<p>In the Arctic tundra, ecological processes are regulated by relatively few species and are thus vulnerable to the changes in resource-consumer interactions expected as a consequence of climate change. In Arctic lake ecosystems, Barnacle geese play a key role in regulating vegetation and nutrient cycling, and are experiencing a demographic increase due to climate warming and increased vegetation productivity. We used remote sensing and C and N stable isotope and elemental analyses in goose faeces to determine whether and how terrestrial primary productivity influences resource use partitioning among individuals, and how this, in turn, affects (i) N concentration in droppings produced by adults and juveniles feeding on different food sources, (ii) the consumption of submerged vegetation by geese, and (iii) the impact of geese on vegetation around lakes. The contribution of grass to the diet of geese was directly related to its productivity around lakes at the beginning of summer. Resource use differed between juveniles, which were highly specialised on grass, and adults, whose diet was more variable and included terrestrial and submerged mosses. In grass-depleted diets, submerged moss was the main food source of geese. Greater consumption of grass resulted in higher concentrations of N in goose faeces, as well as more pronounced reduction of terrestrial vegetation around lakes during summer, as measured through remote sensing. Dietary differences among and between adults and juveniles helped explain mechanisms of intraspecific niche partitioning in geese and produced marked differences in the ecological role of individuals in Arctic lake ecosystems.</p>