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.