Trophic dynamics of Baltic fish species after the 2014 Major Baltic Inflow event

The Baltic Sea is among the ecosystems most affected by global and regional environmental perturbations worldwide, including the drastic expansion of hypoxic areas in recent decades. This has had direct effects on the biota living in the Baltic Sea, such as the reduction of benthic invertebrates, which also represent a food resource for fish communities. In December 2014, the occurrence of a Major Baltic Inflow (MBI) event transporting large amounts of saline, oxygen rich North Sea water to the deep layers of the Baltic Sea may have led to trend reversal in the availability of benthic resources. This raises the question how major fluctuations in the availability of benthic versus pelagic prey resources have affected Baltic fish species. In particular, this is in relation to the commercially and ecologically important fish species Baltic cod (Gadus morhua), which has experienced a strong decline in body condition over the period of extending hypoxia. In this context, better characterisation of benthic versus pelagic feeding by cod and the characterisation of feeding interactions in Baltic fish communities after the 2014 MBI are urgently needed. To address this knowledge gap, I assessed the trophic dynamics of Baltic cod and 12 other fish species in April 2016, using stable isotope analysis (SIA) of carbon (C), nitrogen (N) and sulphur (S). My particular focus lay on ontogenetic patterns of benthic versus pelagic feeding in cod, and on the potential for interspecific competition in three different fish groups, the piscivore predators cod and whiting (Merlangius merlangus), the pelagic fishes herring (Clupea harengus), sprat (Sprattus sprattus) and three-spined stickleback (Gasterosteus aculeatus), and the flatfishes dab (Limanda limanda), flounder (Platichthys flesus) and plaice (Pleuronectes platessa). In total, 296 samples of 17 species from 4 basins and 14 sites were analysed, to ensure a high spatial resolution of the spatio-temporally variable Baltic environment. Results show (1) the overall trophic structure of the community, (2) an ontogenetic diet shift from benthic to pelagic prey in cod, (3) a positive trend of cod condition with an increasing benthos proportion in the diet, (4) a surprisingly low isotopic niche overlap for cod and whiting, (5) little evidence for interspecific competition between the pelagic fishes, and (6) a strong potential for competition within the flatfish community. Sulphur data provided the most informative insights, especially for the assessment of benthic-pelagic diet shifts in this study. This work offers valuable insights into the trophic dynamics of Baltic fish species including non-commercial species that are not routinely assessed with stomach content analyses. Thus, it demonstrates the potential of SIA to routinely assess the overall Baltic food web structure in a relatively convenient and logistically easy way, demonstrated by the spatially resolved data set for both commercial and non-commercial fish species provided by a single research cruise.