Forty years of pollution monitoring in the Wadden Sea using bird eggs: insights from the Trilateral Monitoring and Assessment Program.

As rising global emissions and climate change amplify environmental contamination, long-term monitoring is essential to quantify trends and assess or predict impacts on wildlife. Over the past four decades, such monitoring was part of a Trilateral Monitoring and Assessment Program (TMAP), in which levels of various persistent organic pollutants (POPs) and total mercury were measured in eggs of Eurasian oystercatchers (Haematopus ostralegus) and common terns (Sterna hirundo) breeding across the Wadden Sea. The pollutants examined included six dichlorodiphenyltrichloroethane isomers (Σ DDT), cis- and trans-chlordane and nonachlor (Σ chlordane), α-, β-, and γ-isomers of hexachlorocyclohexane (Σ HCH), up to 62 PCB congeners (Σ PCB62), of which 18 were analysed across the monitoring period (Σ PCB18), hexachlorobenzene (HCB), and total mercury (THg). We investigate whether pollutant levels differ between bird species and regions, and how they changed over time. We find that common terns show higher concentrations of Σ PCB18, HCB and THg than oystercatchers, that levels of most organochlorine compounds, but not THg, are correlated, raising concern about potential synergistic effects, and that river estuaries are pollution hotspots. Encouragingly, concentrations of all pollutants, except Σ chlordane, decreased. Likewise, the proportion of eggs at more than low risk for detrimental effects of THg declined since monitoring began in 1981 (from 74% in the 1980s to 22% in the 2020s). However, several pollutants have shown slight resurgence in recent years, highlighting the need for continued monitoring, with future efforts also including emerging contaminants.