Originally, eutrofication referred to the phenomenon in which nitrogen, phosphorous and other nutrients flowed in from wetlands and gradually accumulated in lakes. In recent years, the centralization of population and industry has caused nitrogen, phosphorous and other nutrients to flow into enclosed coastal seas such as Tokyo Bay, Ise Bay and the Seto Inland Sea as well as lakes, causing algae and other organisms to propagate and resulting in "red tides" and other phenomena that cause a progressive degradation of water quality in these regions.
The result has been a decline in the aesthetic appearance of the lake due to a worsening of the clarity and color of lake water as well as various other problems that include improperly filtered and foul-smelling tap water. "Red tides" and "blue tides" have occurred in ocean regions as well, impairing fishing operations and resulting in other problems.
1. In Lakes
Environmental standards for the nitrogen and phosphorous content of lakes were established in 1982. By 1994, 48 water regions had been categorized, including 44 lakes and two areas of Lake Biwa. Sewage standards for nitrogen and phosphorous were established in 1985, and in June 1998 136 lakes requiring phosphorous restrictions and 124 lakes requiring nitrogen restrictions were added. As a result, restrictions on phosphorous are currently in effect for 1,200 lakes and restrictions on nitrogen are in effect for 201 lakes. Restrictions on nitrogen and phosphorous pollutant loads are also being implemented under the Law to Preserve Lake Water Quality for ten designated lakes including Lake Biwa (only for phosphorous in the case of the Kamafusa Dam Reservoir and Lake Nojiri).
2. In Ocean Regions
Relevant prefectures have provided guidance on reducing nutrients in the Seto Inland Sea since 1980 in accordance with the Law on Special Measures to Preserve the Environment of the Seto Inland Sea. For Tokyo Bay and Ise Bay as well, relevant prefectures and other entities began measures to prevent eutrofication in 1982, and since 1996 the fourth series of guidelines on reducing nutrients has been implemented. In August 1993, environmental and sewage standards for nitrogen and phosphorous in ocean regions were established, and sewage restrictions were implemented for 88 ocean regions that are substantially enclosed and subject to the danger of eutrofication, and for public water regions flowing into these ocean regions.
Environmental standards for total nitrogen and phosphorous content in ocean regions were also established in August 1993, and so the process of categorization is being conducted in the aforementioned enclosed coastal seas on the national and prefectural level. By the end of March 1997, 40 water regions in 15 ocean regions had been designated.