## Materials and Methods
### Experimental Design and Setup  
All the experiments were developed simultaneously in situ during 45 days after an acclimation period of 2 days. Incubations were conducted during July and August, when temperatures in the river are the highest, and microbial metabolic rates of mesophilic microorganisms are assumed to increase accordingly. Previous assays (data not shown) showed that these higher temperatures optimized the effectiveness of treatments within the assayed timeframe. Mesocosoms were made using cylindrical PVC mesocosoms (50 cm diameter and 100 cm high) opened both at its top and its bottom. The mesocosoms had round holes in its upper part to allow water circulation without affecting sediments (Figure 1c), in order to avoid complete water isolation. They were submerged and partially staked into the sediments, in such a way that its mouth was located around 20 cm centimeters below the water surface. The average depth and volume of the sediment enclosed below the overlaying water were \(55\pm 14\) cm and \(108\pm 27\) L, respectively. The immersion in water and the tubes round holes allowed that the flow and natural settling process continued during the experiment. Each of the mesocosoms were assigned to an individual treatment, which are summarized in Table 1, to test (**1**) the effect of the BAP addition, (**2**) the effect of varying the concentration and spatial distribution of the BAP, (**3**) the effect of temporally dosing the BAP, (**4**) the effect of adding electron acceptors for anaerobic respirations, (**5**) the role of the acetoclastic microbial metabolisms, sulfate-reduction, and methanogenesis, and their potential limitations, and (**6**) the occurrence of potential inorganic nutrient limitations. Two additional mesocosoms contained untreated sediments and operated as controls. All 6 experiments were conducted in parallel. The assay solutions were prepared by dissolving the BAP and/or other amended compounds in river water. Each solution was mixed thoroughly for 2 h, then additions were done by injecting each assayed aqueous solution in three points of the sediments of the corresponding mesocosoms to a depth of 50 cm. This proved to be efficient in spreading the solutions inside the sediments confined in the mesocosoms. The detailed setting up of each experiment was as follows:Experiment 1 (effect of the product): This experience aimed to test the efficiency of the BAP in accelerating the digestion of OM accumulated in sediments. This consisted in the addition of a standard injection of 50 g/m\({}^{3}\) of the BAP, which uses the concentration previously assayed in the same river basin that resulted in significant effects [5]. This was the concentration of BAP used for the standard additions in the rest of experiments unless otherwise indicated. Experiment 2 (dosage and spatial distribution of the product): This experiment had optimization purposes for the BAP additions, and was made to assess the dependence of the treatment efficiency on the dose of the product used and its spreading on the sediment. To do this, two concentrations of 25 and 75 g/m\({}^{3}\) of the BAP were assayed additionally to the standard dose already assayed in experiment 1. Furthermore, a treatment was also assayed where the standard dose of 50 g/m\({}^{3}\) was divided into 10 equivalent dosages (5 g/m\({}^{3}\) each) that were injected in 10 different places of the sediment, in order to assess the effect of a higher spatial spread of the inoculum. Experiment 3 (temporal dosage of the product): These treatments also had a BAP optimization target, in this case, to check the requirement of temporal maintenance of the BAP additions in a natural environment with long renewal times.