Zooplankton spatial patterns along the Ob River continuum: Integrating hydrology, chemistry, and landscape controls.

Large Arctic rivers transport substantial amounts of freshwater, carbon (C), and nutrients from land to the Arctic Ocean and play an important role in continental-ocean biogeochemical coupling. Although these systems have been intensively studied with respect to C cycling, greenhouse-gas emissions, and hydrochemistry, the biodiversity and environmental controls of their plankton communities remain less well resolved, particularly along large latitudinal gradients sensitive to permafrost thaw and climate warming. To address this issue, we quantified zooplankton abundance, taxonomic structure, phytoplankton biomass, and nutrient concentrations during summer (July) along a >2000-km main-stem transect of the Ob River, the largest river of western Siberia, extending from the Novosibirsk reach to the southern margin of the permafrost zone. Across the full transect, we identified 118 zooplankton species, with rotifers dominating both richness and numerical abundance. Zooplankton assemblages displayed a discrete-continuous longitudinal pattern shaped jointly by basin-scale landscape heterogeneity and local physicochemical conditions. Community structure and total abundance were strongly influenced by the response traits of dominant rotifers to phytoplankton biomass, pH, and related environmental variables. A central finding is that zooplankton responses to nutrients, temperature, and trophic conditions operated with a predictable ecological lag corresponding to one to two generations (approximately 7-10 days), equivalent to a 300-500 km downstream shift during summer flow. Thus, environmental controls on zooplankton cannot be inferred from instantaneous local measurements alone, but must be interpreted in the context of water travel time and the reproductive dynamics of key taxa. Together, these results provide the first basin-scale mechanistic understanding of zooplankton dynamics in the Ob River and show that large Arctic rivers can serve as sensitive indicators of climate-driven ecological change across northern watersheds.