Carbon source and trophic structure along a depth gradient in Isfjorden, Svalbard

Abstract

Stable isotope analysis has been used to examine marine food webs since the 1980s and has become a valuable tool for studying carbon sources and trophic structures in benthic food chains in the Arctic. Prior to the present study, no one has used stabile isotope analysis to test for a difference in the main carbon source or trophic structure along a depth gradient in the Arctic. Carbon sources (pelagic POM, sediment POM and macroalgae) and consumers (benthic filter feeders, deposit feeders, grazers, scavengers and predators and grazing zooplankton) were collected from Isfjorden, Svalbard, at depths ranging from 0 – 400 m. There was a big overlap in both δ13C and δ15N for most of the carbon sources sampled, making it difficult to identify one single food source to any of the depths. Most benthic primary consumers (filter-feeding bivalves and grazers) were enriched in δ13C relative to pelagic POM, sediment POM and the brown algae Chorda filum at all depths. Most primary consumers fell in between the fractionation rates of the two groups of brown algae (filter-feeding bivalves), or were enriched in δ13C relative to the isotopically lightest carbon source sampled in this study (grazers). This suggests that a) a mixture of multiple carbon sources constitute the diets of most primary consumers, and possibly the entire benthic food web, and b) the benthic primary consumers utilize one or more carbon sources not sampled in this study.The δ15N among primary consumers varied somewhat between stations, but this was not reflected higher up in the food chain. The biggest difference in δ15N was found for the sea urchin Strongylocentrotus sp., which is likely caused by different feeding strategies among specimens inhabiting shallow and deep waters.