We analyzed stable isotopes of carbon and nitrogen to investigate the trophic structure
of the subtidal food web around the Mingan Islands, northern Gulf of St. Lawrence, eastern
Canada. All benthic consumers were enriched in 13C (mean δ13C of
–17.1‰) compared to particulate organic matter (POM: –23.3‰). Nitrogen stable isotope
ratios ranged from 6‰ to 14‰ and the organisms studied fell into three distinct trophic
groups, primary producers, primary consumers (herbivores), and predators. The sea star
Crossaster papposus and the sculpin Myoxocephalus
scorpius, known to be top level predators, had slightly higher δ15N
than other predators. Although the average isotope signature of the sea star
Solaster endeca placed it among regular predators, the δ15N
increased with sea star size and large individuals could be considered as top predators.
The relatively small number of organisms located at intermediate trophic levels suggests a
low level of omnivory in the Mingan Islands’ system, which contrasts with previously
described benthic systems that exhibit a continuum between herbivores and predators. Low
omnivory, in addition to low diversity, suggests that this ecosystem may be relatively
unstable if exposed to natural and/or anthropogenic disturbances such as exploitation and
climate change.