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

Abstract

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 15 N 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 15 N (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 15 N), 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 15 N of their prey, such that a single annual sampling of animals in size classes 16 g was sufficient to determine mean 15 N. For zooplankton and smaller infauna, very frequent sampling would be required to estimate mean 15 N, 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.