沉积物参与下氮磷脉冲式输入对太湖水体营养盐浓度和藻类生长的影响

Influence of Nutrient Pulse Input on Nitrogen and Phosphorus Concentrations and Algal Growth in the Sediment-Water System of Lake Taihu

湖泊沉积物既是氮磷等营养物质的储存库,也是水体营养盐的二次污染源,可以缓冲水体氮磷浓度变化,进而影响水体营养盐的生物可利用性和藻类生长.本文以太湖梅梁湾为研究对象,通过模拟实验研究沉积物参与下外源氮磷脉冲式输入对水体营养盐浓度和藻类生长的影响,并阐明氮磷在沉积物、水和藻类间的迁移转化及再分配过程.结果表明,当以0.30 mg·(L·d)^-1的速率脉冲式输入氮时,实验组(有沉积物)水体氮浓度远低于相应的对照组(无沉积物),沉积物参与下水体氮约以0.144~0.156 mg·(L·d)^-1的速率脱除,根据单位面积估算水体脱氮速率约为40.793~44.193 mg·(m^2·d)^-1,脱氮量约占外源氮的48%~52%;而相应对照组水体约以0.021~0.039 mg·(L·d)^-1的速率脱氮,脱氮量仅占外源氮的7%~13%,可见沉积物-水界面作为浅水湖泊反硝化等脱氮过程的主要场所,对减轻湖泊氮负荷具有重要贡献.当以0.015 mg·(L·d)^-1的速率脉冲式输入磷时,沉积物表现出明显的"汇"效应,约52%~58%外源磷以2.210~2.422 mg·(m^2·d)^-1的速率汇入沉积物,其余约23%~26%外源磷被藻类吸收,约20%~22%则以溶解态存在水体,可见沉积物的参与能有效地缓冲水体磷浓度对外源磷的响应.无外源输入时,沉积物充当磷源,以约0.310~0.468 mg·(m^2·d)^-1的速率释放磷供给藻类生长.薄膜梯度扩散技术(ZrO-Chelex DGT)原位高分辨分析显示,沉积物间隙水中有效态磷浓度远高于上覆水,并与二价铁显著相关,表明受铁结合态磷的影响,沉积物-水界面氧化还原状况发生改变会造成内源磷的大量释放.总的说来,在外源得到有效控制时,沉积物中的磷可以缓慢释放进入上覆水中并供给藻类生长,延滞水体对外源控制的响应.因此,在湖泊蓝藻水华治理时,氮磷协调治理可以起到更快的治理效果.

Lake sediments not only act as a reservoir of nutrients,but are also a source of secondary pollution of nutrients for overlying water,which can buffer the variations in nutrients in overlying water and affect nutrient bioavailability and algal growth.In the current study,a simulation experiment was conducted using sediment cores collected in Meiliang Bay.Our aim was to elucidate the effect of nitrogen(N)and phosphorus(P)pulse input on variations in the water nutrient level and algal growth.We also clarified the migration and redistribution process of N and P between the sediment overlying water and algae.The results showed that the concentration of N in overlying water of the treatment group(with sediment)was much lower than that in controls(no sediment)when N was input at a pulse rate of 0.30 mg·(L·d)^-1.The loss rate of N in the overlying water of the treatment group ranged from 0.144 mg·(L·d)^-1 to 0.156 mg·(L·d)^-1 and that in the control ranged from 0.021 mg·(L·d)^-1to 0.039 mg·(L·d)^-1.On the contrary,the denitrification rate of overlying water in the treatment group ranged from 40.793 mg·(m^2·d)^-1 to 44.193 mg·(m^2·d)^-1,accounting to 48%-52%of the external N loading.In contrast,the denitrification rate of overlying water in controls was from 0.021 mg·(L·d)^-1to 0.039 mg·(L·d)^-1,only accounting for 7%-13%of the external N loading.These results indicated that the sediment-water interface is the main site of denitrification in shallow lakes and plays an essential role in reducing N pollution in lakes.With respect to the pulse input of P at a rate of 0.015 mg·(L·d)^-1,the majority of P(about 52%-58%)was imported into the sediment at a rate from 2.210 mg·(m^2·d)^-1to 2.422 mg·(m^2·d)^-1,and only a small proportion,approximately 23%-26%,was utilized by algae.The remaining P existed in overlying water in a dissolved state.These results implied that the sediment can buffer the external P input as an obvious"sink"effect of nutrients.Our results also showed that the sediment acts as a"source"of P when no external P was added.The release rate of P from the sediment to overlying water was from 0.310 mg·(m^2·d)^-1 to 0.468 mg·(m^2·d)^-1.In situ high-resolution analysis of ZrO-Chelex DGT showed that the DGT-P concentration in the interstitial water was much higher than that in the overlying water,and the concentration of DGT-P was significantly correlated with the concentration of DGT-Fe in interstitial water.These results indicate that changes in the redox potential may cause considerable release of internal phosphorus.In summary,our study showed that internal P in sediments can be released into the overlying water and support the growth of algae when the external nutrients are controlled.As a result,a delayed response was observed in the nutrient concentration in overlying water to external P reduction.Therefore,the dual control of N and P may have a better practical application to mitigate cyanobacteria blooms in shallow lakes.