Effects Of Cyanobacterial Decline On Nitrogen Species At The Sediment-water Interface In Taihu Lake And Its Mechanism

The sediment of eutrophic lakes is important for maintaining lake stability.The algal decomposition will significantly change the physicochemical properties and nitrogen composition of sediment-water interface in a short period of time.Meanwhile,the difference in the composition of the nitrogen conversion microbial community in the sediments of macrophyte-or algae-dominated zones leads to different biochemical response mechanisms in response to the impact of algal blooms.The repeated outbreak and decomposition of algal blooms will change the nitrogen cycling process,which will lead to shift between alternative stable states of lake ecosystem.In this study,taking Lake Taihu as an example,field investigation and laboratory simulation were employed.The nitrogen vertical distribution of sediment-water interface in typical ecotypes were measured via high resolution passive sampling techniques.The effects of the agal decomposition on the physicochemical properties and nitrogen components of macrophyte-or algae-dominated zones were analyzed.The differences in the composition of the microbial community in the sediment nitrogen conversion of macrophyte-or algae-dominated zones were analyzed through molecular biology techniques.The response mechanism of different nitrogen conversion microbial community composition in macrophyte-or algae-dominated zone sediments to the algal decomposition was discussed.The main conclusions are summarized as follows:(1)Field investigations on sediments at different ecosystem in Lake Taihu found that the p H values of sediment were between 6.68 and 8.05.Except for the site disturbed by wind and waves,the oxygen penetration depth(OPD)of the sediments in other lake areas was between 8.0 and 12.7mm.The range of diffusion fluxes of nitrate,nitrite and ammonium at sediment-water interface in difference ecosystem were-67.31?-27.38 mg m^-2-d^-1,-14.30?-2.57 mg m^-2-d^-1 and 0.97?3.43 mg m^-2-d^-1,respectively.The nitrifying bacteria in the sediments of Lake Taihu were mainly Ellin6067.The main denitrifying bacteria are Sphingobium,Bacillus,Gemmatimonas and Rhodobacter.The highest abundance of denitrifying bacteria was recorded in the sediment from depth of 6?9 cm at algae-dominated zones,which was 50.81×10⁴ copies g^-1.(2)The physicochemical properties and vertical distribution of nitrogen in macrophyte-or algae-dominated zones sediments were significantly influenced by the algal decomposition.The algal decomposition experiment showed that the algal decomposition process will significantly reduce the range of p H value of the overlying water from 7.78?8.51 to 7.13?7.94.Meanwhile,the p H value of the sediment has always been maintained between 6.64 and 7.24.The algal decomposition will significantly reduce the oxygen concentration and OPD of sediments in macrophyte-dominated zones(MT)and the algae-dominated open water zones(AT1),while the OPD of sediments in the algae dominated littoral zones(AT2)is always maintained at a low value(2.0?3.0 mm)during the algal decomposition.The concentration of nitrate,nitrite and ammonium in the pore water of AT2 were significantly increased during the algal decomposition.On day 14,the algal decomposition process would significantly increase the downward diffusion flux of nitrite at the sediment-water interface,from-2.27?-1.56 mg m^-2 d^-1 to-23.88?-12.36 mg m^-2 d^-1.On day 42,the algal decomposition process could significantly increase the transformation ability of nitrate and nitrite in MT and AT1.In addition,the dissolved organic matter(DOM)content of sediments with different nutritional levels was significantly different.The content of humus-like DOM in surface sediments of different ecotypes was approximately the same and in deep sediments was MT<AT1<AT2.(3)The different composition of nitrogen transforming microbial community in macrophyte-or algae-dominated zones sediments resulted in different responding mechanisms during the algal decomposition.From the experiment of algal decomposition,it demonstrated that the algal decomposition had no significant impact on the structure of the nitrogen cycling microbial community in sediments,but there were significant differences in the structure of the nitrogen cycling microbial community among the macrophyte-or algae-dominated zones sediments.The total microbial abundance in the sediments was MT<AT1<AT2.The relative abundances of ammonia oxidizing bacteria(AOB),nitrite oxidizing bacteria(NOB)and denitrifying bacteria(DNB)in MT sediments were 33.26%,35.42%and 31.33%,respectively,indicating that the nitrogen cycle process could proceed completely.The relative abundance of AOB,NOB and DNB in AT1 sediments were 36.76%,11.63%and 51.62%,respectively,indicating that the NOB niche was restricted.The relative abundance of DNB in AT2 sediments were reach up to 81.28%.Meanwhile,the relative abundance of AOB and NOB were 15.46%and 3.27%i,respectively,which indicating that the nitrification process was severely restricted in the sediments under strong reduction conditions,and the microbial nitrogen cycling process was dominated by denitrification.