The effect of shifts in fish community structure on PCDD/F temporal variability in common guillemot (Baltic Sea)

After 1970s, polychlorinated dibenzodioxins and dibenzofurans (PCDD/Fs) concentrations peak in Baltic biota, concentrations started to decline following environmental legislation. However, in common guillemot eggs, this decline plateaued in 1990s, despite continued emissions reductions. Here, we test whether these contrasting trends can be explained by environmental and food web structural changes, including prey availability. Analysing temporal variation in the Central Baltic offshore fish community, including guillemot prey, we identified three structural phases: cod and herring dominance (1976-1986), sprat dominance (1987-2001), and stickleback population increase (2002-2021). We linked them with corresponding phases in PCDD/F trends: a steep decline (- 6.4% yr^-1), a plateau (0.27% yr^-1), and a slower decline (- 3.8% yr^-1). Fish community structure shifts were driven by changes in temperature, salinity, zooplankton size, and fishing pressure. We concluded that climate- and human-driven structural changes in food webs cascaded through trophic levels, affecting PCDD/F concentrations in guillemot eggs.