A study on the response of carbon cycle system in the Pearl River Estuary to riverine input variations

Human activities associated with estuaries have already affected estuarine carbon cycles and flows. Especially with rapid development and urbanization, widespread wastewater treatment plants in watersheds have altered the amounts and proportions of riverine nutrients and organic carbon, which has strongly impacted carbon dynamics and budgets in coastal oceans. In this study, a well-validated carbon cycle model was used to reveal the response of carbon dynamics to reasonable estimated disturbance to riverine inputs (i.e., nutrients, dissolved organic carbon (DOC) and particulate organic carbon (POC)) in the Pearl River Estuary (PRE). Scenario results demonstrate the distinct response patterns and spatial variability of carbon dynamics. Specifically, with phytoplankton blooms and biological production closely linked to riverine nutrients, riverine nutrient alterations mainly impact photosynthesis in the PRE, especially in the lower part of the PRE. As DOC fuels bacterial respiration in the water column, heterotrophic respiration is particularly sensitive to riverine DOC disturbances. POC not only relates to light penetration in turbid estuarine waters, which impacts photosynthesis, but also undergoes deposition, which affects the biochemical reactions in the sediment. Thus, perturbations to riverine POC input led to significant changes in both biological production and sediment dissolved inorganic carbon (DIC) release. Multiple linear regression results reveal that both net community production and sediment DIC release flux modulate the variations of air-sea CO2 flux in the PRE, which highlights the roles of terrestrial POC input and deposition in a shallow estuary. When delivered into the PRE, a high proportion of POC tends to be deposited and contribute to sediment DIC release or carbon sequestration, while a significant fraction of riverine DOC is transported offshore to shelf regions. This study underscores the impacts of intensive human activities on carbon cycle system in coastal oceans and the importance of terrestrial organic carbon reduction and nutrient control in upstream basins.