The quality of dissolved organic carbon (DOC) affects both carbon cycling in surface waters and drinking water production. Not much is known about the influence of environmental conditions on DOC quality. We studied the effect of redox conditions on the chemical composition of DOC in a drinking water reservoir by Fourier transform ion cyclotron resonance mass spectrometry in combination with sediment core incubation experiments under manipulated redox conditions. We observed clear differences in DOC quality among oxic epilimnion, anoxic hypolimnion, and sediment porewater. Sediment porewater showed relatively high intensities of polyphenol-like components with H/C ratios of <1 and O/C ratios of >0.6. Consistent with this, anoxic incubation of a sediment core resulted in an accumulation of these components in the overlying water. The observed pattern of DOC quality change can be explained by redox-dependent adsorption/desorption of DOC on iron minerals. Under oxic conditions, the polyphenol-like components bind on freshly formed iron hydroxides, a process that affects both DOC stability in surface waters and treatability during drinking water production.

The trend of increasing dissolved organic carbon (DOC) in surface waters motivated us to gain a mechanistic understanding of DOC exchange at the sediment water interface in lakes. We quantified seasonal DOC and solute fluxes under different redox conditions, in a small drinking water reservoir using sediment core incubations. Processes governing benthic DOC exchange were microbial production of DOC and interaction with mineral surfaces. Mobilization of DOC in anoxic sediments seemed to be closely linked to reductive dissolution of ferric minerals as shown by the strong positive correlation (r 2 5 0.99) between DOC and Fe fluxes. Oxidized surface sediments were an efficient DOC trap where DOC was bound to ferric minerals. Redox conditions appeared to be the primary regulator of the DOC exchange, resulting in sedimentary uptake of DOC (21.8 mmol m 22 d 21 ) only under oxic conditions. DOC production was regulated by temperature, leading to higher DOC fluxes (up to 2.4 mmol m 22 d 21 ) in summer. The sediment was a net sink of DOC (20.2 tonnes yr 21 ) but this was small compared with the annual DOC load (15 tonnes yr 21 ) of the reservoir. However, the benthic DOC flux was about 10-20% of the dissolved carbon flux at the sediment-water interface, making it a significant process in the lake internal carbon cycle. Climate change is supposed to promote reducing conditions at the bottom of lakes, which might increase benthic DOC production in the future.

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Microplastics are ubiquitous in standing freshwater bodies, consequently lakes and reservoirs may be important sinks for these contaminants. However, the mechanisms governing the deposition of microplastics and their interactions with the sediments are understudied. We demonstrate how aggregation-based transport facilitates the sinking and infiltration of buoyant microplastics into freshwater reservoir sediments by employing experiments with intact sediment cores. Buoyant polyethylene microplastics were rapidly (1–4 h) incorporated into sinking iron-organic aggregates, followed by swift deposition into sediments. Ingression of microplastic bearing flocs into sediments was completed within 6 days and led to stable deposition of the incorporated particles for at least 2 months. Most microplastics were deposited in the top 2 cm of the sediments and few particles (5–15%) were re-released into the water. Our results show at least 85% burial of microplastics, indicating the significant role of freshwaters with low flow velocities in reducing microplastic loads to the oceans.