汤溪水库蓝藻水华发生的影响因子分析

Analysis of factors affecting cyanobacteria bloom in a tropical reservoir (Tangxi Reservoir,China)

汤溪水库位于北回归线附近的热带地区,是广东省东部的大型供水水库。1997年来该水库频繁在10～11月份发生蓝藻水华,但在2003年,蓝藻水华提前在7月份发生。对影响蓝藻水华发生的主要因子进行了分析,探讨了2003年蓝藻水华提前发生的原因。汤溪水库的营养物质主要来源于流域降水形成的地表径流,其中5月份降水多,水库营养盐输入量大;该水库浮游植物生长基本不受N、P营养盐限制,具备蓝藻水华发生的N、P营养盐基础。在4～11月份,水库表层水温为25～35℃,适合蓝藻水华的发生,水温与蓝藻、微囊藻的丰度具有显著相关性,较高的水温是蓝藻水华发生重要条件之一。在具备充分营养盐与适合水温条件下,水体稳定性是控制汤溪水库蓝藻水华发生时间的关键因子。2003年全年的降水仅为多年平均值的76.8%,且汛期7月份降水量只有64mm,远小于往年的同期降水量,汛期反常的水体稳定性导致了蓝藻水华的提前发生。

Tangxi Reservoir, located in the east of Guangdong Province, China, near latitude 23.5 degrees north, is an important drinking-water reservoir for local people. From latter 1990＇s, Tangxi reservoir has been in an unhealthy eutrophic state, and cyanobacteria bloom was frequently observed. As climatic characteristics in the latitude Tangxi reservoir located, suitable water temperature for the occurrence of cyanobacteria bloom（25 - 35 C）was always observed from April to November, and generally, flood season, during which weak water column stability due to high precipitation did not offer the occurrence of cyanobacteria bloom suitable physical condition, occurred from April to September. Thus, controlled by both water temperature and precipitation, cyanobacteria bloom in Tangxi reservoir was most likely to occur in October or November. From 1997 to 2002, cyanobacteria blooms in Tangxi reservoir occurred during every October or November. While in 2003, cyanobacteria bloom was observed in July. In this study, water pH value, nutrients, water temperature and water column stability were discussed as important factors affecting cyanobacteria bloom, and the critical factor that cause the earlier cyanobacteria bloom occurrence was determined. Results demonstrated that, pH values through 2003 ranged from 8.4 to 9.5, and these values may favor the competition advantage of cyanobacteria. In the period of mass cyanobacteria bloom, significant positive correlation between pH values and the cyanobacteria abundances indicated that high pH values and the growth of Cyanobacteria may promote mutually. Nutrients in upper layer of the water column, mainly loaded by land runoff, had close positive correlation with precipitation. N and P concentrations in the reservoir were heavy enough for occurrence of cyanobacteria bloom. Up-layer water temperature, ranging from 16 C to 35 C through the study period, had significant correlation with abundance of phytoplankton or cyanobacteria, indicating that water temperature was an important factor affecting cyanobacteria bloom. Compared to the past three years （from 2000 to 2002）, total annual precipitation in 2003, which was only 65.0% of that for 2000, was much lighter. Thermal stratification of the water column was found from March to November due to light annual precipitation and high water temperature through the year. More abnormally, precipitation distributed in July 2003 was only 64mm, which was only 18.6%o of that for the same periods in 2000 and close to values for October or November in the past three years. Light precipitation in July 2003 led to strong water column stability and long water retention time. Based on adequate N, P and suitable water temperature in the flood season, strong water column stability induced by abnormally light precipitation in July 2003 may be the cause of earlier occurrence of eyanobacteria bloom in the Tangxi Reservoir.