4.1 Understanding niche differentiation and resource
partitioning with CSIA
Our results show that the δ13CEAAfingerprinting method holds considerable potential for identifying
feeding differences in marine habitats. In our two westernmost Baltic
locations, the Kiel Bay and the Arkona Basin, we were able to identify
niche differentiation among all putative functional groups, as well as
most species. This differentiation is in agreement with previous
knowledge based on traditional methods like stomach content analysis,
e.g. Hislop et al. (1997). Species with similar modes of feeding
clustered closely. It is surprising, however, that seastars clustered
very differently than bivalves, considering that blue mussels are
considered of major importance in their prey (Sommer, Meusel & Stielau
1999). Such ‘mismatches’ do not pertain to limitations of the
fingerprinting method, but rather to the extend a study has sampled and
analysed all relevant endmembers. For example, seastars also feed on
other invertebrates such as sponges, snails, and isopods (Anger et
al. 1977). In addition, primary consumers integrate more recent
photosynthates in their tissue than higher level consumers. Taken
together, our results highlight the potential of
δ13CEAA fingerprinting to elucidate
the dietary niches of marine consumers, and how fluxes of carbon and
nutrients from primary producers to detritus and consumers structure
marine ecosystems (Cebrian 1999; Lartigue & Cebrian 2012).
The highly dynamic and complex nature of marine food webs can make it
challenging to assess trophic relationships between consumers and
producers, particularly on a taxon specific level (Woodward et
al. 2005; Armengol et al. 2019). The clear spatial and trophic
group differences observed in our study underscore the potential of
δ13CEAA fingerprinting to determine
the trophic basis of production, i.e. how particular production sources
are linked to consumers, and specifically selective grazing and
assimilation of phytoplankton and detrital resources. Since
phytoplankton assemblages influence the food web structure, it will be
critical for future studies to establish a reference phytoplankton
library based on well characterized in situ algal assemblages and
single species cultures. As demonstrated in this study, laboratory
cultures of bacteria, phytoplankton and other potential endmembers can
be used as a proxy for in situ samples, which means that the
fingerprinting approach works well for tracing inconspicuous sources.
Increased application of this method to identify the taxonomic groups
fuelling production on higher trophic levels could improve our
understanding of trophic links in many marine food webs and reduce the
current bias towards larger prominent species feeding on clearly
identifiable food items.