4.2. Assessing spatial differences in marine consumers and food webs with CSIA
Spatial isotope differences of marine consumers can inform about underlying differences in the organic matter at the base of food webs, as well as migration patterns of individuals (Hansson et al.1997; McMahon, Berumen & Thorrold 2012; Torniainen et al. 2017). The geographically distinct δ13CEAAfingerprints of herring and sprat observed in our study points to limited mixing among schools from the different locations, i.e., spatial population structuring, and to the presence of different isotopic baselines (e.g., different phytoplankton assemblages) among locations. This corresponds well with monitoring studies of phytoplankton in the Baltic Sea, highlighting the change in assemblages along the environmental gradient (Wasmund et al. 2017) (Fig. 2). The additional observation of substantial variability within the same locations for both sprat and herring could be related to size-related differences in feeding (Last 1989; Kleppel 1993; Casini, Cardinale & Arrhenius 2004) as well as differences in migrations, both between areas (Aro 1988; Jørgensen et al. 2005; Gröhsler et al. 2013) and in the case of herring between coastal and offshore areas during spawning runs, e.g., (Šaškov et al. 2014). The lower spatial differentiation observed after pooling the two species is in line with a previous study in the Baltic; it found indiscernible fingerprints among regions after pooling multiple zooplankton species that differ in their dietary preferences and response to the trophic state (Pejler 1983; Gorokhova et al. 2016; Eglite et al. 2019). Our finding suggests that with further development, δ13CEAA fingerprinting have the potential to complement conventional tags and bulk isoscape approaches to track migrations in offshore system (St. John Glew et al.;Chittenden et al. 2013; Soto, Wassenaar & Hobson 2013; Torniainen et al. 2017). It could also provide further and much needed insight into feeding ecology and response to changing physio-chemical conditions (Casini, Cardinale & Arrhenius 2004; Kulke 2018).