Trophic studies in a high-latitude fjord ecosystem — a comparison of stable isotope analyses (δ¹³C and δ¹⁵N) and trophic-level estimates from a mass-balance model

Abstract

Stable isotopes of δ 13 C and δ 15 N were used to examine food sources and trophic structure of 65 taxa, representing 19 ecological groups, in a high-latitude ecosystem. Discrimination was made between pelagic and benthic carbon sources, where feeding in most cases reflected the habitat. Trophic levels from these analyses, TL N , were compared with corresponding levels estimated by an Ecopath mass-balance model, TL E , constructed independently of the isotope data. The good correlation between the two methods (r 2 = 0.72) supports the diet composition and the grouping of taxa into ecological groups in the model. However, when estimates diverged, this was often explained by the analyses of few taxa, taxa that were not the most representative for the group, or the analyses of specimens from a limited size range. Some assumed detrivores were assigned high TL N in favour of an abundant microbial community in the sediments. High TL N estimates for many invertebrate taxa, combined with relatively low TL N for fishes, suggest that parts of the benthic food web are decoupled from the classical food web.