风场对太湖梅梁湾水华及营养盐空间分布的影响

为了解风场对湖泊表层蓝藻水华及营养盐空间分布的影响,以太湖梅梁湾为例,在蓝藻水华期间开展表层粒子漂流实验,研究风场对水体表层物质的推移规律,并开展全水域水体表、中、底层密集布点采样,测定水体藻类叶绿素a、氮、磷、高锰酸盐指数、溶解性有机碳、溶解氧等水质指标,探讨风场驱动下大型浅水湖泊蓝藻水华及营养盐时空分布特征.结果表明,在平均风速1. 9 m·s~(-1)和2. 3 m·s~(-1)的情况下,表层粒子的平均漂移速度分别为3. 0 cm·s~(-1)和5. 0 cm·s~(-1);风场对表层水体蓝藻水华的空间分布具有决定性影响,能够引起蓝藻水华在空间上较高的异质性;蓝藻水华物质的空间变化对水体颗粒态氮、磷、有机质和溶解氧等水质指标产生较大影响,表、中、底层颗粒态氮和磷、高锰酸盐指数与叶绿素a浓度的空间分布一致,而溶解态氮、磷浓度及溶解性有机碳的分布与叶绿素a浓度分布不尽相同;蓝藻水华物质在风场作用下的再分配对水体溶解氧产生复杂的影响,底层溶解氧平均值低于表层与中层,可能对沉积物营养盐释放产生影响;依据高密度布点调查估算,仅表层20 cm,梅梁湾水域的蓝藻干物质赋存量约396 t,远大于蓝藻打捞工程的清除量.研究表明,鉴于水华期间风场作用下对蓝藻水华漂移的巨大影响,在湖泊水质调查采样方法及数据分析时应充分考虑蓝藻水华漂移的影响因素;防控湖泛灾害的蓝藻打捞作业对湖体蓝藻水华赋存量的清除能力有限,只能对岸边带湖泛的预防产生影响.

Wind Field Influences on the Spatial Distribution of Cyanobacterial Blooms and Nutrients in Meiliang Bay of Lake Taihu, China

Wind field is a very important physical factor controlling the formation of cyanobacteria blooms. A surface particle tracking drift experiment was carried out to study the influence of wind field on the surface current in Meiliang Bay of Lake Taihu during the algal bloom season. For this, chlorophyll-a, nitrogen, phosphorus, the permanganate index, dissolved organic carbon (DOC), and dissolved oxygen (DO) were measured in surface, middle, and bottom waters of the Meiliang Bay during the cyanobacteria bloom period to test how wind field affects the temporal and spatial distribution of cyanobacterial blooms and biomass stock in the water column. The results showed that the average drift velocities of surface particles were 3.0 cm·s^-1 and 5.0 cm·s^-1 when wind speed averaged 1.9 m·s^-1 and 2.3 m·s^-1, respectively. The wind field determined the spatial distribution of cyanobacterial blooms in surface waters and led to a high spatial heterogeneity of cyanobacterial blooms. The spatial redistribution of cyanobacterial blooms exerted an important influence on water quality indexes such as particulate nitrogen, phosphorus, organic matter, and dissolved oxygen. The concentrations of particulate nitrogen, phosphorus, the permanganate index, and chlorophyll-a showed a similar vertical distribution pattern. Cyanobacterial blooms were less influenced by the distribution of dissolved nitrogen and dissolved organic carbon from external pollution, while long-term legacy loading played a more important role. This meant that the spatial distributions of dissolved nitrogen and dissolved organic carbon were different from that of chlorophyll-a. Because the redistribution of cyanobacterial blooms, as affected by wind fields, has a complex effect on the dissolved oxygen in the water column, the dissolved oxygen concentration decreased with depth, which may affect the release of soluble nutrients from the sediment. The cyanobacterial biomass stock in the surface water was estimated according to the survey of high-density sites. The dry matter of cyanobacteria in the surface 20 cm of Meiliang Bay was approximately 396 tons on the day of sampling. The results from the present study indicated that the factors influencing cyanobacterial blooms should be considered in sampling methods and the analysis of lake water quality due to the significant influence of wind fields on bloom drift. The collection of cyanobacteria has limited effect on the removal of the algal bloom biomass in whole lake, only being effective at prevention of the event of black spots in lake shore.