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Sediment Quality Assessment and Management: Insight and Progress Edited by M. Munawar© 2003 Ecovision World Monograph Series Aquatic Ecosystem Health & Management Society Surface water recruitment of bloom-forming cyanobacteria from deep water sediments of lake Biwa, Japan K. Ishikawa1, 2*, S. Tsujimura2, M. Kumagai2, H. Nakahara1 1Kyoto University, Division of Applied Biosciences Graduate School of Agriculture, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, 606-8502 Japan. * 2 Lake Biwa Research Institute, 1-10 Uchidehama, Otsu, Shiga, 520-0806 Japan. Keywords: toxic cyanobacteria, initial growth, algal recruitment, seeding Introduction Lake Biwa is a large, deep, mid-latitude temperate lake (Fig. 1), and the water in the lake is phosphorus-limited (Tezuka, 1992). Several genera of cyanobacterial blooms have appeared during most summers since 1983 in the enriched South Basin (Ichise et al., 1987), and in the harbours of the North Basin from 1994 onward (Yoshida et al., 1996). Since 1997 there have also been occasional cyanobacterial blooms in offshore surface waters of the North Basin (Kumagai et al., 1999). Although the present cyanobacterial toxic concentrations in the water are below the levels that damage human health (Watanabe, 1999), the risk of increasing toxic water blooms is a major concern, because the lake supplies approximately 14 million people with their drinking water. Since there were few reports of cyanobacterial bloom in a deep lake like Lake Biwa, research on the developing mechanism of cyanobacterial blooms offshore was required. In the last three decades, a great deal of research on the annual cycle of bloom-forming cyanobacteria has been progressing, and the role of hibernating colonies and the role of akinetes in the sediments as a seed population for following summer blooms has been investigated (e.g. Sirenko et al., 1969; Reynolds and Rogers, 1974; Preston et al., 1980; Fallon and Brock, 1981; Reynolds et al., 1981; Trimbee and Harris, 1984; Takamura et al., 1984). Barbiero and Welch (1992) found that in Green Lake (USA), Gloeotrichia echinulata populations were heavily 288 subsidized by benthic recruitment, however the maximum standing crop was negligible for the co-occurring toxic cyanobacterial species Microcystis aeruginosa. Hansson et al. (1994) suggested that algae on the sediment surface rise into the water column in response to some depth-related cue. Although many variations of recruitment patterns, depend on species, sites, grazing and other environmental conditions have been shown (Hansson et al., 1994; Hansson, 1996; Head et al. 1999), it is not clear how to establish an assessment protocol on seed populations in the sediment. In any case, information on recruitment potential in the lake is seen to be of essential value to the prevention of toxic blooms since sediment populations are important as an inoculum for future blooms. Several factors have been thought to effect the germination and the recruitment of overwintering cyanobacteria. Light energy might promote their germination on the sediment (Reynolds et al., 1981). Water temperature, anoxic condition and phosphorus loading might effectively increase their growth rate (Trimbee and Prepas, 1988; Reynolds, 1973; Murphy et al., 1998). Gas vesicles in cyanobacteria are responsible for much of their biological success (Fogg, 1969) although in the deep regions Microcystis colonies with collapsed gas vesicles due to high water pressure may have little chance of recruitment from the sediments (Tsujimura et al., 2000). However, it is reasonable to suppose that Microcystis recruitment in the lake was physiologically associated with some of these factors. In this study, we would like to determine the limits of viability of bloom-forming cyanobacterial colonies on the bottom sediments of the lake and thus their recruitment potential as a seed population to surface water. Hypotheses regarding factors mentioned above for cyanobacterial germination under lake conditions were considered and tested here. Study site Lake Biwa (34° 58’- 35° 31’ N, 135° 52’- 136° 17’) (Fig. 1) is the largest lake in Japan (total surface area; 670 km2, total volume; 27.5 × 109 m3). The lake consists of the North and the South Basin. The former has a surface area of 615 km2 and mean depth of 43 m and the latter has a surface area of 55 km2 and mean depth of 3.5 m. The lake has a single outlet at the south end while 125 rivers flow in around its circumference. The population in the surrounding catchment has expanded in the past 40 years, resulting in a deterioration of water quality, and the lake is currently meso-trophic within the North Basin and eutrophic in the South Basin. To cover the...


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