Using stable isotopes to assess carbon and nitrogen turnover in the Arctic sympagic amphipod Onisimus litoralis

Food web studies based on stable C and N isotope ratios usually assume isotopic equilibrium between a consumer and its diet. In the Arctic, strong seasonality in food availability often leads to diet switching, resulting in a consumer's isotopic composition to be in flux between different food sources. Experimental work investigating the time course and dynamics of isotopic change in Arctic fauna has been lacking, although these data are crucial for accurate interpretation of food web relationships. We investigated seasonal (ice-covered spring vs. ice-free summer) and temperature (1 vs. 4 degrees C) effects on growth and stable C and N isotopic change in the common nearshore Arctic amphipod Onisimus litoralis following a diet switch and while fasting in the laboratory. In spring we found no significant temperature effect on N turnover [half-life (HL) estimates: HL-N = 20.4 at 4 degrees C, 22.4 days at 1 degrees C] and a nonsignificant trend for faster growth and C turnover at the higher temperature (HL-C = 13.9 at 4 degrees C, 18.7 days at 1 degrees C). A strong seasonal effect was found, with significantly slower growth and C and N turnover in the ice-free summer period (HL-N = 115.5 days, HL-C = 77.0 days). Contrary to previous studies, metabolic processes rather than growth accounted for most of the change in C and N isotopic composition (84-89 and 67-77%, respectively). This study provides the first isotopic change and metabolic turnover rates for an Arctic marine invertebrate and demonstrates the risk of generalizing turnover rates based on taxon, physiology, and environment. Our results highlight the importance of experimental work to determine turnover rates for species of interest.