Relationship between formation process of water quality and natural pH recovery, and response estimation model for water quality on acid lake

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Slow rise of pH value was observed in Lake Inawashiro since 1995. Until now, the cause of this phenomenon is not made clearly. Although mass of primary production with nanophytoplankton smaller than 20 μm dominated to the total amount, it was decreasing until 2003, and it is unclear that this is due to decreasing dissolved inorganic carbon or due to limiting of photon. In this study, we attempted to determine limiting factor and analysis water response model of quality in Lake Inawashiro. Ratio of carbon to nitrogen (C/P) and carbon to phosphorus (C/P) were 1±2 and 330±110 in surface layer and 5 m layer, respectively in 2004. Those in 2005 were, 8.5±1.0 and 354±82 in surface layer, and 8.7±1.1 and 324±51 in 5 m layer. As C/P ratio was so higher in both years, phosphorus is limiting factor in Lake Inawashiro. In addition, inorganic carbon was also limiting factor as pCO_2 was in low level although this lake was acid lake. Therefore, primary production was in low level as inorganic carbon and phosphorus lacking. As vertical distribution in inorganic carbon including CO_2 was did not change from surface to deep layer, supply of CO_2 from atmosphere is not important as compared with supply with inorganic carbon loading from inflowing river.