Carbon and nitrogen isotopic records of effects of urbanization and hydrology on particulate and sedimentary organic matter in the highly urbanized Pearl River Delta, China

•Spatio-temporal patterns of OM apportionment were identified by C and N isotopes.•The signal of urbanization can be recorded in the organic matter.•Runoff was the dominant factor affecting seasonal change of SPM source distribution.•Sewage discharge due to urbanization could facilitate C and N transformations.