The corner of Bao'an Lake (Credit: IHB) 

The research group led by Prof. WANG Hongzhu from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences revealed that high nitrate loading may have dual effect on sediment phosphorus (P) release. The study was published in Water Research.    

Control of sediment P loading is one of the main bottlenecks encountered in eutrophication management, especially in shallow lakes. Even when external P sources are effectively controlled, internal P release from the sediment to the overlying water can sustain eutrophication for decades. 

Several restoration approaches have been developed to reduce the sediment P loading. Nitrate addition treatment is one of the methods that has proved to be effective in preventing sediment P release in stratified lakes. However, it is unclear whether nitrate addition to shallow lakes can effectively inhibit P release from sediments.  

To understand this, researchers conducted a nitrate-addition experiment in a mesocosm system simulating shallow lake conditions. 

The results indicate that high nitrate may have a dual effect by inhibiting sediment P release through improving the sediment's oxidation and increasing P release by promoting phytoplankton growth. 

Combining total P concentration data and in situ measurement of labile P at the sediment-water interface, they found that nitrate had no effect on the sediment P release when it was maintained at high levels. This is likely due to the counteracting effects of promotion and inhibition.  

After nitrate addition was terminated, nitrate dropped to nearly zero, followed by a significant P release in the high nitrate treatments occurred. This is probably due to a decrease in the inhibitory effect caused by nitrate. 

The promotion effects of nitrate loading on sediment P release were dose-dependent, being relative clear when nitrate > 5–7 mg/L.  

The findings suggest that adding nitrate to the water is not recommended for shallow lakes to control P release from sediment, especially under the increased pressure that global climate change will have on lake ecosystems.