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Expanding marine predator isotope work: community-level metrics, mercury isotopes, and data-model linkages

Ecological applications of stable carbon and nitrogen isotope analysis are rapidly expanding through the analysis of metadata. Recent efforts of the IMBeR CLIOTOP Task Team on Marine Predator Isotopes have focused on finalising an open-access data paper and three research papers that present novel findings of global scale, spatial and temporal analyses on migratory tuna muscle isotopes. The task team recently met in Brest, France to discuss novel ways to expand our ecological understanding of the ocean. The main approaches included formulating community level metrics, using isotopes of mercury, and finding practical ways to link isotopic data with ecosystem and biogeochemical models.

IMBeR CLIOTOP Task Team on Marine Predator Isotopes Lydie Couturier, Chris Somes, Joan Navarro, Nathalie Bodin, Anne Lorrain and Heidi Pethybridge (left to right). Other participants in the discussions were David Point and Marta Coll.

Carbon and nitrogen isotopes are increasingly used to infer community-scale metrics, such as niche breadth and dietary overlap between consumers at the local to regional scales (e.g. Abrantes et al., 2010). Few studies have used isotope community metrics at broader (ocean basin to global) scales despite its potential strength to test macroecolocial principles and hypotheses (e.g., trophic diversity, complexity and redundancy). The task team undertook SIBER analyses to derived and compare community metrics based on tuna muscle isotopes (Figure 1). A number of challenges were identified, highlighting the need to formulate new metrics that account for mismatches between the scale(s) of interest. Future work will involve relating isotope community-scale metrics with environmental data to better project climate impacts on oceanic food webs.

Assessing the stable isotopic composition of mercury and methylmercury toxin is a fairly new, yet growing, field of research with many potential ecological and biogeochemical applications (Masbou et al., 2015). Mercury isotopes can, for example, be used to assess foraging habitat of predators (Blum et al., 2013). The CLIOTOP task team are to work with David Point, leader of the recently funded ANR project MERTOX, set up to better understand marine ecosystems and mercury cycling at the global scale. Task team members will to provide samples for future analyses of mercury and carbon isotopes embedded in the methylmercury toxin accumulated by pelagic predators in the MERTOX framework.

Models of biogeochemical and ecological processes are only as good as the empirical data used to identify important ecological processes, estimate parameters and assess model performance (Pethybridge et al., 2018). The task team discussed how isotope data could be more widely used to assist international modelling efforts, including the international inter-model comparison project (FISH-MIP). Future work will likely include comparing tuna isotope data with future developments of the global EwE model (EcoOcean) (Christensen et al. 2015), and the development of a roadmap on how to bridge models and data. Aligned with these efforts, emphasis was placed on ways the team could expand its marine predator database by including meso-predator isotope data and communicating with other marine isotope groups.

The task team thanks the participating institutions (IRD, CSIRO, ICM-CSIC, and GEOMAR) and IMBeR for sponsoring this workshop.

References:

  • Abrantes, K.G., Barnett, A., Bouillon, S., 2014. Stable isotope‐based community metrics as a tool to identify patterns in food web structure in east African estuaries. Functional ecology, 28(1), pp.270-282.
  • Blum, J.D., Popp, B.N., Drazen, J.C., Choy, C.A., Johnson, M.W., 2013. Methylmercury production below the mixed layer in the North Pacific Ocean. Nat. Geosci. 6, 879–884. Christensen, V., Coll, M.,
  • Buszowski, J., Cheung, W.W., Frölicher, T., et al., 2015. The global ocean is an ecosystem: simulating marine life and fisheries. Global Ecology and Biogeography, 24(5), pp.507-517.
  • Masbou, J., Point, D., Guillou, G., Sonke, J.E., Lebreton, B., Richard, P., 2015. Carbon Stable Isotope Analysis of Methylmercury Toxin in Biological Materials by GC Isotope Ratio Mass Spectrometry. Anal. Chem. 87, 11732–11738.
  • Pethybridge, H., Choy, C.A., Polovina, J., Fulton, E. 2018. Improving ecosystem models with biochemical tracers. Annual Reviews Marine Science, January 2018.