Diel cycles of reduced manganese and their seasonal variability in the Marque River (northern France)

Highlights: • High frequency monitoring of electrolabile Mn^{2 +} in a small river was performed. • Diel and seasonal variations were observed over two one-month periods. • Photoreduction of MnO_x and biotic oxidation of Mn^{2 +} partly control Mn behaviour. • Biodegradable organic matter input during storms impacted Mn^{2 +} concentrations. Electrolabile reduced manganese (II) has been monitored by voltammetry during two periods of one month in summer 2014 and at the end of winter 2015 in a small river (the Marque River) located in northern France and going through a suburban area with agricultural activities. Diel variations, evolution within the one-month periods and seasonal differences have been observed. Taking into consideration the multiple physical, biological and chemical reactions regulating manganese speciation in aquatic systems, it has been demonstrated that manganese speciation is probably controlled by the competition of two antagonist reactions: the photoreduction of manganese oxides (in broad sense and represented thereafter by MnOx) and the biotic oxidation of Mn(II). Depending on the season, the biological activity in the river and the amount of luminosity reaching the MnOx, either the production of reduced labile Mn(II) or the precipitation of MnOx can become the dominant process. Other punctual events such as the drop of oxygen concentration due to large inputs of biodegradable organic matter and eutrophication phenomena, rainy events and high luminosity periods can also affect the behaviour of dissolved Mn(II) in the Marque River and so, of other contaminants.