Carbonate dissolution in the turbid and eutrophic Loire estuary

Abstract

We measured particulate and dissolved organic carbon (POC and DOC), chlorophyll, oxygen, partial pressure of CO2, pH, total alkalinity (TAlk) and particulate inorganic carbon (PIC) during a late summer cruise in the eutrophic Loire estuary. These parameters reveal an intense min- eralisation of organic matter in the estuarine maximum turbidity zone (MTZ) that results in oxygen deficits (down to 20% of the saturation level) and high CO2 oversaturations (pCO2 up to 2900 µatm). Several facts revealed the occurrence of carbonate dissolution in the Loire MTZ: large amounts of alkalinity were produced in the upper estuary, increasing its transfer to the ocean by 30%; the calcu- lated saturation index showed a net undersaturation for aragonite and a slight undersaturation for calcite in the MTZ; and PIC decreased from 2.1% (% dry weight) in riverine suspension to 0.4% in the MTZ. A stoichiometric approach is used to assess the coupling between aerobic respiration and carbonate dissolution, where apparent oxygen utilisation, excess CO2, TAlk and dissolved inorganic carbon are compared quantitatively. About 20% of the CO2 generated by respiration was involved in carbonate dissolution. The loss of PIC at the river-estuary transition quantitatively corresponds to the amount of authigenic calcite precipitated upstream in the highly eutrophic river. This suggests that CO2 exchange with the atmosphere along the eutrophic river-estuary continuum is buffered by carbonate precipitation in the autotrophic river and its dissolution in the heterotrophic estuary.