The dependence of Microcystis aeruginosa growth on nitrogen and phosphorus concentration and nitrogen/phosphorus mass ratio （N/P ratio） in the medium, which could be regarded as the initial total nitrogen/phosphorus mass ratio （TN/TP ratio） under the existence of Cyclotella sp. was examined through monoculture and monoxenic culture experiment. Furthermore, the relationship between M. aeruginosa dominance and low TN/TP ratio in eutrophic lake was discussed based on the results obtained in this study and reference data. The monoculture as well as monoxenic culture experiment under high nitrogen and phosphorus concentration showed that the growth of M. aeruginosa was affected by nitrogen－deficient condition, and depended on N/P ratio rather than nitrogen and phosphorus concentration. On the other hand, in the monoxenic condition under low nitrogen and phosphorus concentration, the growth depended on both N/P ratio and phosphorus concentration. As the possibility of generally accepted fact that the low N/P ratio （TN/TP < 29） favors the appearance of water bloom, we suggested that low N/P ratio might be a result of water bloom, which would reflect the cellular N/P ratio of water bloom followed by assimilation of nitrogen and phosphorus in lake water.

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The most important item is accurate identification when we use algae as environmental indicators. In this study, we attempted taxonomical investigation of Navicula salinarum Grunow which is indicator diatom of water containing salt. This taxon is recorded including 11 infraspecific taxa. We examined each taxon in detail and following 4 taxa, Navicula salinarum Grunow var.salinarum f.salinarum, Nav. salinarum Grunow var. salinarum f.minima Kolbe, Nav. salinarum Grunow var. nipponica Skvortsov, Nav. salinarum Grunow var. rostrata (Hust.) Lange-Bert. are considered to be infraspecific taxa. In addition, Navicula salinarum Grunow var. gracilior Lacsny is recognized as new rank, Navicula gracilior H. Fukush., Kimura & Ts. Kobay. stat nov. judging from a morphology of the valve.

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Chemical oxygen demand (COD) in Lake Biwa has not been improved during the past quarter century in spite of decreases in organic loadings from watershed and total phytoplankton biovolume. In addition, an increase in cyanobacteria has been observed in Lake Biwa. Most of cyanobacteria have a large volume of gelatinous sheath, which is usually called extracellular polysaccharides (EPS) and may play a significant role in COD in the lake. Therefore, we analyzed long-term variation of phytoplankton biovolume and EPS volume at Imazuoki, Nagahamaoki and Imazuokichuo-points in the northern basin of Lake Biwa from 1980 to 2009. The EPS volume in 2000－2009 increased approximately three times compared with that in 1980－1989 at Nagahamaoki-point, although the phytoplankton biovolume in 2000－2009 decreased to 85％ of that in 1980－1989. Furthermore, the ratio of EPS volume to the phytoplankton biovolume at Nagahamaoki-point reached 112 in 2000－2009 while that in 1980－1989 remained 31. At Imazuoki and Imazuokichuo-points, the ratio increased from 41 to 109 and from 40 to 120 during the same period, respectively. These results suggest that the large amount of EPS produced by phytoplankton could be one of the major sources of organic matter in Lake Biwa.

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