Characteristics and mechanisms of nitrogen exchanges across the sediment-water inter-face of the macrophyte-and algae-dominated areas under the influence of suspended particulate matter deposition
The continuous enrichment and deposition of suspended particulate matter(SPM)aggravate the internal loadings from the sediment of eutrophic lakes.In this study,a monthly field investigation for one year was conducted in the macrophyte-dominated area(MDA)and algae-dominated area(ADA)of Lake Taihu.The nitrogen(N)contents,fractions and sediment exchange char-acteristics of the sediment-water interface(SWI)in water samples,SPM and sediments were analyzed.The fraction transformation and transport of N across the SWI affected by the SPM deposition in both areas were investigated.The results showed that the annu-al variation ranges of total nitrogen(TN)in the water of the MDA and ADA were 0.50-1.51 mg/L(average was(0.97±0.23)mg/L)and 1.77-6.12 mg/L(average was(3.67±1.36)mg/L),respectively.The ADA showed higher concentrations and sea-sonal variations of various N fractions than those of the MDA.The annual average TN concentrations in the SPM of the MDA and ADA were(6998.48±1413.87)and(5162.63±2205.13)mg/kg,and in the sediment of the MDA and ADA were(4219.75±216.62)and(3187.88±103.07)mg/kg,respectively.The MDA showed significantly higher TN concentrations in both the SPM and the sediment than those of the ADA,with significantly higher TN in the SPM than in the sediment of both areas.NH3-N was the dominant fraction of available N in the SPM and sediment of both areas.The release of NH3-N from the sediment in the ADA mainly occurred in summer,while that in the MDA mainly occurred from summer to autumn.The continuous deposition of the SPM with high concentrations of available N was believed to be an important reason for the transformation of N concentrations and frac-tions in the sediment.The proportion of available N in the ADA(5.17％)was significantly higher than that in the MDA(4.34％)during algal bloom seasons.The shift from a macrophyte-dominated system to an algae-dominated system might increase the concen-tration of SPM in the water,thereby intensify the resuspension of sediment particles.Consequently,the alternated deposition and resuspension processes of SPM would exacerbate the N exchanges across the SWI.Therefore,more attention should be paid to the role of SPM during future management of internal N loadings in lakes.
Lake Taihumacrophyte-dominated areaalgae-dominated areasuspended particulate matterinternal nitrogen loading
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