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

Abstract

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.

Feather‐based measures of stable isotopes and corticosterone reveal a relationship between trophic position and physiology in a pelagic seabird over a 153‐year period

“…Rather than being a recent event, our long‐term data show a steady enrichment in 13 C, indicating a gradual shift from the late 1800s to the present, whereas background variation in both δ 13 C and δ 15 N in Atlantic marine foodwebs has remained relatively stable over time (MacKenzie et al . ). Although relatively small, and potentially influenced by anomalous data from the 1960s to 1980s (see below), the temporal shift in δ 13 C values was opposite to the Suess effect, suggesting that the shift was stronger than appears at first glance.…”

Ecogeochemistry potential in deep time biodiversity illustrated using a modern deep-water case study

“…Isoscapes based on the isotopic composition of carbon in vegetation can be effective over wide bioclimatic provinces, particularly across gradients of C3-and C4-dominated vegetation [35]. In marine systems, the carbon and nitrogen isotopic composition of phytoplankton (and thus zooplankton) varies with temperature, phytoplankton taxonomy and nutrient source, providing predictable isotopic gradients across ocean basins [36] and in restricted shelf seas [37,38] which can and have been used to infer movements in a variety of marine organisms [4,39]. Application of modern isoscapes to infer movements in deep time studies depends on temporal stability of the relationship between predictor variables and isotopic values over time, and the quality of available predictor variable information.…”

Moult location and diet of auks in the North Sea inferred from coupled light-based and isotopebased geolocation

“…To date, the limited number of detailed and spatially explicit, regional and in situ sample-based marine isoscapes (Jaeger et al 2010, MacKenzie et al 2014, Radabaugh & Peebles 2014, Vander Zanden et al 2015 has limited the application of these techniques in the marine environment. Geolocation using δ 13 C and δ 15 N ratios of animal tissues is further complicated as the isotopic composition of a consumer's diet reflects both the spatial origin of the prey items and their trophic level.…”

“…We draw on light-based geolocator data from breeding individuals for which we also had stable isotope data for flight and body feathers. We interpreted isotope data with reference to recently developed δ 13 C and δ 15 N iso scapes for the North Sea (MacKenzie et al 2014, True man et al 2017. Our primary aims were to de termine population-level foraging locations and quantify individual diet variability during annual moult by combining 2 complementary geolocation techniques.…”

“…N values in consumers within the North Sea are relatively stable over time (MacKenzie et al 2014, Trueman et al 2017. Seasonal variations in isotopic compositions of primary producers and low trophic level consumers are probable (Kürten et al 2013) but are likely attenuated during successive predator− prey interactions so that higher trophic level consumers (such as the jellyfish used to construct the isoscape and seabirds) inherit isotopic compositions that reflect a seasonal average value.…”