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
lengthening in mesozooplankton is carried over to fish. The TP of cod from
the western Baltic, feeding mainly on decapods, was equal to the global
mean value (4.1, secondary carnivore). Only cod from the central Baltic,
mainly feeding on zooplanktivorous pelagics, had a higher TP (4.8,
near-tertiary carnivore), indicating a strong carry-over effect of
FNC-caused trophic lengthening from mesozooplankton. In contrast, the TP
of molluscivorous flounder (3.2 ± 0.2 in both areas), associated with the
benthic food web, was unaffected by trophic lengthening. This suggests
that FNC blooms cause a large loss of energy in zooplanktivorous but not
in molluscivorous mesopredators. If FNC blooms continue to detour energy
at the base of the pelagic food web, the TP of cod will not return to
global mean values and the fish stock not recover. Monitoring the TP of
key species can identify fundamental changes in ecosystems and provide
useful information for resource management.