RESEARCH PAPER Seasonal changes in the microbial population of the water column and sediments of the Ongagawa River, northern Kyushu, Japan Syouhei Nishihama Æ Akira Haraguchi Æ Tomonori Kawano Æ Keisuke Michiki Æ Kohji Nakazawa Æ Takuya Suzuki Æ Kazuya Uezu Æ Kazuharu Yoshizuka Received: 29 May 2007 / Accepted: 20 December 2007 / Published online: 29 February 2008 Ó The Japanese Society of Limnology 2008 Abstract The total viable count, population density of Escherichia coli and coliform bacteria, and nitrogen in the microbiomass (microbiomass-N) in sediments were moni- tored monthly at 12 points in the Ongagawa River basin from June 2002 to May 2006. The measurement of the sediment microbiomass-N was used for evaluation of the sediment’s microbial population in the river ecosystem. An extraordinarily high population of E. coli was observed during the season when there was stagnant water in the basin, with a high population and an insufficient drain diffusion system, and, thus hydrological water control is indispensable to prevent rapid E. coli growth. Microbio- mass-N in sediments showed a negative correlation or independent fluctuation in relation to the bacterial popu- lation in the water column of the river. Seasonal changes in extracted nitrogen (N) in river sediments did not show correspondence with microbiomass-N in sediments. The microbiomass-N in sediments changed independently of the bacterial population in the river water, indicating that the high population of bacteria in the water does not lead to a high microbial population in river sediments. Ordination of the microbial parameters by canonical correspondence analysis (CCA) showed that microbiomass-N in sediments was quite different from other parameters. Relatively higher H + (lower pH), PO 4 3- concentration and dissolved oxygen (DO) were the determinant parameters of higher microbiomass-N in sediments. A relative microbial abun- dance between the water column and sediments as well as each of the microbial populations in the water column and sediments could be a quantitative parameter for evaluating the biochemical processes of stream water. Keywords Biochemical environment Á Escherichia coli Á Microbial population Á Microbiomass-N in sediments Á Ongagawa River basin Introduction The biochemical processes in river ecosystems, especially metabolism by microorganisms and aquatic macrophytes, constitute one of the determinant factors of the chemical environment of river water and the consequent water quality. Since biochemical processes by the microbial community is the dominant means for decomposition of organic components in water, the microbial community in water plays a key role in water purification by removing organic pollutants in the river ecosystem. Biochemical processes in the river water column have been commonly investigated in order to obtain information concerning the capacity for self-purification of water. Biochemical oxygen demand (BOD) is one of the common parameters evalu- ating the microbial activity in water (Stumm and Morgan 1995). Although BOD is determined by the numbers of both microorganisms and biodegradable organic substances in the water column, the oxygen consumption rate at the initial stage of organic decomposition, i.e., initial oxygen S. Nishihama Á A. Haraguchi (&) Á K. Nakazawa Á T. Suzuki Á K. Uezu Á K. Yoshizuka Faculty of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Kitakyushu 808-0135, Japan e-mail: akhgc@env.kitakyu-u.ac.jp T. Kawano Graduate School of Environmental Engineering, The University of Kitakyushu, Kitakyushu, Japan K. Michiki Instrumentation Center, The University of Kitakyushu, Kitakyushu, Japan 123 Limnology (2008) 9:35–45 DOI 10.1007/s10201-008-0236-6