太湖水华成因及控制途径初探

ALGAL BLOOM IN TAIHU LAKE AND ITS CONTROL

1990年8～12月对太湖水华9次调查表明,水华主要由漂浮性蓝藻—微囊藻组成,夏季SE风时其漂移集聚是西北湖区形成严重水华的主要原因。这种漂移使得藻类所含营养物逆吞吐流方向传输,形成了一种“生物营养滤器”,加速了太湖尤其是西北湖区的富营养化进程。太湖外源N、P负荷量分别为12.0和0.85g/(m^2·a),足以引起富营养化;表层5cm底泥中含丰富的N、P,其释放也成为湖水中部分营养的来源。因2～8月藻类总生物量的增长基本遵守Logistic方程dN/dt=N·r(1—N/K),故对藻类控制可从N、r入手,即通过收获藻类达到控制藻类总量和营养输出的双重功效;提高水位增加非光合层厚度,有效地降低水柱中藻类生产力;建立有水生植被的水质保护区也是一种局部藻类控制方法。

In Taihu Lake, algal bloom is caused by blue-green algal microcystis, occuring in summer and autumn, and distributing mostly in the northwestern area. This lake has nutrient loading of 12. 0 g N/(m2 ?a)and 0. 85g P/(m2 ?a), and a nutrient store of 62. 3 g N/(m2 ?a)and 41. 0 g P/(m2 ?a)m in the 5 cm surface layer of sediments. Due to its shallowness and strong disturbance of wind-waves, the lake provides sufficient nutrients for the algae. In summer, the algae has a daily P/B ratio of 0. 92, a net productivity equal to 20960 t/d of fresh algae, and consumes as much as 167. 3 t N and 15. 85t P every day. Wind-crowding effect of the buoyant algae results in heavy alage bloom in the northwestern area, and it forms a 'special biological nutrient filter' which prevents nutrients in the lake water from being brought out by the outflow currents and enhances nutrient accumulation in the lake. Strong disturbance of wind-waves can break algal bloom and limit the light availability of algae. Sharp decrease of macro-zooplankton may enhance algal bloom in the lake. The algal community developed following Logistic Equation from Feb. to Aug. in 1991. Although it is difficult to reduce K by nutrient control in a short time, it may be possible to reduce N or r by algal removal or by regulating some ecological factors in the lake. By harvesting algae, not only can the amount of algae in the lake be effectively controlled, but also large amount of nutrients be removed. By elevating water level, r may be reduced because the net productivity may be.decreased by adding a respiratory layer in the bottom. Protecting zooplankton to increase its grazing pressure on algae may be a good way to reduce r; but it needs further studies to prove if silver carp can be used to contol blue-green algae in this lake.