A critical assessment of marine predator isoscapes within the southern Indian Ocean

Carpenter-Kling et al. Movement Ecology https://doi.org/10.1186/s40462-020-00208-8 (2020) 8:29 RESEARCH Open Access A critical assessment of marine predator isoscapes within the southern Indian Ocean Tegan Carpenter-Kling1,2* , Pierre Pistorius1,2, Ryan Reisinger1,3, Yves Cherel3 and Maëlle Connan1 Abstract Background: Precise and accurate retrospective geolocation of marine predators via their tissues’ isotopic composition relies on quality reference maps of relevant isotopic gradients (“isoscapes”). Additionally, a good working knowledge of any discrimination factors that may offset a marine predator’s isotopic composition from baseline isotopic values, as well as tissue specific retention rates, are imperative. We provide a critical assessment of inter-specific differences among marine predatorlevel isoscapes within the Indian Sector of the Southern Ocean. Methods: We combined fine-scale GPS tracking data and concurrent blood plasma δ13C and δ15N values of eight seabird species (three albatross, two giant petrel and three penguin species) breeding at Marion Island to produce species- and guild-specific isoscapes. Results: Overall, our study revealed latitudinal spatial gradients in both δ13C and δ15N for far-ranging seabirds (albatrosses and giant petrels) as well as inshore-offshore gradients for near-ranging seabirds (penguins). However, at the species level, latitudinal spatial gradients were not reflected in the δ13C and δ15N isoscapes of two and three, respectively, of the five farranging species studied. It is therefore important when possible to estimate and apply species-specific isoscapes or have a good understanding of any factors and pathways affecting marine predators’ isotopic composition when estimating the foraging distribution of marine predators via their tissues’ stable isotope compositions. Conclusions: Using a multi-species approach, we provide evidence of large and regional scale systematic spatial variability of δ13C and δ15N at the base of the marine food web that propagates through trophic levels and is reflected in the isotopic composition of top predators’ tissues. Keywords: Geolocation, Stable isotope ecology, Southern Ocean, Seabirds, Procellariiformes, Penguins Background Some of the greatest threats faced by land-breeding marine predators are experienced at sea. These include bycatch-risk and changes in food availability as a result of competition with fisheries and climate change [1–3]. Therefore, to implement effective conservation-based marine spatial planning there is a growing need to better * Correspondence: 1 Marine Apex Predator Research Unit (MAPRU), Department of Zoology, Institute for Coastal and Marine Research, Nelson Mandela University, Port Elizabeth, South Africa 2 DST-NRF Centre of Excellence at the FitzPatrick Institute of African Ornithology, Nelson Mandela University, Port Elizabeth, South Africa Full list of author information is available at the end of the article understand the at-sea distribution of marine predators [4–6]. This has led to an impressive growth in the number of tracking studies in recent years (reviewed in [7]), often with the general aim of providing policy-relevant information on important habitat for the respective study species [8]. However, dataloggers are still cumbersome for small species (e.g. some burrowing seabird species) and deployment of loggers on study animals requires significant amounts of time in the field, especially when instruments need to be retrieved. Stable isotope ecology as a tool for retrospective geolocation of predator foraging grounds has relatively recently emerged as an alternative and complimentary method to conventional tracking studies [9, 10]. Stable isotope analysis of body © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Carpenter-Kling et al. Movement Ecology (2020) 8:29 tissues is relatively cheap, less demanding in terms of field time and as a result allows for easy sampling of a greater number of individuals than would often be incorporated in tracking studies (e.g. [11]). The precision and accuracy of retrospective geolocation of marine predators based on their isotopic composition is primarily reliant on two important factors. Firstly, the reliance on the availability of reference maps of the relevant isotopic gradients, known as “isoscapes” [12]. In marine systems, marine predator movement is commonly inferred by linking the ratios of the stable isotopes of carbon (13C/12C; δ13C), and to a lesser extent nitrogen (15N/14N; δ15N), of their tissues to known gradients of δ13C and δ15N values present at the base of their food webs (e.g. [13–15]). Across the global oceans, there is a strong negative latitudinal gradient in the δ13C values of phytoplankton, from the equator towards the poles [16], as well as from inshore benthic habitats to offshore pelagic habitats [17–19]. Whereas gradients of δ15N are not as strong or predictable, the δ15N values of phytoplankton tend to be lower or higher in areas of nitrogen fixation (e.g. pelagic oceans) or denitrification (e.g. upwelling regions around coastlines), respectively [20, 21]. Secondly, a good working knowledge of potential discrimination factors which may offset a consumer’s isotopic composition from baseline isotopic values is required. These discrimination factors may vary with diet composition, isotopic averaging as well as physiological fractionation through intermediate trophic levels, isotopic turnover rates and physiological transformation in the consumer [9, 12]. This includes tissue-specific retention times, as isotopic turnover of different tissues varies greatly [22]. Previously, studies which estimated oceanic δ13C and 15 δ N isoscapes have largely used organisms close to the base of the food web (e.g. [16, 23, 24]) or particulate organic matter [25–27]. The isotope ratios of organisms near the base of the food web (e.g. phytoplankton) or particulate organic matter are (...truncated)