Abstract: Kelp-derived detritus is a potential food item at the base level of marine food webs. A good knowledge of the factors that influence stable isotope composition in kelps is essential for reliable food web models. The variation of stable isotope signatures was analysed in three species of kelp in Kongsfjorden, Svalbard. Between 15 and 2.5 m depth, towards higher photon fluence rate, Alaria esculenta was gradually enriched in ¹³C while ¹⁵N did not change. The carbon/nitrogen ratio decreased with depth. 2- and 3-year-old kelp individuals had significantly higher δ¹⁵N values in their blades than 5- and 6-year-old ones. C/N ratio in blades remained statistically the same over the range of 2–6 years old plants. A two-way ANOVA did not show any interactive effects between depth and age group. A. esculenta as well as Saccharina latissima and Laminaria digitata exhibited enrichment in heavy carbon isotopes in the blades compared to the stipes, while within each species, δ¹⁵N values remained the same between blades and stipes. A more detailed analysis of kelp blades showed that independent of location in the fjord, year or season, and species, young tissue was more enriched in ¹³C than meristem and that decaying apical tissue was depleted relative to young tissue. The degree of this ¹³C depletion varied with kelp species. Meristem tissue appeared most uniform in the blades compared to young and decaying tissues and its δ¹³C signature was characteristic for each species with no significant difference in δ¹⁵N values. Decaying tissue showed no difference in δ¹³C between species. Carbon and nitrogen contents along the blades were species-specific and varied with growing location and year or season. Some advice is given on the temporal, spatial, and tissue-related choice of samples for different research objectives.