) to national Korean stream biomonitoring records from 2011 to 2023. A strict tri-taxon core of 13,386 paired site-years was constructed in which epilithic diatoms, benthic macroinvertebrates, and fish were recorded at the same site, in the same year, and in both survey rounds. For each assemblage, Hellinger-CDI measured community displacement between survey rounds using Hellinger-transformed relative-abundance vectors. Mean CDI was highest for diatoms (0.988), intermediate for macroinvertebrates (0.903), and lowest for fish (0.636). Pairwise cross-assemblage synchrony was near zero, and split-half reliability correction and a structural random-pairing null model indicated that this decoupling was not explained by sampling noise, taxon richness, dominance, or the structural ceiling of Hellinger distance alone. Response classes showed that mixed/intermediate and single-assemblage pathways were more common than whole-ecosystem high-CDI reorganization. The framework identifies which biological compartment—biofilm surface, benthic habitat, or fish connectivity—expressed the strongest survey-round displacement. Multi-taxon CDI provides an assessment layer for stream biomonitoring, but should be interpreted as rainfall-linked community displacement rather than direct rainfall causality until paired environmental-driver matrices are integrated.</p>