Bulk Carbon and Amino Acid nitrogen isotope data from Baltic cod (Gadus morhua) and European flounder (Platichthys flesus) muscle tissue samples from the western and central Baltic Sea

Eutrophication, increased temperatures and stratification can lead to massive, filamentous, N2-fixing cyanobacterial (FNC) blooms in coastal ecosystems with largely unresolved consequences for the mass and energy supply in pelagic and benthic food webs. Mesozooplankton adapt to not top-down controlled FNC blooms by switching diets from phytoplankton to microzooplankton, resulting in a directly quantifiable increase in its trophic position (TP) from 2.0 (herbivore) to as high as 3.0 (carnivore). If this process in mesozooplankton, we call trophic lengthening, was transferred up to higher trophic levels of a food web, a large loss of energy could result in massive declines of fish biomass. We used compound-specific nitrogen stable isotope data of amino acids (CSIA) to estimate and compare the nitrogen (N) sources and TPs of cod and flounder (mesopredators) from areas with influence of FNC blooms (central Baltic Sea) and without it (western Baltic Sea). We tested if FNC-caused trophic leng..., The fishes were collected during four cruises to the western Baltic Sea (ICES subdivision (SD) 22) and central Baltic Sea (SD 24-25) in January/February 2019 and 2020 and caught by bottom trawl, with a Bacoma cod end (2019) and a TV30 #520 trawl gear (2020) following the standards of the Baltic International Trawl Survey (ICES, 2017). On board, the fish were killed, identified to species level, counted, and measured. The otoliths were extracted and a sample of white muscle tissue was taken from behind the third dorsal fin of 30 cod individuals and from the tail muscle of 21 flounder individuals. All samples were frozen immediately at −20°C for later stable isotope analyses.
The frozen cod und flounder muscle tissue samples were cleaned mechanically and with distilled water to remove surface contaminants, freeze dried (Christ Alpha 1-4), and then ground and homogenized for further processing. For CSIA, ~10 mg of each dried sample was transferred into a heat-resistant borosilicate vial, m..., , # Bulk Carbon and Amino Acid nitrogen isotope data from Baltic cod (*Gadus morhua*) and European flounder (*Platichthys flesus*) muscle tissue samples from the western and central Baltic Sea

[https://doi.org/10.5061/dryad.12jm63z48 ](https://doi.org/10.5061/dryad.12jm63z48)

There are currently four TP models available that are suitable to estimate the TP of vertebrates (Germain et al., 2013; Nielsen et al., 2015; Bradley et al., 2015; Brault et al., 2019). They were tested and partly modified to identify, which TP model is most suitable for cod and flounder, respectively. Specifically, CSIA data of cod and flounder from the western Baltic Sea were chosen to validate and calibrate the TP models based on the assumption that their TPs at this reference site should be close to the global mean TP value for cod of 4.1 ± 0.2 and for flounder of 3.3 ± 0.2, respectively (Froese and Pauly, 2022). The rationale for this is that the TP of cod and flounder from the western Baltic Sea should not be...