Spatial models of carbon, nitrogen and sulphur stable isotope distributions (isoscapes) across a shelf sea: An INLA approach

Abstract

Spatial models of variation in the isotopic composition of structural nutrients across habitats (isoscapes) offer information on physical, biogeochemical and anthropogenic processes occurring across space, and provide a tool for retrospective assignment of animals or animal products to their foraging area or geographic origin. The isotopic differences among reference samples used to construct isoscapes may vary spatially and according to non‐spatial terms (e.g. sampling date, or among individual or species effects). Partitioning variance between spatially dependent and spatially independent terms is a critical but overlooked aspect of isoscape creation with important consequences for the design of studies collecting reference data for isoscape creation and the accuracy and precision of isoscape models. We introduce the use of integrated nested Laplace approximation (INLA) to construct isoscape models. Integrated nested Laplace approximation provides a computationally efficient framework to construct spatial models of isotopic variability explicitly addressing additional variation introduced by including multiple reference species (or other recognized sources of variance). We present carbon, nitrogen and sulphur isoscape models extending over c. 1 million km2 of the UK shelf seas. Models were built using seven different species of jellyfish as spatial reference data and a suite of environmental correlates. Compared to alternative isoscape prediction methods, INLA‐spatial isotope models show high spatial precision and reduced variance. We briefly discuss the likely biogeochemical explanations for the observed spatial isotope distributions. We show for the first time that sulphur isotopes display systematic spatial variation across open marine shelf seas and may therefore be a useful additional tool for marine spatial ecology. The INLA technique provides a promising tool for generating isoscape models and associated uncertainty surfaces where reference data are accompanied by multiple, quantifiable sources of uncertainty.

Tracking long‐distance migration of marine fishes using compound‐specific stable isotope analysis of amino acids

“…Our results demonstrate that an isotope tracking method using copepod‐based δ 15 N Base isoscapes coupled with retrospective isotope analysis of vertebral sections has the potential to provide new insights into the migration routes of highly migratory marine fishes. So far, several studies have published marine isoscapes of light elements such as δ 13 C, δ 15 N, and δ 34 S (McMahon et al 2013; St. John Glew et al, 2019). However, bulk tissue analysis of these elements is generally unsuitable for migration tracking.…”

Habitat and resource segregation of two sympatric seals in the North Sea

“…This interaction highlights that the nutritional intake of Se by herbivores may differ considerably, depending on their geographical area and so their spatial distribution (Schomburg and Arnér 2017). δ 13 C and δ 15 N values are lower in offshore areas compared to coastal areas in the North Sea (MacKenzie et al 2014;Glew et al 2019).…”

“…In Scotland, grey seals were sampled on the Isle of May (56°11ʼN, 2°33ʼW) in October -November 2016 and harbour seals were sampled on Orkney (59°05'N, 2°97'W) in April -May in 2017 (Map 1). Both sites presented similar environmental characteristics such as sea surface and bottom temperatures and similar isotopic baseline values (MacKenzie et al 2014;Glew et al 2019). All Scottish seals were adults.…”

“…Total volatile selenide concentrations are linearly dependent on the primary productivity (Amouroux and Donard 1996). We hypothesise that natural Se maritime emissions decrease from inshore to offshore waters in the North Sea as primary productivity decrease in offshore and deeper waters (Richardson and Pedersen 1998;Škaloud et al 2006;MacKenzie et al 2014;Glew et al 2019). Thus, Se biotransformation by phytoplankton and therefore Se emissions will be less important in offshore areas compared to inshore areas.…”

“…δ 15 N values show a sequential and predictable enrichment from one trophic level to the next, and is a good indicator of the trophic position of the animal (Minagawa and Wada 1984;Newsome et al 2010). Moreover, the δ 13 C and δ 15 N values may give an indication on the foraging distributions of seals as their values decreased from coastal and open sea areas in German and Scottish parts of the North Sea (MacKenzie et al 2014;Glew et al 2019). In addition, isotopic ratios allow identification of the food resources on which a predator depends with the use of mixing models (SIAR).…”