蓝藻腐解对水中营养盐影响的模拟研究

为研究不同密度藻类腐解过程对水体营养盐再分配的影响,在太湖藻类聚集区采集沉积物柱状样进行室内加藻模拟实验,共设置B、C、D 3组加藻密度分别为2500、7500和15000 g·m-2的处理组,监测蓝藻腐解过程中上覆水体的溶解氧(DO)、氧化还原电位(Eh)、氨氮(NH_4~+-N)、总氮(TN)及总磷(TP)等理化指标的变化情况.结果表明,蓝藻腐解会使水体DO和Eh迅速下降,最终分别维持在0.1 mg·L~(-1)左右和-300~-400 mV之间,水体处于厌氧强还原状态;同时,厌氧强还原条件又加速了蓝藻死亡腐解过程,促进了藻体营养盐向上覆水的扩散,B、C、D组营养盐含量分别于实验第6、第10和第14 d达到峰值,TN平均增长速率分别为26.67、43.41和67.82 mg·L~(-1)·d~(-1),TP平均增长速率分别为3.30、5.53和8.35 mg·L~(-1)·d~(-1),NH_4~+-N浓度最大值分别为对照组的7、51、125倍,水体形成明显的氮磷污染负荷,且蓝藻衰亡导致的上覆水TN浓度升高持续时间较TP浓度升高持续时间要长.研究表明,蓝藻水华腐解过程中向上覆水体释放了大量的有机物和可溶性营养盐,降低了水体透明度,加剧了水体富营养化状况,诱发湖泛发生,也为蓝藻水华再次爆发提供了物质基础.

Simulation of cyanobacteia decay's impacts on nutrients in Water

To reveal the influence of algal decay processes with different densities on the redistribution of dissolved nutrients, sediment cores were sampled from algae accumulation region in Lake Taihu and were incubated with algae in the laboratory. A series of cyanobacterial densities of 2500, 7500 and 15000 g·m^-2, set as B, C and D group, respectively, was added to continuously monitor the concentration variations of dissolved oxygen (DO), oxidation redox potential (ORP), ammonium (NH₄⁺-N), total nitrogen (TN), total phosphorus (TP) and other physical-chemical parameters of the overlying water. Results show that DO and ORP rapidly decreased during the algal decay processes and eventually maintained at 0.1 mg·L^-1 and -300~-400 mV, respectively, suggesting the overlying water in a anaerobic and strong reduction state. Simultaneously, it accelerated the decomposition of cyanobacteria and promoted the release of cyanobacterial nutrients to overlying water. The average nutrient contents of B, C and D groups peaked on the 6th day, the 10th day and the 14th day, particularly, the average increasing rate of TN was 26.67, 43.41 and 67.82 mg·L^-1·d^-1 and TP was 3.30, 5.53 and 8.35 mg·L^-1·d^-1, respectively. The maximum concentration of NH₄⁺-N was 7, 51 and 125 times of that of the control group, respectively. The algal decay processes caused an obvious pollution load of nitrogen and phosphorus to the overlying water, additionally, the duration of TN concentration increase in the overlying water was longer than that of TP. These observations clarified that the release of large amounts of organic matters and dissolved nutrients during the processes of cyanobacteria bloom decay. It reduced the transparency of lake water, intensified the eutrophication and induced the black bloom and provided the material basis for the re-emergence of cyanobacteria bloom.