Study on the Possible Cause of Water Blooming and the Bloom-Prevention Technology in Lake Qiandaohu

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.

Biological manipulation of eutrophication in West Yangchen Lake

This paper aims to share findings on nonclassical biomanipulation from a field study on eutrophic West Yangcheng(WY)Lake.In the forty years before 2009,cyanobacteria frequently bloomed in WY Lake,an event which has not occurred since 2009.The stocking density of filter-feeding carp was estimated to be more than 50 g/m^(3) in July after 2009,approximately double the amount observed before 2009,and the ratio of silver carp to bighead carp(SBR)increased from 0.8 to more than 1.6.Compared with the water quality between 2004 and 2007,the total nitrogen(TN)and the chlorophyll-a(Chl-a)concentration decreased while total phosphorus(TP)and transparency(SD)increased significantly after 2009(P<0.05).The decreasing Chl-a concentration was observed in Jul,Jun,May,Jul,and Mar in 2009,2010,2011,2016,and 2017,respectively.The Chl-a concentration was significantly negatively correlated with TP(P<0.05),and no significant correlation with TN was observed(P>0.05).A significant negative correlation was observed between fish yield and SD(P<0.05).The results also showed that filter-feeding carp more effectively restrained algae growth when SD was low.These results indicated that a high density of filter-feeding fish with increased SBR interfered with the driving effects of nutrients and temperature on the growth of algal biomass and then restrained bloom of algae.It may be necessary to take SD into consideration for determining the appropriate fish stocking density for the biomanipulation of algae.