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

[Display omitted] •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. The deltaic and coastal region is an important area for transportation, production and transformation of organic matter (OM). However, the high degree of urbanization and seasonal variability of runoff in this region significantly impact source apportionments and biogeochemical processes of the OM. In this study, stable isotopes of carbon (C) and nitrogen (N) in both suspended particulate matter (SPM) and sedimentary organic matter (SOM) were measured in the highly urbanized Pearl River Delta (PRD), China. The results revealed that source apportionments exhibited spatial variations due to different degrees of urbanization and seasonal variations due to seasonal changes in runoff. High urbanization could increase the contribution of sewage-derived OM in the SPM and decrease the proportion of soil OM in the SOM. Increased precipitation and runoff during wet season could result in a greater contribution of sewage-derived OM (55%), and decreased runoff during the dry season resulted in an increased proportion of phytoplankton (51%) in the SPM. Urbanization and runoff also affected C and N transformation processes of OM. An increased volume of sewage discharge due to urbanization provided nutrients for phytoplankton growth, promoting photosynthesis and assimilation processes. Increased light intensity due to decreased precipitation and runoff during the dry season could facilitate photosynthesis by phytoplankton. This study suggested that OM could record the impacts of urbanization and hydrology on C and N transformation processes in the rivers, further improving knowledge on the C and N cycling processes in highly urbanized river ecosystems.