Agriculture Minister Yoshio Yatsu on Tuesday announced that he will open the water gates of a dike closing off a large part of Isahaya Bay to see if it improves water quality in the Ariake Sea.

Yatsu made the decision in the evening after a report from an advisory council to the Agriculture, Forestry and Fisheries Ministry that recommended the government open the gates.

Water quality in the Ariake Sea has deteriorated since the dike was installed in 1997.

The dike is part of a controversial land reclamation project that has been strongly protested by environmental groups. Early this year, the environmentalists received some unexpected ammunition for their fight when the area’s prized nori crop came in badly damaged.

Nori is used to wrap various kinds of sushi, and nori from the Ariake Sea is one of the most-favored varieties.

Area farmers have recently begun to demand that the central government cancel the reclamation project and open the gates so the environment can return to its natural state in which water is purified by the tideland ecosystem.

The advisory council was set up to study the bad nori crop. The chair, Makoto Shimizu, an honorary professor at the University of Tokyo, said the probe would require the water gates to stay open for several years.

Although the proposal does not say when the gates should be opened, Yatsu said he would respect the committee’s proposal and make certain that the gates are opened.

After receiving the report, Yatsu told an evening news conference that opening the gates would become necessary in the course of an investigation that would take two years.

Isahaya Bay is part of the Ariake Sea, a nearly land-locked body surrounded by the prefectures of Nagasaki, Kumamoto, Saga and Fukuoka. Seaweed farmers believe the sea has been polluted by an inflow of nutrient-rich waters since the dikes sealed off part of the bay for reclamation in 1997.

In the report, the panel said it is necessary to leave the gates open for “at least a few years” to monitor any subsequent changes to the area’s ecosystem.

The panel made its recommendations after examining the findings of a ministry research team that gathered and analyzed samples of sludge and water from the area.

The study, which began March 15, examined water quality, organic materials and nutrient salts such as nitrogen and phosphorus in sludge and soil on both sides of the 7,050-meter-long dike, which has created a 1,710-hectare reservoir.

Meanwhile, the Environment Ministry said Tuesday that its own research, conducted Feb. 23 and Feb. 24, shows that the nutrient salts seaweed needs to grow have plunged to almost undetectable levels throughout most of the inland sea.

It also said quantities of “chlorophyll a” — a substance found in phytoplankton — were unusually high in many of the 24 points surveyed in the sea. Phytoplankton are consumers of nutrient salts.

In one area deep inside the bay, the ministry detected 20 micrograms of chlorophyll per liter, a concentration close to that of red tides, the massive phytoplankton outbreaks that kill fish and other marine life by releasing poison into the water.

A microgram is one-millionth of a gram.

“These findings,” the report says, “prove that the overgrowth of diatom plankton has resulted in a reduction in the nutrient salts in the sea, thereby causing the poor growth of nori seaweed.”

Diatom plankton are a type of phytoplankton.

“There is a possibility that the fall in the number of these seabed creatures, which consume the nutrient salts, has resulted in an overabundance of nutrient salts in the seawater, thus causing the overgrowth of phytoplankton,” the ministry said.

The reclamation project was begun in 1986 to create 1,840 hectares of farmland. The eight water gates were shut in 1997.

The advisory council recommended that the ministry open the gates after ensuring that seawater flowing back into the dike does not damage local residents living near the tideland.

Agriculture officials said the timing of the gate opening must be made carefully because the inflow of sea water may kick up sludge that has accumulated at the bottom and damage the entire sea.

The government and assembly of Nagasaki Prefecture are still opposed to opening the gates. So far, 237 billion yen has been spent on the reclamation project.

When the 3,500-hectare section of Japan’s largest wetland area was closed off, the government said the gates were needed to reclaim more farmland and prevent flooding.

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