三峡水库支流拟多甲藻水华的形成机制

三峡水库支流的拟多甲藻水华发生已成为近年的常态,但是其发生机制依然不清楚。2009年到2011年期间,对拟多甲藻水华频发的童庄河回水区进行重点监测,并扩展到三峡水库长江干流及26条支流未淹没区和回水区浮游植物的四季调查。结果表明,四季调查中,拟多甲藻出现率、平均密度、最大密度和最大优势度,4月最高,相同季节,回水区最高。2—4月,童庄河回水区各点拟多甲藻密度均为升高到下降变化,最高密度自上游至下游大幅下降。2010年,拟多甲藻水华于2月最先在童庄河回水区上游出现,逐步向中下游发展,维持一定规模和时间后消退。与此同时,长江干流拟多甲藻出现率、密度和最大优势度大幅低于童庄河回水区。童庄河回水区发生(2005年、2007年和2010年)比不发生(2009年)拟多甲藻水华年份,三峡水库1—3月日出库流量(平均值4489.9—5623.3 m3/s)较小、1—2月水位日变幅(平均值0.148 m、0.246 m)和2月水位日升降(平均值-0.223 m)较大。研究认为,藻类在适宜水温时形成水华,2—4月支流回水区水温适宜、氮磷含量满足甲藻营养能力和生活习性需要。拟多甲藻能否形成水华,首先取决于回水区上游是否存在满足要求的水动力条件。其次,三峡水库出库流量、水位波动等水文条件,通过改变回水区上游水动力条件和回水区流速,影响水华能否形成及形成时间、程度、范围和维持时间。由于尺度效应不同,三峡水库相同水文条件对不同位置支流拟多甲藻水华形成的影响不一样。

The mechanism for occurrence of Peridiniopsis blooms in the tributaries of Three Gorges Reservoir

Recent occurrence of Peridiniopsis blooms has become a routine phenomenon in the tributaries of the Three Gorges Reservoir, however the mechanism under them is still unknown. In the present study, ecological surveys on phytoplankton were intensively performed in the Peridiniopsis blooms occurring Tongzhuang River, then extending to the main stem of the Yangtze River and 26 tributaries in Three Gorges Reservoir during 2009 to 2011. The results showed that the occurring frequency, average and maximum densities, and maximum dominance of Peridiniopsis sp. were highest in backwater areas of the tributaries in April. During February to April, the density of Peridiniopsis sp. had a pattern with initial increase and later decreased at each sites in backwater area of Tongzhuang River, and the highest density of Peridiniopsis sp. dramatically decreased from the upstream to the downstream. In 2010, Peridiniopsis bloom firstly occurred in the upstream of backwater area of Tongzhuang River in February, gradually developed into the downstream, and disappeared after a long duration. The occurring frequency, density, and maximum dominance of Peridiniopsis sp. in main stem of Yangtze River were far lower than those in backwater area of Tongzhuang River on the same seasons. Compared with the year without occurrence of Peridiniopsis bloom, the years with Peridiniopsis blooms were characterized with lower daily water discharge out of the Three Gorge Reservoir (average values range from 4489.9 to 5623.3 m3/s) from January to March, and with higher daily water level amplitude in January (average value is 0.148 m) and February (average value is 0.246 m) and daily water level lift (average value is -0.223 m). The results demonstrated that water temperature in backwater area of tributaries from February to April was optimal to the occurrence of the Peridiniopsis blooms, indicating the importance of water temperature triggering the blooms. The nitrogen and phosphorus levels were shown to meet the capability of nutrient and life habit of Peridiniopsis sp.. It was concluded that formation of Peridiniopsis blooms in the Three Gorges Reservoir should depend on the following conditions: hydrodynamic conditions in upstream of backwater area, hydrological conditions such as water discharge out of Three Gorges Reservoir and water level variation by changing flow rates in backwater area and hydrodynamic conditions in its upstream to influence the occurrence, timing, extent, scale and duration time of the blooms. Due to the effects by the different scales, the influence of same hydrological conditions of the Three Gorges Reservoir on occurrence of Peridiniopsis blooms at different sites were not the same.