Microzooplanktonic tintinnids as reliable bioindicators for monitoring the Kuroshio and Oyashio currents: insights from biogeography, death rate and primacy ratio

Despite the recognized potential of numerous pelagic plankton groups as bioindicators for the Oyashio and Kuroshio Currents, precise delineation of current boundaries and elucidation of associated ecological functions remain challenging, primarily due to disordered species distributions and difficulties in assessing population viability. To address this, we combined ship-based observations with daily satellite remote sensing data to evaluate the bioindicator utility and ecological roles of tintinnids across the major pathways of the Kuroshio-Oyashio Extension (KOE). Our analysis delineated three distinct biogeographic regions: the Oyashio Current-influenced Region (OCR), the Kuroshio Current-influenced Region (KCR), and a Mixed Area Region (MAR), each characterized by a unique assemblage of core and frontline tintinnid species. By synthesizing data on tintinnid biogeography, death rate, and remote sensing data, we quantitatively defined the northern boundary of the Kuroshio Current at 37.32°N and the southern boundary of the Oyashio Current at 45.30°N. We further demonstrated that eddies drive discontinuities in species distributions. Contrary to Bergmann's Rule, the tintinnid mean body size was smaller in the colder OCR than in the warmer KCR waters. Latitudinal trends from the KCR to the OCR revealed a decrease in species redundancy alongside an increase in primacy ratios (an innovative bioindex we propose) jointly indicating lower ecosystem stability in the KCR compared to the OCR. Multivariate biota-environment analyses identified temperature and salinity as the principal limiting factors structuring tintinnid communities in the KOE. These findings confirm the reliability of tintinnids as bioindicators and advance our understanding of physical-biological coupling in this critical oceanic frontal zone. • Precisely identification of the Kuroshio-Oyashio boundary confirmed tintinnid as reliable bioindicator via biogeography and mortality traits; • Tintinnid body size contradicted Bergmann‘s Rule, being smaller in cold-eutrophic Oyashio than in warm-oligotrophic Kuroshio region; • Decreasing species redundancy and rising primacy ratios (new index) from south to north indicate lower ecosystem stability in OCR than KCR.