Using water age to study the biogeochemistry of nutrients in a large-river estuary and the adjacent shelf area

Whether a large-river estuary and the adjacent area behave as a water sink or as a source for important terrestrial biogeochemical compounds is a challenging question. Such aquatic systems are very complex and can be overwhelmed by physical circulation and estuarine mixing. In this study, physical oceanography and marine biogeochemistry were combined to address this question. Seven cruises were conducted in the Changjiang River (Yangtze) Estuary and the adjacent East China Sea shelf in February–March and in July over a three year period (2014 to 2016). During each cruise, nutrient concentrations (NO₃⁻, SiO₃²⁻, PO₄³⁻, NH₄⁺, and NO₂⁻) and salinity were measured at the surface, middle, and bottom layers of 75–106 sampling stations, and water age was derived from numerical modeling. Two-variable regression analysis among salinity, water age, and nutrients was applied to calculate the specific variation rates of nutrient concentration versus water age. A positive rate value indicated that the water was a source of nutrients, and a negative value indicated the site as a sink. In both seasons tested, calculations for the five nutrients analyzed showed that all biogeochemical processes occurring in Changjiang Diluted Water (salinity <31.5) could distinctly change terrestrial fluxes from the Changjiang River in both seasons. Thus, the proposed method has the potential to elucidate the nonconservative behaviors, net fluxes, and budgets of important biogeochemical compounds through large-river dominated estuarine, coastal, and shelf areas worldwide.