Testing the long-term stability of marine isoscapes in shelf seas using jellyfish tissues

Maps of the spatial distribution of stable isotope ratios across wide geographic areas (isoscapes) are increasingly used to study mechanisms of nutrient flux, movements of animals, and to improve trophic information derived from stable isotope analyses. Isoscapes are usually constructed from reference samples collected from known geographic positions, a time consuming and costly process. In this study, we test the temporal stability of isoscapes of carbon and nitrogen isotopes across the North Sea over a ten-year period. Using jellyfish tissues as reference organisms, we show that hydrodynamic and biogeochemical processes controlling the distribution of carbon and nitrogen isotope values, and thus that the underlying isoscapes, are temporally stable. Remarkably, broad geographic variations in stable carbon and nitrogen isotope distributions across the North Sea are consistent with previously published variations seen in archaeological cod bones from the ninth to seventeenth centuries, despite dramatic changes in land use in the past 1,000 years. Stable isotope-based studies of trophic interactions or movements in animals with lifespans of a year or more in shelf ecosystems can consequently be referenced to previously published isoscape models, and do not require construction of temporally explicit isotope baseline corrections. Scyphomedusan jellyfish are excellent reference organisms for ecological stable isotope analyses in pelagic ecosystems, due to their widespread distributions, well-defined life histories, and fast tissue growth.