Past and present carbon dynamics in the Scheldt estuary as traced by changes in C speciation and isotopes

The Scheldt Estuary (Belgium-The Netherlands), located in one of the most densely populated areas of Europe (over 500 inh./km²), is a highly eutrophic system currently recovering from hyper-eutrophication thanks to improved waste water treatment efforts. Consequently, the carbon cycle in the Scheldt estuary underwent a series of consecutive changes. We analyzed the evolution of organic carbon content and carbon-to-nitrogen ratio in suspended particulate matter, of inorganic carbon speciation (carbon dioxide, bicarbonates and carbonates) and of the 13C/12C isotopic composition of particulate organic and dissolved inorganic carbon, at monthly resolution over the past 20 years. Results show a long term trend superimposed on seasonal patterns. Since the 1998, allochtonous organic carbon inputs decreased progressively in the upstream river basin of the estuary with the reduction of untreated sewage releases, while a simultaneous increase of oxygen and pH, a decrease of DIC (and especially of CO2), and an increase of δ13C-DIC are observed. These all point to reduced respiration and increased primary production in the estuary. Consequences of changes on the global C budget of the estuary are estimated and discussed.