Using stable isotopes and chemical markers to understand the history of Negombo Lagoon, Sri Lanka

•The biomarkers and stable isotopes reveal the changes at Negombo lagoon.•Switch from terrestrial to a marine signal in the late 19th century.•PAH and oil hopanes pollution reached a peak concentration at the 1980s.•Marine and terrestrial biomarkers reached the peak maxima at the 1980s.•Reduction in terrestrial biomarkers in the last decade due to mangrove exploitation.