Causes of animal extinction in small irrigation reservoirs in Japan and requirements for future re-establishment

In the past, small irrigation reservoirs were constructed only for rice paddy irrigation in Japan. Many of the watersheds for these irrigation reservoirs were forests. These watersheds have been gradually developed into farmland or housing areas. After World War II, the use of chemical fertilizers, such as ammonium sulphate, became popular in upland fields in Japan. As a result, there are high concentrations of nitrates in groundwater and spring water under upland fields. Hence, irrigation reservoirs that collect water from these watersheds have been polluted by nitrates. High nitrates concentrations in irrigation reservoirs have been found where the watershed is located close to tea yards. However, there has been minimal research into these reservoirs in the fields of limnology and agricultural engineering. We have therefore tried to verify the water‐quality characteristics in irrigation reservoirs from the point of view of water‐quality conservation. In the process of the investigation, we encountered a quite significant reservoir called the Tanno Reservoir that has not seen any forms of life. We examined the causes of this lack of life, as well as measures that could be taken for the remediation of such a reservoir. This paper discusses and clarifies the results of these studies.