Abstract
Estuarine zones are regarded as the ecotones connecting the rivers and lakes. Sediment dredging is a conventional treatment technology that is widely used to remove the internal loading in estuarine zones worldwide. However, what is the characteristic of nutrient release in the gate-controlled estuary and how long this practice is effective are still unclear. Hence, sediment and water samples were collected from dredged and undredged regions around the gate-controlled estuary of northern Taihu Lake for laboratory experiments, in which they were subjected to different temperatures, depths, and disturbance levels. The total nitrogen (TN) and total phosphorus (TP) concentrations of the dredged region were lower than those from the undredged region under stable hydrodynamic conditions. A high dynamic release rate (R) of nutrients in the dredged sediments (RTN = 164.75 mg/m2·d and RTP = 5.83 mg/m2·d) existed under dynamic release conditions (stirring speeds: 90 and 120 r/min). The effect of disturbance and temperature on release rate and nutrient form was completely different for the static and dynamic release cases. The nutrient loads from dynamic release were 4–17 times greater than those from static release. For unstable hydrodynamic conditions, the release rate from the bottom sediment exceeded that from the surface sediment in the undredged region. These results indicated that, under stable hydrodynamic conditions, dredging improves long-term water quality. However, dredging alone in unstable hydrodynamic conditions may not remove the potential risk of internal release in the long term. Specific ecological and engineering measures should be combined with dredging practice to restore estuary habitats and minimize the release of internal pollutants.
Highlights
Sediment is considered an important nutrient source and/or sink in waterbodies [1,2,3]
A better water quality was observed in the dredged region, but its correction of the Nemerow pollution index (CNPI) values were still classed as moderate pollution (CNPI > 1), indicating that other sources of nutrients may be present in this area. e field investigation and data analysis revealed that gate construction in the estuary may have aggravated the pollution level. e gate blocks some of the connectivity between the rivers and lakes, preventing nutrient cycling, material transportation, and energy exchange [36, 38, 54]
At high temperatures (30°C), the release rate from the bottom sediment exceeded that from the surface sediment. is indicated that a high risk of nutrient release still remained when the surface sediment was removed
Summary
Sediment is considered an important nutrient source and/or sink in waterbodies [1,2,3]. When hydrodynamic conditions are stable and nutrients and other pollutants are introduced into lakes by river systems, sediments can serve as a sink through the adsorption, sedimentation, and mineralization of particles [4, 5]. These natural conditions are continually changed by wind-wave action, flooding/flushing, dissolved oxygen, pH, water temperature, and anthropogenic activities [6,7,8,9], which may cause nutrients to be released from sediments and suspended matter [10].
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