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