Body-size dependent temporal variations in nitrogen stable isotope ratios in food webs

We describe seasonal variation in nitrogen stable isotope ratios of marine animals (zooplankton, benthic infauna, benthic epifauna and fishes) in size-fractionated classes spanning 6 orders of magnitude in body mass. Variation in δ15N over the course of the seasonal production cycle was significantly greater in smaller animals and declined continuously with increasing body size. The duration and magnitude of a decrease in δ15N (as observed in the zooplankton community at the start of the production cycle) was reduced and attenuated in larger animals at higher trophic levels (higher mean δ15N), likely reflecting different rates of energy propagation in different food web pathways and the transfer of energy to animals with slower turnover times. Animals ≥16 g wet body mass integrated seasonal variation in δ15N of their prey, such that a single annual sampling of animals in size classes ≥16 g was sufficient to determine mean δ15N. For zooplankton and smaller infauna, very frequent sampling would be required to estimate mean δ15N, and we show that measurements of food web properties such as trophic position and predator-prey size ratios would be biased when based on single samples of these small animals.