Abstract

We used stables isotope analysis to investigate the sources of primary productivity (phytoplankton, kelp and benthic microalgae) and the trophic structure of benthic food webs in the region of the Mingan Islands, northern Gulf of St. Lawrence. Benthic consumers were rich in ¹³C seemingly indicating a strong dependence on ¹³C-rich benthic primary producers (kelp and benthic microalgae) as opposed to ¹³C-depleted phytoplankton as a food source. However, suspension and deposit feeders taken along a depth gradient, and individuals suspended in cages away from shore, were even more enriched in ¹³C than individuals on the bottom nearshore, suggesting that inputs from nearshore benthic primary producers are not the cause of the enriched carbon ratio in these organisms. A review of previously reported isotopic ratios from arctic and temperate regions showed that deposit and suspension feeders are almost always enriched compared to the ¹³C-depleted ratio of phytoplankton, regardless of their distance from ¹³C-rich primary producers (measured as depth). Factors other than the feeding on ¹³C-rich benthic primary producers must explain the carbon ratios in benthic consumers (e.g. possibly strong selective feeding on enriched particles). Finally, three distinct trophic groups characterize the benthic communities in the northern Gulf of St. Lawrence, primary producers, primary consumers (herbivores) and predators, with only a few organisms falling outside of these groups. The sea star Crossaster papposus and the sculpin Myoxocephalus scorpius, known to be top level predators, were positioned at a slightly higher level than other predators. The relatively small number of organisms located at intermediates trophic levels indicates a low level of omnivory in this system.

Alternate abstract:

We used stable isotope analysis to investigate the sources of primary productivity (phytoplankton, kelp and benthic microalgae) and the trophic structure of benthic food webs in the region of the Mingan Islands, northern Gulf of St. Lawrence. Benthic consumers were rich in ¹³C seemingly indicating a strong dependence on ¹³C-rich benthic primary producers (kelp and benthic microalgae) as opposed to ¹³C-depleted phytoplankton as a food source. However, suspension and deposit feeders taken along a depth gradient, and individuals suspended in cages away from shore, were even more enriched in ¹³C than individuals on the bottom nearshore, suggesting that inputs from nearshore benthic primary producers are not the cause of the enriched carbon ratio in these organisms. A review of previously reported isotopic ratios from arctic and temperate regions showed that deposit and suspension feeders are almost always enriched compared to the ¹³C-depleted ratio of phytoplankton, regardless of their distance from ¹³C-rich primary producers (measured as depth). Factors other than the feeding on ¹³C-rich benthic primary producers must explain the carbon ratios in benthic consumers (e.g. possibly strong selective feeding on enriched particles). Finally, three distinct trophic groups characterize the benthic communities in the northern Gulf of St. Lawrence, primary producers, primary consumers (herbivores) and predators, with only a few organisms falling outside of these groups. The sea star Crossaster papposus and the sculpin Myoxocephalus scorpius, known to be top level predators, were positioned at a slightly higher level than other predators. The relatively small number of organisms located at intermediate trophic levels indicates a low level of omnivory in this system.