千岛湖水华发生与主要环境因子的相关性分析

CORRELATION BETWEEN BIOMASS REDUCTION OF SILVER CARP AND BIGHEAD CARP AND THE OCCURRENCE OF ALGAL BLOOMS IN LAKE QIANDAOHU

藻类水华的发生通常被认为是水体富营养化的结果或表征。然而这种对水华发生机制的经典理解却不能很好地解释千岛湖在营养盐较高时没有发生水华而在较低时却发生了水华的现实。通过对千岛湖发生水华前后与其相关的主要生态因子所做的调查和分析结果表明,千岛湖发生大面积水华时的水体营养盐(TN和TP)含量、水位和银鱼生物量等指标均没有与水体历年(1993～1997年)未发生水华时的相应指标存在显著差异(P>0.05);同时也有证据表明,水体浮游动物密度在水华发生前后出现显著差异的可能性很小,且水体也仍处于中营养程度。只有鲢鳙生物量在发生水华时显著小于没有发生水华时的历年生物量平均值。此外,温度也常被认为是诱发水华暴发的一个重要环境因素,但分析认为,水温是周期性变动因子,千岛湖历年的水温并没有显示1998和1999年有任何异常。综合上述分析后认为,千岛湖1998、1999年大面积水华的发生,可能与水体中鲢鳙生物量的显著减少有密切关系。但银鱼的大量存在,遏制了浮游动物的数量,使低鲢鳙生物量和低浮游生物量同时出现成为可能,从而也可能成为导致水华发生的原因之一。

The Occurrenec of algal blooms has generally been considered as the result or the symptom of aquatic eutrophication. However, this traditional interpretation of the blooming mechanism could hardly explain why algal blooms did not occur when the nutrients were at a higher level while they did occur when the nutrients were at a lower level in Lake Qiandaohu. The results from this study through the investigation and analysis on the main ecological factors associated with algal bloom occurrence before and during the blooms that occurred in 1998 and 1999 showed that the average concentrations of nutrients （TN and TP）, water levels and mean biomasses of icefish in the blooming years （1998 and 1999） were not significantly different from those before the blooming years （1993-1997） （P〉0.05）. They also showed that the biomass of zooplankton when the algal bloom occurred was not significantly lower than those before. The lake was also in a mesotrophic state. The only significant difference noticeable was the biomasses of the two filter feeding sliver carp and bighead carp, which were significantly lower in the blooms years than those of the previous years. Besides, water temperature has often been considered as one of the major causes of algal blooms, however, historical records from the local hydrological authorities have not showen any irregular variation of water temperature during those blooming years in Lake Qiandaohu. Thus it could be concluded that the lower biomass of silver carp and bighead carp might play a crucial role in relation to the algal blooms that occurred in the two years. While the existence of higher biomasses of icefish in 1998-1999 made it possible for the concomitant of lower biomasses of both zooplankton and the two Chinese carps, which weakened the top-down forces in the water body, and might be responsible for the occurrence of algal blooms in the two years. Thus the existence of high biomasses of icefish might be another cause of the algal bloom.