Ice algae vs. phytoplankton: resource utilization by Arctic deep sea macroinfauna revealed through isotope labelling experiments

Climate change is expected to change future Arctic marine primary production (PP) by reducing ice algal and increasing phytoplankton contributions. As most benthic macrofauna depend on PP from the euphotic zone for food, they could be vulnerable to changes in their food supply. To investigate the differential utilization of ice algae and phytoplankton food by benthic macroinfauna, isotope labelling experiments on dual 13C-15N labelled ice algae and phytoplankton were carried out at 2 sites in the Canadian Arctic. After 4 d, all animals collected at North Water Polynya (NOW; 709 m) and Lancaster Sound (LS; 794 m) exhibited isotope labelling. The C:N ratio of the macrofaunal biomass-specific uptake showed that all taxa were N-limited, and the uptake of algal C and N was often decoupled. Overall, the 2 macroinfaunal communities had different responses to the food items: in LS the accumulative biomass-specific uptake of phytoplankton C and N of all fauna was higher than uptake of ice algae, whereas in NOW ice algal C was more readily utilized. When taxa were examined individually, differences in food utilization by polychaetes, bivalves and crustaceans were site-specific, with no taxa exclusively exhibiting higher rates of ice algal uptake. The dietary plasticity observed between these sites suggests that benthic macroinfauna are able to efficiently utilize both ice algae and phytoplankton as a food source, and that the replacement of ice algae with phytoplankton food may not alter faunal feeding rates or their role in benthic nutrient cycling.