Effects of increasing organic matter loads on pore water features of vegetated (Vallisneria spiralis L.) and plant-free sediments

The effects of organic enrichment on pore water chemistry of bare and Vallisneria spiralis L. colonized sediments were investigated. Substrates of three organic levels were created by adding different amounts of powdered fish feed (0, 5 and 10 g/l of sediment, respectively) to homogenized sediment and incubated with and without plants. Redox potential (Eh), reduced compounds (CH4, Fe2+, Mn2+) and nutrients (PO43−, NH4+) were analyzed at time zero and after 6, 10, 13 and 17 days. In control microcosms V. spiralis sediments displayed significantly higher Eh and lower CH4, Fe2+, Mn2+, PO43− and NH4+ concentrations than bare ones. In organic enriched microcosms methanogenesis became the main degradation pathway when other electron acceptor pools were depleted. However, lower levels of interstitial Fe2+, Mn2+ and PO43− were found in vegetated sediments compared to bare ones and this difference was maintained during the whole experimental time. Root oxygen release in the rizosphere seemed to be the main responsible of this outcome, as also suggested by the nitrification potential assay, indicating the maintenance of oxic microniches. V. spiralis can act as an engineer species in urban, organic impacted sediments due to its high tolerance against reduced conditions, which makes this macrophyte an interesting option in aquatic ecosystems restoration programs.