The Distribution Characteristics Of Nutrients In The Sediments Of The Typical Accumulation Area Of ​​algae Blooms In Lake Taihu

Nitrogen,phosphorus and other nutrients in sediments might come from land-based sources and lake debris particles.After the physical,chemical and biological processes,these nutrients can released into the overlying water,which make the eutrophication more serious.Even though the exogenous pollution has been controlled,the accumulation and deposition of algae might become the main source of water pollution.The study on the distribution characteristics of nutrients in sediments of the typical algae accumulation area will guide the treatment of eutrophic lakes.In this paper,the typical algae accumulation area in the west of Taihu lake was selected as the key research area.Sediment cores were collected during the vigorous growth period of cyanobacteria.The sediment sampling consisted of two sections:（1）from the trench to the lake,including the trench,the trench peripheral and the lake,（2）from the reed community in natural slope to the lake,with the concentration of TN,NH₄⁺-N,NO3--N,TP,IP,Fe/Al-P,Ca-P and TOC were determined and analyzed to discuss the distribution characteristics.Results were as followed:（1）Distribution characteristics of the nitrogen in sediments:in the horizontal direction,compared with the offshore area,the trench had weak hydrodynamic conditions,large amounts of water algae gathered from the lake area,while the backtracking was relatively hard,which led to the serious accumulation of algae residue in the trench.Nitrogen could release with the decomposition of algae residue.The content of TN in the surface 2 cm was up to 3825.22 mg/kg,significantly higher than that in the trench peripheral（772.23 mg/kg）and the lake（661.29 mg/kg）.From the reed community area to the lake,the content of nitrogen in the surface had a gradually reduce.Both the reed community area and the trench were parallel to the dam.But,without the blocking functions of the offshore dam and wave-dissipating stakes,cyanobacteria might have a relative easy imput but hard output from the reed cluster,which made the TN concentration in the reed community area higher than the trench area,the content of TN was up to 4693.90 mg/kg.In the vertical direction,the content of nitrogen was high in the sediments of the trench from the surface to the bottom.The TN concentration ranged from 2642.50 mg/kg to 4983.30 mg/kg with an average of 3064.80 mg/kg.NH₄⁺-N was averaged of 1171.60 mg/kg,which was higher than previous reports in western and northern in Taihu lake.The surface concentration of TN and NH₄⁺-N reached 772.23 mg/kg and 143.13 mg/kg in the sediments of the outside of the trench.While,4 cm below the surface sediment,the TN and NH₄⁺-N decreased to 504.49 mg/kg and 97.23 mg/kg respectively with the effect of wind waves and hydrodynamic force.Under the influence of the lake power and wind waves,accumulation of algal residues reduced in sediments of the lake,the water-sediments interface had a large exchange capacity,which led to the decrease of nutrients load,the nutrient content decreased,the average concentration of TN and NH₄⁺-N were 661.29 mg/kg and 50.39 mg/kg.Therefore,the typical algae accumulation area in the west of Taihu lake was beneficial for the nitrogen accumulation,more and more attention should be paid to in the management of eutrophic lakes.The reed community area in natural slop was parallel to the dam,and had gently terrain slopes.With neither the effect of the closure by trench nor wave-breaking piles,in this sense,the cyanobacteria gathered with the wind direction and accumulated under the effect of reed blockage.The concentration of TN and NH₄⁺-N reached 6585.34 mg/kg and 1148.25 mg/kg in the surface layer.Respectively,which were much higher than those in the trench.However,different from the trench,the accumulated algae debris was relatively thin and TN and NH₄⁺-N decreased sharply to 530.70 mg/kg and 111.88 mg/kg in the below 2 cm.（2）The distribution characteristics of phosphorus concentration in surface sediments were in consistent with the distribution of nitrogen.In the horizontal direction,the content of TP in the surface 2 cm was 982.46 mg/kg,significantly higher than that in the trench peripheral（877.29 mg/kg）and the lake（829.03 mg/kg）,higher than the reported TP content in the sediments of other eutrophic lakes,and the west and northwest coast of Taihu.The content of TP in the reed community area in natural slop was 1316.56 mg/kg,higher than that in the trench.In the vertical direction,the TP content in the trench ranged from 527.55 mg/kg to 1184.39 mg/kg,vertical change was small.Fe/Al-P accounted for 55%of TP concentration,while Ca-P accounted for less than 18%generally.It indicated that the sediment phosphorus in the trench was dominated by active phosphorus.Extending from the trench to the direction of the lake,the TP content in the surface sediments of the trench peripheral was 877.29 mg/kg.Below the surface 2cm,TP content was sharply reduced to less than 532.82 mg/kg.Concentration of TP in the lake was obviously lower than that of the trench and the outside of the trench,and gradually decreased from the surface to the bottom.Concentration of TP（1316.56 mg/kg）in the reed community area was higher than in the trench,below the surface quickly dropped to below 418.49 mg/kg and significantly lower than that in the trench.（3）The concentration of TOC in the surface sediments was in consistent with the distribution of nitrogen and phosphorus.In the reed community area,the highest surface content of TOC was 82303.00 mg/kg.Vertically,the concentration of TOC in the trench ranged from 25263.00 mg/kg to 39365.00 mg/kg with an average of 25515,00 mg/kg.In the trench peripheral,the sediment TOC was 13877.50 mg/kg,which was less than that in the trench.Extending from the outside the trench to the lake,the content of TOC decreased significantly from the surface to the bottom,and lower than in the trench and the trench peripheral.The surface sediments of TOC in the natural reed cluster was higher than that of the trench,the lower is rapidly decreased,significantly lower than the trench.In this paper,the reed community area and the trench in the west coast of Taihu were areas for the accumulation of cyanobacteria.These places were the reservoirs of nutrients,which might provide endogenous pollution for the lake water if the management was not timely.This study might get primary knowledge of the nutrients Ioad of the typical algae accumulation areas in Taihu,and in turn provided basis for scientific management of eutrophication.