Arctic marine ecosystems are biodiversity hotspots essential for commercial exploitation and ecosystem management. These typically nutrient-limited environments, experience seasonal fluctuations in resource availability driven by variation in light, sea-ice cover, and terrestrial runoff. Climate change alters temperature and precipitation patterns, resulting in increased terrestrial inputs and reduced sea-ice extent, which effect on timing and quantity of resource inputs into marine food webs. However, the impact of these changes on resource partitioning among species, and their implication for species coexistence and productivity, remain understood.

As an initial step toward addressing this gap, this study investigates how seasonal variations in terrestrial inputs and phytoplankton influence isotopic niche structure and niche partitioning of dominant zooplanktonic and benthic species in Kongsfjorden, Svalbard Islands. Sampling was conducted in early summer, when resource levels are low, and late summer, during peak resource availability. Individual stable isotope analysis was used to characterise species' isotopic niches and overlap.

Results reveal significant niche partitioning of both within and between zooplanktonic and benthic species. Benthic species maintained stable carbon isotopic signature throughout the summer, reflecting benthic and sympagic inputs. Conversely, zooplanktonic species shifted to more negative carbon isotopic values in late summer, reflecting increased assimilation of phytoplankton-derived carbon, while a mix of pelagic and sympagic sources appeared to support early summer diets. As predicted by optimal foraging theory under resource-abundant conditions, pelagic species reduced isotopic niches breadths and greater overlap in late summer. Our findings suggest a limited direct contribution of terrestrial runoff to marine consumers in both time periods.

This study presents the first seasonal, individual-level isotopic profiles of key species in Kongsfjorden, providing valuable insights into how climate-driven shifts in resource inputs may influence niche partitioning of species. These dynamics have important implications for nutrient transfer within the Arctic food web and the ecosystem services it sustains.