2.2. Fauna sampling
Sampling for this study took place during research cruise AL476 with research vessel ALKOR in April 2016 (see sampling stations in Fig. 1). All specimens were measured (total length or diameter to the nearest mm, mass to the nearest g), ca. 0.5 cm3 of muscle tissue was taken for isotope analysis and immediately frozen at -20C on board of the vessel for further analyses. Our sampling was designed with our two main research questions in mind: 1) can δ13CAA fingerprints differentiate feeding niches at functional group and species levels; and 2) is it possible to obtain geographically distinct fingerprints for widely distributed zooplanktivorous fishes. For the first research question, we collected the following species in the two westernmost sites, Kiel Bay and the Arkona Basin: planktivores (herring: Clupea harengusLinnaeus 1758) and (European sprat: Sprattus sprattus Linnaeus 1758), pelagic piscivore (Whiting: Merlangius merlangus Linnaeus 1758), suspension feeders (Ocean quahog: Arctica islandicaLinnaeus 1767, Blue mussels: Mytilus edulis Linnaeus 1758), benthic predatory flatfish (Common dab: Limanda limanda Linnaeus 1758, European flounder: Platichthys flesus Linnaeus 1758, European plaice: Pleuronectes platessa Linnaeus 1758), benthic predators (Common starfish: Asteria rubens Linnaeus 1758), and scavengers (Red whelk: Neptunea antiqua Linnaeus 1758). For the second research question, we also samples herring and sprat in the two easternmost sites, Bornholm Basin and Gdansk Basin. For further sample characteristics, see Table 1 for a summary and Supplementary S1 for detailed information.