Standardizing freshwater macroinvertebrate taxonomy: a Lithuanian operational taxa list for ecological quality assessments and biodiversity research.

Taxonomic discrepancies pose a significant barrier to national data synthesis and accurate biomonitoring reporting. We compiled a standardized operational taxa list of freshwater macroinvertebrates known to occur in Lithuania, with the goal of improving comparability and ecological quality assessment reliability of biomonitoring data. A national list of freshwater macroinvertebrates was created by synthesizing literature and biomonitoring data. We standardized the synthesized taxa list according to contemporary nomenclature and integrated ecological and genetic information for each species. Using a rule-based approach, we assigned operational taxonomic units to each validated taxon according to the taxonomic level required within each of the Lithuanian river macroinvertebrate index's constituent metrics. We tested the impact of operational taxonomic unit assignment on the calculation of ecological quality using national biomonitoring data from 401 sites (931 samples) collected between 2014 and 2022. Three out of four tested metrics were practically indistinguishable from the originally reported ecological status values, with the final ecological status classes matching in 91.4% of samples. For the fourth metric, which encompasses the proportional contributions of Malacostraca (formerly Crustacea), we found taxonomic inconsistencies in originally reported results, with recalculated values for this metric being consistently lower. Our findings indicate that Lithuania's current ecological assessment method is not adversely sensitive to changes in taxonomic resolution, even when standardization led to a 61.3% reduction of taxonomic richness across all groups of macroinvertebrates. The present study is a step toward more reliable and consistent biomonitoring in Lithuania, especially with the anticipated uptake of DNA-based monitoring methods.