Pathways of Redox Processes in a Seasonally Anoxic Lake, Aydat Lake, France

Abstract

The classical sequence of successive > favourable electron acceptors (02, NO3, MnOx, FeOOH, SO]-), utilized by microorganisms to decompose organic matter (primary redox reactions), in lacustrine or coastal marine sediments (Froelich et al, 1979), occurs also in water column. However, the interpretation of the distribution of reactions by-products is not straight forward. Though, this sequence at a first glance would appear with depth, species distribution can alternatively be related to the interplay between transport processes and secondary inorganic redox reactions (Van Cappellen and Wang, 1996). In this paper, we discuss the seasonal distribution of redox species, the location and pathways of redox processes in a eutrophic lake studied by biologists and geochemists for more than 10 years (Sarazin et al., 1995). Aydat lake located in the French Massif Central, 25 km SW from Clermont Ferrand originates from the damming of the Veyre river by a basaltic flow which occurred 7500 years ago. The lake surface is 6• 105 m 2, its volume 5 • 106 m 3, its maximum depth 15 m. The water residence time is about 7 months. The lake is typically eutrophic and develops an anoxic hypolimnion from May to November.