A trophic niche is more than diet: it’s the phenotypic route through which organisms acquire energy and redistribute nutrients. Because trophic niches are shaped by selection, resource availability and behavioural trade-offs, they determine the identity of consumed resources, trophic position, the carbon pools that are mobilised, and the nutrients returned to receiving ecosystem compartments through excretion. Thus, variation in the trophic niche of species can alter stoichiometry, microbial activity and ecosystem-level nutrient cycling. Here, Arctic tundra lakes were used as a proof of concept to show how climate-driven niche shifts can propagate from individual foraging decisions to carbon and nitrogen cycles. Sea-ice loss increased the use of goose eggs by polar bears due to reduced predation on seals, modifying the reproductive success and social structure of Barnacle geese (Branta leucopsis). In geese, brood status and dominance regulate access to high-quality grass patches versus mosses and aquatic vegetation, producing strong intraspecific dietary differences and changing nitrogen return through faeces, which was higher in grass-rich diets. Goose-derived nitrogen inputs then improved the stoichiometric quality of basal resources in lake food webs, promoting a shift in the omnivorous tadpole shrimp Lepidurus arcticus from carnivory to herbivory–detritivory. This accelerated the transfer of carbon from basal storage compartments into consumer pathways. At the ecosystem scale, nitrogen enrichment in lakes, in combination with warming, increased greenhouse-gas emissions, shifting Arctic lakes from carbon sinks to carbon sources. Together, these linked responses describe a “niche cascade” from climate to carbon: trophic decisions propagate through nutrient cycles. Optimal foraging predicted each switch observed in the case study, namely seals to eggs, grass to moss, animal prey to basal resources, suggesting that trophic niches can make nutrient cycles more predictable because they determine which carbon pools are consumed, which nutrients are returned, and how climate-driven changes propagate across food webs.