A hypothesis was formulated to explain the possible cause of water bloom occurring inLake Qiandaohu in 1998 and 1999. We tested this hypothesis with a 3-year in situ fieldstudy. The results showed that the reconstruction of the silver carp and bighead carppopulations, without other measures of nutrient control, could prevent the recurrence ofalgal bloom in the lake successfully. This result could serve as an evidence to thesuggested hypothesis for water blooming: The drastic decline of the filter feedingsilver carp and bighead carp in the lake, rather than the nutrients overloading, wasmainly responsible for the algal bloom. According to this study, we suggest a generalhypothesis to the ecological mechanism of algal blooming: The insufficient grazing fromthe phytoplanktivores (top-down control) to the algal reproduction from nutrientsavailable (bottom-up effect) is the radical cause of water blooming, while conventionally,it is primarily attributed to the enrichment of nutrients. Besides, this study showedthat stocking silver carp and bighead carp in lakes could improve water quality, whichis also contrary to the conventional opinion. Finally, this study provided a cost-effective and practicable approach to control water bloom for the large-sized reservoirs,especially when water blooming occurred locally. A net-enclosed aquaculture zone (NEAZ)can be established in the nutrients-exposure area of the waters and stocked with the twocarps, water bloom could be controlled and prevented.
