Biogeochemical generation of dissolved inorganic carbon and nitrogen in the North Branch of inner Changjiang Estuary in a dry season. (15th October 2017)

• Record Type: Journal Article
• Authors: Zhai, Wei-Dong; Yan, Xiu-Li; Qi, Di
• Is Part Of: Estuarine, coastal and shelf science. Volume 197(2017)
• Page Start: 136
• Page End: 149
• Publication Date: 2017-10-15
• Subjects: Estuarine processes -- Partial pressure of CO2 -- Carbonate system -- Nutrients -- Changjiang Estuary; Estuarine oceanography -- Periodicals; Coasts -- Periodicals; Estuarine biology -- Periodicals; Seashore biology -- Periodicals; Coasts; Estuarine biology; Estuarine oceanography; Seashore biology; Periodicals; 551.461805
• DOI: 10.1016/j.ecss.2017.08.027
• Languages: English
• ISSNs: 0272-7714

Abstract

Abstract: We investigated the surface water carbonate system, nutrients, and relevant hydrochemical parameters in the inner Changjiang (Yangtze River) Estuary in early spring 2009 and 2010. The two surveys were carried out shortly after spring-tide days, and covered both the channel-like South Branch and the freshwater-blocked North Branch. In the North Branch, with a water residence time of approximately one month, we detected remarkable partial pressures of CO2 ( p CO2 ) of 930–1518 μatm with a salinity range of 4.5–17.4, which were substantially higher than the South Branch p CO2 values of 700–1100 μatm at salinities of less than 0.88. The North Branch p CO2 distribution pattern is unique compared with many other estuaries where aquatic p CO2 normally declines with salinity increase. Furthermore, the biogeochemical additions of ammonium (7.4–65.7 μmol kg −1 ) and alkalinity (196–695 μmol kg −1 ) were identified in salinities between 4 and 16 in the North Branch. Based on field data analyses and simplified stoichiometric equations, we suggest that the relatively high North Branch p CO2 values and estuarine additions of dissolved inorganic nitrogen/carbon in the mid-salinity area were strongly associated with each other. These signals were primarily controlled by biogeochemical processes in the North Branch, combining biogenic organic matter decomposition (i.e. respiration), ammonia oxidation, CaCO3 dissolution, and CO2 degassing. In the upper reach of the South Branch, notable salinity values of 0.20–0.88 were detected, indicating saltwater spillover from the North Branch. These spillover waters had minor contributions (1.5–6.9%) to the springtime nutrient, dissolved inorganic carbon, and alkalinity export fluxes from Changjiang to the adjacent East China Sea. This is the first attempt to understand the biogeochemical controls of the unique p CO2 distributions in the North Branch, and to evaluate the effects of saltwater spillover from the North Branch on dry-season export fluxes of biogenic elements to the adjacent East China Sea. Graphical abstract: Highlights: Coupled chemical dynamics of North Branch carbonate and DIN systems were studied. Water residence time of ∼30 d enhances estuarine respiration and CaCO3 dissolution. The North Branch biogeochemistry contributes to Changjiang element export fluxes. Biogeochemical controls of p CO2 in inner Changjiang Estuary were better understood.