Interactions between sediment characteristics and oxygen conditions at the sediment–water interface of reservoirs: influences on nutrient dynamics and eutrophication

Dissolved oxygen (DO) level at the sediment–water interface is one key factor controlling redox-sensitive processes, such as nutrient cycling. Microcosm experiments with sediment collected from three reservoirs were performed to quantify the influences of water column oxygenation (oxic, anoxic, oxygen fluctuation), sediment characteristics (grain size distribution, total nitrogen and total phosphorus contents, microbial activities), and their interactions on nutrient fluxes from sediments to the water column. Algal growth bioassays were also performed using water from the microcosms to determine which conditions produced the most favorable growth conditions. Anoxic conditions increased the release of dissolved inorganic nitrogen (DIN), mainly as ammonium and phosphates, compared to the other DO conditions. Such effects were likely due to an inhibition of the nitrification–denitrification coupling process for DIN and a reductive dissolution of Fe (III) oxides for phosphates. Following this increased nutrient availability, algal growth in the bioassays was the highest in water collected from microcosms exposed to anoxic conditions. Under both oxic and anoxic conditions, the percentage of fine sediment particles led to decreasing DIN and phosphates fluxes by reducing the nutrient diffusion rate from sediments to the water column. Finally, both DO and sediment grain size controlled the contribution of sediments to reservoir eutrophication.