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• Taxonomic resources of the pristine rivers in the Biebrza National Park. • Five groups of organisms examined: benthic diatoms, phytoplankton, macrophytes, macrozoobenthos, and ichthyofauna. • Documented pool of taxa and estimated the predicted richness. • Valuable biodiversity resources, including protected plant and animal species. • High alpha diversity (within-site diversity) but relatively low gamma diversity (total diversity across all sites). The study focuses on the pristine rivers of the Biebrza National Park (BNP), an extensive area of wetlands, which holds significant biodiversity importance on a European scale. The aim of this work was to identify the taxonomic resources of the river network of this area and to compare the biodiversity of various aquatic organisms, and moreover, evaluate the field sampling effort required to identify the whole pool of taxa inhabiting the survey area. Fieldwork was carried out in 2021 at 25 survey sites, where the taxonomic compositions of five groups of organisms were identified: benthic diatoms, phytoplankton, macrophytes, benthic macroinvertebrates, and ichthyofauna and identified taxa have been presented in the appendix. The biodiversity indices of these groups were calculated, i.e. the species richness, and the Shannon and Simpson indices. The entire predicted biodiversity of these groups of organisms was also estimated. The Chao2 asymptotic taxonomic richness estimator for incidence data was used to calculate the additional number of survey sites that would be required to discover 80%, 90% and 100% of the anticipated taxonomic richness in each of the five groups of organisms. With the information on the species inhabiting the Biebrza Valley that this survey yielded, the biodiversity at a single site (alpha diversity) and within the entire area surveyed (gamma diversity) was estimated. The phytoplankton (475 taxa) and the benthic macroinvertebrates (308) showed the greatest diversity in taxonomic richness. In addition, there were 151 species in the benthic diatom group, 72 macrophyte taxa, and 24 fish species. The analysis showed, moreover, that if the fieldwork were continued, another 110 phytoplankton and 104 benthic macroinvertebrates taxa would be recorded for BNP, along with 40 further species of benthic diatoms, 9 more macrophyte species, and 2 more fish species. Alpha diversity was found to be high but gamma diversity was relatively low due to the homogeneous nature of the rivers surveyed (low beta diversity), effective conservation measures, and minimal human disturbances. [ABSTRACT FROM AUTHOR]