Study on Effects of Seasonal Denitrification and Ammonium Removal Processes in Nitrogen Cycling of Lake Kasumigaura

概要

Many lakes worldwide have faced several environmental problems due to increasing anthropogenic activities. Lake Kasumigaura is the second largest lake in Japan and suffers from severe water blooms every summer and causes musty odor problems. Nitrogen (N) is one of the most important nutrients because it controls the primary production in the lake and eutrophication. Among the nitrogen compounds, nitrate and ammonia are the most critical pollutants because of their eutrophication and toxicity to aquatic organisms. Therefore, removing nitrate and ammonia from water is an important research issue in nitrogen cycling. The important biological nitrogen removal process from the biosphere is denitrification which converts nitrate to nitrogen gas by microorganisms.

　Furthermore, the biological ammonia removal performance with low ammonia load (especially when less than 2.0 mg N/L) is important in drinking water treatment plants (DWTPs). Therefore, the knowledge of seasonal changes of denitrification and ammonium removal processes in nitrogen cycling of the lake is greatly important. However, the information is still limited. Therefore, this study was conducted to clarify the characteristics of the seasonal changes of nitrogen cycling in a eutrophic lake, Lake Kasumigaura, especially nitrogen compounds removal from the lake by microbial denitrification and ammonia removal from the lake by biofilms.

　The acetylene blockage method measured seasonal denitrification and nitrous oxide production. Quantitative real-time PCR (qPCR) analysis was applied to determine the abundance of total bacteria and functional genes (nirS and nosZ genes). The denitrification and nitrous oxide rates were higher in winter than summer. The denitrifying functional genes (nirS and nosZ) were seasonally different (p <0.05) and the greatest in Spring, followed by autumn, winter, and summer. The abundances of 16S rDNA and denitrifying functional genes were seasonally significantly different among four seasons (p < 0.05). The denitrification, nitrous oxide rates, and abundance of denitrifiers in the surface layers (0-2 cm) were higher than bottom layers and tended to decrease with increasing depths.

　Laboratory incubation experiment was carried out at in situ temperature monthly to verify the difference of ammonium removal rates with low ammonia inflow (1.0 mg/L) under dark conditions. The ammonia can be effectively removed within 2-day incubation when the temperature was as low as 11℃, while no significant ammonia removal could be confirmed at 7ºC. Molecular analysis showed that canonical ammonia oxidizers (AOA and beta-AOB) may not be responsible for the observed ammonia abatement in the sampled drinking water treatment plant (DWTP) as well as in laboratory incubation vials.

　The surface layer was the hot zone for the denitrification processes. Microbial ammonia assimilation or comammox process may be the probable ammonia removal pathway in low- ammonia conditions such as DWTPs. Nitrogen compounds removal by microbial denitrification and ammonia removal by biofilms can abate lake water's eutrophication, ammonia, and nitrate toxicity.