Weak Cross-Assemblage Concordance of Survey-Round Community Dissimilarity in Korean Lotic Waters: A Matched National Biomonitoring Analysis

National biomonitoring programs increasingly survey multiple biological assemblages, but whether one biological element can serve as a surrogate for the magnitude or ranking of repeated-survey compositional displacement remains unresolved under strict site-year-round matching. We analyzed Korean National Aquatic Ecological Monitoring Program records from 2011 to 2023 using a strict core of 13,386 site-years from 3016 matched lotic sites in which epilithic diatoms, benthic macroinvertebrates, and fish were recorded in both monitoring rounds. For each assemblage separately, we calculated standard Hellinger dissimilarity between Round 1 and Round 2 relative-abundance vectors. Spearman rank concordance was −0.012 (−0.030 to 0.005) for diatom–macroinvertebrate, 0.034 (0.015 to 0.052) for diatom–fish, and −0.014 (−0.034 to 0.005) for macroinvertebrate–fish; corresponding Pearson correlations ranged from −0.010 to 0.041. Within-site-centered associations, correlations among site-level mean dissimilarities, broader pairwise matched datasets, and alternative relative-Euclidean and Bray–Curtis metrics all remained weak. Thus, under the present monitoring design, the magnitude or ranking of between-round compositional displacement in one assemblage provided little information about the corresponding displacement in another. The three assemblages were surveyed by separate specialist teams within the same nationally prescribed round-specific seasonal windows rather than by required same-day co-sampling. The result is therefore interpreted at the operational survey-round scale; temporally linked environmental covariates and the replicate information needed to quantify observation error were not analyzed.