Nitrogen exchange across the sediment-water interface after dredging: The influence of contaminated riverine suspended particulate matter.

Dredging has been widely implemented in shallow lakes to reduce internal nitrogen (N) loading. The suspended particulate matter (SPM) coming from polluted rivers usually contains high levels of N and ultimately deposits on the dredged sediment surfaces near the river mouth. To study the influence of the riverine SPM on N exchange across the sediment-water interface (SWI) after dredging, a 360-day experiment was carried out comparing un-dredged and dredged sediments from Lake Chaohu, China. Dredged treatments showed a significant increase (p < 0.01) in total N concentrations in the sediments, while the deposition of SPM had little influence on labile NH-N concentrations. In addition, NH-N concentrations in pore-water and NH-N fluxes were significantly lower in dredged than in un-dredged sediments, despite the deposition of SPM. The oxygen production rates and the oxygen penetration depth in the dredged sediments were both higher than those in the un-dredged sediments. The increase of Nitrospira in dredged sediments was consistent with their decreased NH-N concentrations and fluxes across the SWI. Therefore, the oxidizing condition, increased oxygen production/consumption rates and Nitrospira relative abundance across the SWI were believed to be correlated with the low N exchange rates in dredged sediments. Dredging for reducing internal N loading in a river mouth area is therefore feasible, although the influence of the riverine SPM should be taken into account when aiming to achieve a long-term internal N loading reduction.