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 composition was analysed in three kelp species growing in Kongsfjorden, Svalbard. The parameters considered were depth, age, and algal anatomy: Between 15 and 2.5 m depth, towards higher photo fluence rate, Alaria esculenta was gradually enriched in δ13C while δ15N did not change. 2&3-year-old algae had significantly higher δ15N values in their blades than 5&6-year-old ones. A two factor analysis did not show any interactive effects between depth and age class. A. esculenta as well as Saccharina latissima and Laminaria digitata exhibited enrichment in heavy carbon isotopes in the blades.