Characterizing niche differentiation among marine consumers with amino acid δ13C fingerprinting
Thomas Larsen1†, Thomas Hansen2, Jan Dierking2
1Max Planck Institute for the Science of Human History, Kahlaische Strasse 10, 07745 Jena, Germany
2GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
Email: larsen@shh.mpg.de
Key words: Baltic Sea, carbon stable isotopes, diet partitioning, fish diets, food web reconstruction, migration tracking, phytoplankton, predator-prey dynamics
Abstract
Marine food webs are highly compartmentalized and characterizing the trophic niches among consumers is important for predicting how impact from human activities affect the structuring and functioning of marine food webs. Biomarkers such as bulk stable isotopes have proven to be powerful tools to elucidate trophic niches, but they may lack in resolution, particularly when spatio-temporal variability in a system is high. To close this gap, we investigated whether carbon isotope (δ13C) patterns of essential amino acids (EAAs), also termed δ13CAA fingerprints, can characterize niche differentiation in a highly dynamic marine system. We tested the ability of δ13CAAfingerprints to differentiate trophic niches among six functional groups and ten individual species in the Baltic Sea. We also tested whether fingerprints of the common zooplanktivorous fishes, herring and sprat, differ among four Baltic Sea regions with different biochemical conditions and phytoplankton assemblages. Additionally, we investigated how these results compared to bulk C and N isotope data for the same sample set. We found significantly different δ13CAA fingerprints among all six functional groups. Species differentiation was in comparison less distinct, due to partial convergence of the species’ fingerprints within functional groups. Herring and sprat displayed region specific δ13CAA fingerprints indicating that this approach could be used as a migratory marker. Bulk isotope data had a lower power to differentiate between trophic niches, but may provide more easily interpretable information about relative trophic position than the fingerprints. We conclude that δ13CAA fingerprinting has a strong potential to advance our understanding of ecological niches, and trophic linkages from producers to higher trophic levels in dynamic marine systems. Given how management practices of marine resources and habitats are reshaping the structure and function of marine food webs, implementing new and powerful tracer methods are urgently needed to improve the knowledge base for policy makers.
1. INTRODUCTION
Direct pressures on marine systems such as increasing temperatures, eutrophication, introduction of non-indigenous species and overfishing are affecting the performance of individual species and the structure of entire systems. Examples of these consequences include the malnutrition of ecologically and commercially important fish species (Eero et al. 2015), niche shifts following the introduction of non-indigenous species (Ojaveer et al. 2017), and evidence for system wide shifts in many regions (Alheit et al. 2005). In this context, identifying organic matter sources at the base of the food web is key for understanding resource partitioning and trophic niche differentiation across time and space.
How marine communities differentiate and partition resources among species are often poorly understood due to the complexity of marine food webs and methodological constraints. Diet identification has traditionally relied on visual taxonomic assessment of stomach and faecal contents (Hyslop 1980), but visual assessments are now increasingly complemented with DNA metabarcoding (Bowser, Diamond & Addison 2013). While the taxonomic resolution of these methods can be high, they only provide instant snapshots of ingested diets provided that the identifiable fragments or DNA sequences are intact. Obtaining intact sequences can be logistically challenging when assessing multiple species over space and time. In comparison, it is possible to integrate dietary histories with stable isotope ratios, since the diet derived building blocks for animal tissues are sourced over time. Stable isotopes of elements can be informative of diet sources because lighter stable isotopes enter reactions and physical processes at faster rates than heavier stable isotopes, resulting in different isotope ratios among different organic pools. The rate by which elements shifts their isotopic ratios during trophic transfer differ greatly: elements such as carbon and sulfur are used as source tracers because they hardly discriminate (Mittermayr et al 2014) in contrast to nitrogen that is used as a marker of trophic position (Vander Zanden and Rasmussen 1999). However, isotope ratios of whole (“bulk”) tissues often lack source specificity because of variable, and at times, unpredictable isotope discriminate processes and isotope baseline values for different systems (Post 2002; Fry 2006). To overcome these limitations, ecologists are increasingly using compound specific isotope analyses (CSIA), in which stable isotope ratios are determined for individual compounds, as a complementary approach (Whiteman et al. 2019).
CSIA of protein amino acids has emerged as one of the most promising approaches to trace the origins and fate of food sources (McClelland & Montoya 2002; O’Brien, Fogel & Boggs 2002). Amino acids (AAs) are among the major conduits of organic carbon in food webs, and well suited as a source tracer because metazoans cannot synthesize the carbon backbones of about half of the 20 protein AAs de novo. Instead, metazoans depend on essential amino acids (EAA) from food sources (McMahon et al.2010) or bacterial symbionts (Larsen et al. 2016b). EAA are powerful source tracers because δ13CEAA values remain largely conserved through trophic transfer and because the producers of these EAA, algae, bacteria, fungi and vascular plants each generate unique δ13CEAA patterns or fingerprints (Scott et al. 2006; Larsen et al. 2009; Larsen et al. 2013). See the Textbox for an illustrative explanation of the δ13CEAA fingerprinting technique. Thus, by analysing δ13CEAA ecologists can circumvent the problem of variable and unknown isotopic fractionation during trophic transfer, but the ability of fingerprints to resolve primary production sources is still unclear. Larsen et al. (2013) compared two dozen species of laboratory cultures comprising of diatoms, cyanobacteria, crysophytes, chlorophytes and haptophytes to macroalgae, seagrass, fungi, bacteria, and terrestrial vascular plants and found that of all these groups, phytoplankton displayed the largest intragroup variability in δ13CEAApatterns across species and types. Despite some unresolved questions for applying δ13CEAA fingerprints in marine environments, they have been applied successfully to track habitat use of fishes with distinct ontogenetic migration patterns (Vaneet al. 2018), resource and habitat use in marine systems (McMahon, Berumen & Thorrold 2012), and proportional contributions of primary production sources to marine consumers (Vokhshoori, Larsen & McCarthy 2014; Elliott Smith, Harrod & Newsome 2018; Rowe et al.2019). A recent study on mesozooplankton in the Baltic Sea has shown promise in distinguishing between interannual algal assemblages (Egliteet al. 2019). Taken together, these results indicate that δ13CEAA fingerprints may be able to provide detailed insights into ecological niches of consumers to a much larger extend than previously realized.
Exploring further use of CSIA to elucidate changes in basal resources and ecological niches are particularly pertinent for regional seas because of their rapidly warming sea surface temperatures and increasing stressors from anthropogenic activities such as eutrophication and overfishing, with corresponding changes in food webs (Reusch et al. 2018). In this study, we selected the western and central Baltic Sea as a study area because it is a brackish inland sea characterized by strong differences in phytoplankton composition (Gasiūnaitė et al. 2005; Wasmund et al. 2017; Eglite et al. 2019) driven by a gradient in hydrographic-hydrochemical conditions (Naumann et al. 2017). In this sea, food web related processes have been identified as driver of changes in ecosystem composition (Möllmann et al.2009) and declines of key commercial species (Casini et al. 2016; Reusch et al. 2018). Compared to fully marine systems, this brackish system is characterized by a relatively low diversity (Ojaveeret al. 2010), and a tight coupling of benthic and pelagic food webs (Griffiths et al. 2017). Across the gradient, the small pelagic fish species herring and sprat are the dominant zooplanktivores, and of large commercial value (Ojaveer et al. 2018). As zooplanktivores, these species are also natural integrators of pelagic planktonic production.
To test the power of CSIA to identify niche differentiation among marine consumers in the spatially variable Baltic Sea, we obtained δ13CAA values for a range of species from different functional groups including suspension feeders, planktivores, benthic predators and scavengers. Furthermore, to assess the power of the method to identify differences across larger spatial scales, we obtained δ13CAA values for herring and sprat from four locations along the Baltic Sea gradient (Fig. 1). We first assessed the power of δ13CEAA fingerprints to identify (1) trophic niche differentiation among functional groups and among species, and (2) the presence of spatial patterns among planktivorous fishes, positing that different δ13CEAAprofiles of phytoplankton assemblages would propagate via mesozooplankton to zooplanktivore fishes. Finally, we assessed the potential of bulk δ13C and δ15N data to provide complementary information about modes of feeding and trophic position.
2. MATERIAL AND METHODS