Short-term effect of oxic to anoxic transition on benthic microbial activity and solute fluxes in organic-rich phytotreatment ponds

Manipulative experiments to test the short-term effect of oxygen depletion events on microbial activity and benthic fluxes in organic-rich sediments were carried out in March and June 2004. Oxic\u2013anoxic transitions were induced by prolonged dark incubation of sealed sediment cores collected in phytotreatment ponds. Benthic fluxes of oxygen (O2), carbon dioxide (CO2), inorganic nutrients, and free sulfides were measured before (oxic) and after (anoxic) the transition occurred. A multifactorial design was employed for monitoring esoenzymatic activity, heterotrophic bacterial production, total prokaryotic abundance, actively respiring bacterial cells, and the biochemical composition of sedimentary organic matter. The oxic to anoxic transition resulted in a significant increase of esoenzymatic activity and bacterial production in March, due to the profound modification of the benthic community and the release of labile organic compounds which followed the onset of anoxia. In parallel, net efflux rates of dissolved inorganic carbon (DIC) and ammonium (NH4 ?) sharply decreased, soluble reactive phosphorus (SRP) influx reversed, and sulfide was buffered within the oxidized sediments. From March to June, ponds evolved toward oxygen deficit and reducing conditions in the upper sediment horizon, losing benthic fauna and biogeochemical buffering capacity. Thus, the oxic to anoxic transition had a much smaller effect on microbial activity and net flux exchange, while S2- was consistently delivered from the sediment to the water column. Overall data from this study suggest that the shortterm response of benthic microbial activity and solute fluxes to anoxic events may have a significant impact on sediment biogeochemistry (e.g., at the oxic\u2013 anoxic interface), and that this impact may vary greatly depending on the sediment features, mainly its organic content and redox condition