Increased eutrophication offshore Hong Kong, China during the past 75 years: Evidence from high-resolution sedimentary records

Total organic carbon (TOC), total nitrogen (TN), stable isotopes ( δ 13C and δ 15N), lipids and biogenic silica (BSi) from a high-resolution sediment core east of Hong Kong span the interval from 1925 to 2001. Organic matter (OM) within this core (E2) is derived from both marine phytoplankton and terrestrial material as deduced from the range of values of δ 13C org (− 24.0‰ to − 22.4‰) and δ 15N (2.9‰ to 5.3‰) and lipid biomarkers. Diatom and dinoflagellate productivity which is reflected in the biogenic silica (BSi) and dinosterol concentrations respectively, increased gradually starting in 1940 and accelerated after 1965, especially between 1980 and 2000, indicating that algal blooms and/or red tides caused by eutrophication increased during this time. The abundance of coprostanol, which reflects domestic sewage discharge, and the terrestrial biomarkers (long-chain fatty acids and fatty alcohols and sitosterol) exhibit similar temporal changes with the primary production, showing that the enhanced eutrophication resulted from increased anthropogenic activities in the northern coastal waters of the South China Sea (SCS) in recent decades.