Nitrogen and Phosphorus Metabolism in Lake Water/Mud Ecosystems and Ecological Succession as Factors responsible for Cyanobacterial Bloom.

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Field surveys and nutrient enrichment cultures were conducted for the North Basin to reveal the possible factors responsible for the development of cyanobacterila bloom in oligotrophic Lake Biwa, with a special reference to the nitrogen and phosphorus metabolism at lake water/mud interaction system and successional interaction in phytoplankton community.Based on my previous studies on the positive role of nutrients regeneration from the bottom sediments on development of cyanobacterial population, the field survey was focused on the sequential changes s of nitrogen and phosphorus in both lake water and phytoplankton. Pronounced decrease in nitrate concentration and noticeable increases in particulate nitrogen and cyanobacterial abundance were observed in association with increasing water temperature from late spring toward mid summer. These findings and a parallel decrease of dissolved organic nitrogen suggest the summer development of cyanobacterial population could be supported by ni … More trogen regeneration from dissolved organic matter through microbial activity at the lowest level of nitrate. Lowest nitrate condition is also effective to limit the growth of other algal species and permits the growth of cyanobacteria. In the nutrient enrichment cultures using the lake waters from the surface of the North Basin, significant positive effects on algal growth were observed for cyanobacterial population at higher temperature and external phosphorus enrichment, for green algae at nitrogen enrichment and for diatoms at lower temperature.These results imply that higher temperature and lower nitrate environment could be essential for the competitive development of cyanobacterial population and an external phosphorus supply to the water under this environment should stimulate the cyanobacterial growth. Lake water/sediments ecosystem at littoral zone of Lake Biwa is characterized by higher temperature and stimulated phosphorus liberation from the sediments during summer, and hence should provide the sites with frequent development of cyanobacterial bloom. Less