"PRIMA Learning" Stable Isotope Workshop in Cape Town in February 2023
The analysis of stable isotopes in an organism’s tissues provides information about its diet and trophic position and has become a useful tool for researching trophic ecology. Stable isotopes of carbon and nitrogen are most-commonly analysed and provide information about the primary production at the source of an organism’s trophic pathway and the relative trophic level of that organism within that pathway, respectively. Because stable isotope analysis (SIA) is now widely applied, an international workshop to provide post-graduate students and early researchers with an introduction to using this technique for the analysis of trophic interactions in the marine environment was held in Cape Town on the 20th-23rd of February 2023.
The stable isotope workshop was organised as part of the PRIMA Learning collaborative project between the University of Bergen (UiB), Norway and the University of the Western Cape (UWC), South Africa. The project collaborates with the EAF-Nansen Programme, which is funded by the Norwegian agency for development cooperation (Norad) and implemented by the Food and Agriculture Organization of the United Nations (FAO) in close collaboration with the Institute of Marine Research (IMR), Norway.
The workshop was co-organised by Carl van der Lingen (Department of Forestry, Fisheries and the Environment [DFFE] and UWC) and Natalya Gallo (UiB) with logistical and technical support from Michael Brown (UWC) and was held at the Iziko South African Museum in Cape Town. The workshop comprised (i) introductory lectures by the co-organisers and local scientists with expertise in SIA including Prof. Judy Sealy (University of Cape Town [UCT]), Associate Prof. AJ Smit (UWC), and Mr. M. Horton (UCT); (ii) a tour of the Light Stable Isotope Laboratory, UCT; and (iii) guided, hands-on analysis by participants of actual SI data collected from several marine fish and invertebrate species sampled from the Benguela and California upwelling ecosystems as well as from Norwegian fjords. A total of 17 post-graduate students and early researchers from African and Norwegian universities and research institutions participated in the workshop (Figure 1).
The first morning consisted of welcomes by Dr Wendy Black, Chief Curator for Social and Art History and standing in for Dr Wayne Florence, Director Research and Exhibitions for the Iziko Museum, and by Prof. Mark Gibbons for the PRIMA Learning project. These were followed by an overview of the workshop by the co-organisers and brief introductions by the participants in which they shared their expectations of the workshop. Presentations describing what stable isotopes are and how they are measured (J. Sealy); the basic principles of using SIA to understand trophic ecology and foodweb structure and how to interpret results (C. van der Lingen); and the collection, preparation and measurement of stable isotopes (N. Gallo) were then given. A guided tour of UCT’s Light Stable Isotope laboratory was given (J. Sealy) during the afternoon in which participants were shown the final part of sample preparation (precisely weighting dried tissue samples on microbalances and sealing them in tiny tin capsules) and the instruments (isotope ratio mass spectrometers) used to measure their stable isotope signatures. An ice-breaker dinner for participants was then held at a local pizza restaurant.
Day two consisted of lectures describing how to analyse stable isotope data in R (M. Horton) and estimating dietary composition using stable isotope data and mixing models (AJ Smit), followed by an overview by the co-organisers of the data analysis process to be followed and descriptions of the various SI datasets available for analysis. Participants were divided into four Groups, each of which selected one of the datasets (Southern Benguela demersal, Southern Benguela pelagic, California demersal and Norwegian fjords) on which they worked for the remainder of the workshop. Each group first analysed SI data for a single species and investigated intraspecific differences (e.g. ontogenetic, spatial, and environmental differences, etc), then analysed SI data for a second species and investigated intra- and inter-specific differences, and subsequently analysed yet further data to assess community trophic structure, identify possible foodwebs, and conduct ecosystem comparisons. Each Group provided regular feedback on their progress, culminating in presentations delivered and discussed on the last day of the workshop (Figure 2) and submission of a short report (examples of the Group outputs are shown in Figure 3).
Figure 2: Students give their final presentations on the last day of the workshop.
Participants provided feedback on the workshop after the Group presentations. The expectations of most participants had been met and they felt well-equipped and, in many instances, keen to apply their new knowledge. The students stated that the workshop was well catered to the different experience levels in the course – from complete beginner to some experience with stable isotope sample preparation and analysis. Participants also provided several useful suggestions for future modification of the workshop. These included lengthening the workshop by a day, getting more hands-on lab practice in sample preparation, modifying the structure of the workshop so that presentations are spread across all days, allowing more time for guidance on the interpretation of stable isotope results from the group projects, and introducing analytical methods in R more gradually throughout the workshop including through providing a list of needed R packages and practice scripts in advance of the course.
Figure 3: Outputs of Group analyses conducted on stable isotope data showing (a) an isospace plot (δ15N on the y-axis and δ13C on the x-axis) for three small pelagic fish species (red = anchovy Engraulis encrasicolus, green = round herring Etrumeus whiteheadi, and blue = sardine Sardinops sagax) in the Southern Benguela ecosystem (from Seimberge, Fee, Stokkeland and Rbiai); (b) mean (±95% CI) δ15N values for three demersal fish species (red = Pacific grenadier Coryphaenoides acrolepis, green = Pacific hake Merluccius productus, and blue = longspine thornyhead Sebastolobus altivelis) from the California Current ecosystem (from Andersen, Digre, Ratusinski and Zacharias); (c) an isospace plot with fitted standard ellipses and convex polygons for three demersal fish species (green = blackmouth dogfish Galaeus melastomus, blue = roundnose grenadier Coryphaenoides rupestris, and red = velvetbelly lantern shark Etmopterus spinax) from Norewgian fjords (from Riska, Asmul, Brady and Vumazonke); and (d) scatterplots of δ15N versus fish total length for deepwater hake (Merluccius paradoxus) off the west (blue) and south (red) coasts of South Africa in the Southern Benguela ecosystem and Pacific hake (M. productus) from the California Current (green) ecosystems (from Kadila, Melaa, Aspelund and Nerlie).