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

Abstract

Negombo Lagoon is a highly valued estuary in Sri Lanka due to its high socioeconomic value, but this estuary has undergone many changes since the early 1800s due to human impact. Records of carbon and nitrogen stable isotopes, and alkane, alcohol, polycyclic aromatic hydrocarbon (PAH) and hopane biomarkers from the mid-19th century to 2012 in a sediment core collected from Negombo Lagoon indicate that the lagoon went through two major changes during this 160-year time frame. The first change occurred from 1850 to 1900, where a switch from predominately terrestrial sources of organic material to marine sources is observed, after the Hamilton Canal was built in 1804 to connect the open ocean to the lagoon. The second change occurred post 1940s during Sri Lanka’s industrialization period where high anthropogenic inputs are seen in the estuary, specifically as increases in PAHs and oil-related, diagenetic hopanes. Overall, the Negombo Lagoon appears to be moderately polluted by PAHs with the highest concentrations of parent PAHs measured in the 1980s and likely derived from pyrolytic sources of solid fuel biomass.