This research was carried out to elucidate the impacts of dissolved oxygen (DO) concentration to phosphorus release mechanism from soil into water in lakes. PO4-P contributed to the growth of the total phosphorus (T-P). PO4-P was steadily increasing because water was accumulating PO4-P. T-P was closely related to turbidity, pH, and DO. We found that DO had decreased because DO was consumed in organic matter decomposition, and that the resulting anaerobic decomposition occurred whenever water had run out of DO. We also found that pH had decreased sharply by production of organic acid by the anaerobic decomposition and that T-P decreased because a decrease in pH removed turbidity by precipitation. T-P was dissolved without microbial decomposition. This mechanism was of great importance in lakes because phosphorus is released from soil into water.