Carbon System Simulation in the Pearl River Estuary, China: Mass Fluxes and Transformations

A carbon cycle model is built to describe the behavior of carbon materials in the Pearl River Estuary (PRE), China. The distributions and transformations of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) are simulated in the water column and sediment for the year 2006. The terrestrial carbon input is the dominant factor that determines the seasonal variation of carbon, while physical and biochemical processes contribute to the spatial‐temporal circulation of carbon. The simulation results reveal that the PRE acts as a net source for atmospheric CO2 throughout the year, and it buffers the export of DIC from the river and sediment to the adjacent system via the DIC consumption by primary production. As a consequence of biochemical processes, the PRE exports more organic carbon to the sediment and adjacent marine ecosystems than the amount that it receives from upstream river reaches. POC burial in sediment and refractory DOC export to the adjacent marine ecosystems are the main carbon fixation pathways in the PRE. The total amount of carbon fixation in PRE is estimated 6.92 × 1010 mol C year−1. Carbon fixation analysis shows that (1) 20.4% of the POC deposited into sediment is fixed through burial, while the remaining continues to participate in the circulation of carbon materials, and (2) the combined effects of river discharge and monsoon dominate the amount and direction of refractory DOC exports. Plain Language Summary Carbon cycle in estuaries is an important part of the global carbon cycle. Due to the intensive human impact and complicated biogeochemical processes, there exist great uncertainties in estimating carbon fixation and difficulties in illustrating the entire carbon cycle in estuaries. In this study, we chose the Pearl River Estuary as the study area and used a carbon cycle model to simulate the distribution and transformation of carbon materials. Our study found that the carbon cycle in the PRE is very active. Even though CO2 undergoes continuous evasion from the sea surface to the atmosphere, the PRE fixes 6.92 × 1010 mol C year−1 via particulate organic carbon burial into deep sediment and refractory dissolved organic carbon export to adjacent marine systems. In different seasons, with river discharge and monsoon changes, carbon fixation ability and the direction of carbon export flux vary as well. Key Points The entire carbon system of Pearl River Estuary is simulated in this study C