Study of odor-producing Planktothrix sp. in Miyun Reservoir, regarding the ecological niche identification, regulation and control strategy

MIB producing potential of Planktothrix sp.

The production of odorant methylisoborneol (MIB) in water bodies by Planktothrix sp. have not been understood very well. Through a four-year investigation in Miyun Reservoir, a huge mesotrophic drinking water reservoir known to have the MIB episodes, we found that the Planktothrix sp. bloomed during September and October causing the high levels of MIB in the reservoir. The concentration of MIB and the biomass of MIB-producing cyanobacteria Planktothrix sp. were measured (n=887) at different sites and depths during different seasons. The results indicated that the shallow region of the reservoir is the major habitat for Planktothrix sp. due to that the light is able to penetrate down to the relatively high concentrations of nutrients close to the sediments. Quantile regression analysis between Planktothrix sp. biomass and MIB concentration shows that the risk of MIB exceeding the odor threshold (15 ng L⁻¹) in water was as high as 90 % when the Planktothrix sp. density was more than 4.0 × 10⁵ cells L⁻¹, while the risk was reduced to 10 % when the Planktothrix sp. density remained below 1.6 × 10⁴ cells L⁻¹. This study will improve the understanding of the environmental behaviors of Planktothrix sp., and can provide useful information for better management of drinking water lakes/reservoirs experiencing the taste and odor (T&O) problems caused by deep living cyanobacterial species.

Niche identi􏰀ication of Planktothrix sp.

Water quality problems caused by harmful algae in drinking water reservoir have raised great concerns. According to a four-years investigation in Miyun Reservoir through 2009 to 2012, the prediction of odor-producing Planktothrix sp. were performed based on the ecological niche theory and redundancy analysis, in case of that the reservoir accepts water with increasing TN-loading from Danjiangkou Reservoir through South-to-North Water Diversion Project in China this year. The result shows insolation and TN are the two key variables in the niche space for Planktothrix sp. and Microcystis sp., the dominate species in summer. In insolation-TN plane, the orbit of aquatic environment cross over the niche space of Planktothrix and Microcystis chronologically in Miyun Reservoir. In addition, the growth potential of the odor-producing Planktothrix sp. will probably increase after the water from Danjiangkou Reservoir 􏰀low into Miyun Reservoir according to the simulation result. This study is helpful for the understanding of algal competition and succession based on the ecological niche theory, and can provide an important tool to assess the risk of T&O problem therefore conducting the source water management.

Regulation and control of Planktothrix sp.

The control and management of harmful algae bloom in source water is critical to drinking water safety, especially for the algal toxins and taste & odor issues. Due to traditional solutions of controlling algae were mainly based on physical or chemical mechanisms, they were greatly limited by their disadvantages including high energy consuming or toxic to other aquatic organisms. According the previous result, the niche space of Planktothrix sp. has been determined, and the key factors governing its growth were identified as the light availability and nutrient concentration in water. It implies the water depth is a comprehensive factor: low water depth have high risk. According to this result, the high risk zone of suffering T&O problem has been determined. By performing the GIS based technology, the dynamic changes of high risk zone were simulated under different water level, and the recommend water level of Miyun Reservoir were determined aiming to minimize the T&O risk. The result showed that the risk (less than 10 %) of suffering T&O problem can be signi􏰀icant reduced if the water level rising to 147.7 m a.s.l. in Miyun Reservoir. In addition, the application of water level strategy were discussed for different types of reservoirs/lakes. This strategy can be applied combined with the South-to-North Water Diversion Project, and could be a possible solution for the T&O problem in Miyun Reservoir.