A New Approach for a Fertile Seto Inland Sea

Improvements are being made to the water quality of the Seto Inland Sea, once called the “Dying Sea,” through a variety of water quality conservation measures such as wastewater regulations. However, as new issues such as declining fish catches and frequent occurrences of color-fading in nori (dried seaweed) have come to the forefront, special measures have been approved for the management of nutrients to ensure the diversity and productivity of estuarine ecosystems. It can be said that this large shift in the water environment administration is completely different from wastewater regulations to date, namely, the simple reduction of pollutants such as nutrients.

Improvements to Water Quality in the Seto Inland Sea

The Seto Inland Sea became increasingly polluted during Japan's period of rapid economic growth, at one point deteriorating to the point that it became known as a "dying sea." Measures to combat this situation included effluent regulations and total loading reductions under the Water Pollution Control Law and the Law concerning Special Measures for Conservation of the Environment of the Seto Inland Sea¹ (hereinafter referred to as the Seto Law), which resulted in improvements to the quality of the water. However, in recent years, new issues have come to light, such as declining fish catches and frequent color fading in nori (dried seaweed). This article will introduce the history of the various efforts to restore the Seto Inland Sea and the new system enacted this year, which aims to create a more fertile sea through a focus on nutrient management.

Water Quality Targets for Environmental Conservation and Their Achievement

When conserving and restoring any environment, it is essential to define the desired state of the environment, i.e., the goals being aimed for. For example, the first Seto Law aimed to conserve water quality and natural landscape in response to rapidly worsening water pollution in the Seto Inland Sea between 1965 and 1974. The first measure to improve water quality involved using chemical oxygen demand (COD) as a parameter for environmental water quality standards (EWQS) to prevent organic pollution². To achieve these standards or targets, various effluent regulations based around COD have been implemented. However, it became clear that COD alone was not enough to prevent eutrophication like the outbreaks of red tide. Therefore, nutrients (total nitrogen and total phosphorus), which are the limiting factors of eutrophication, were added to EWQS, and their effluent has also been regulated.

So, what happened to the water quality as a result? According to the Results of the FY 2019 Water Quality Survey of Public Water Areas, the compliance rate for EWQS in COD in the Seto Inland Sea excluding Osaka Bay — as well as Osaka Bay itself — were still low at 77% and 66.7%, respectively. However, the EWQS compliance rates for total nitrogen and total phosphorus were extremely high at 98% and 100%, respectively. Compared to the compliance rates of 0% and 60% for total nitrogen and total phosphorus, respectively, when the standards were introduced (1995-1996), these results indicate that effluent control and other measures have been successful.

From Conserving Water Quality to Restoring the Seto Inland Sea

Despite EWQS having been largely achieved, biological productivity is stagnant, as revealed through declines in fish catches and frequent color fading in nori. There is a growing awareness that water quality conservation alone will not restore the once bountiful Seto Inland Sea.

Alongside the previous notion of focusing on water quality conservation, the 2015 revision to the Seto Law also includes the concept of "making the sea rich in biodiversity and productivity, and maximizing its diverse values and functions." It additionally mentions that measures for environmental conservation in the Sea "shall be implemented in accordance with the respective conditions of baysand other open sea areas." The two previous goals of conserving water quality and natural landscape were replaced by four new goals: "conservation, restoration and creation of the coastal environment," "conservation and management of water quality," "conservation of natural and cultural landscape ," and "ensuring sustainable use of fishery resources." In particular, the revision clarifies conservation, regeneration, and creation of the coastal environment, including the conservation of submerged aquatic vegetation (SAV) and tidal flats, as new goals for restoring a bountiful Seto Inland Sea. At the same time, the perspective of managing water quality in accordance with regional and seasonal characteristics was also added. Additionally, a new major goal was set to ensure the sustainable use of fishery resources by promoting the growth of aquatic plants and animals while considering the perspective of biodiversity and harmony with the environment.

Since these changes were made, it was pointed out that the impact on fishery resources due to nutrient deficiencies such as nitrogen and phosphorus, which were reduced through past efforts, and the decrease in SAV and tidal flats due to development, may worsen further. This situation has also been coupled with environmental changes such as the increase in water temperatures due to climate change. Furthermore, there has been increasing concern about the adverse effects of marine debris, including plastic litter, on the marine environment and its ecosystems.

In response, a further amendment to the Seto Law was enacted on June 3, 2021³. The change adds a new objective: conserving the environment of the Seto Inland Sea by adding special measures for nutrient management in order to ensure biological diversity and productivity. Additionally, the law's basic principles now include the notion that the environmental conservation of the Sea must account for increases in water temperature and other environmental impacts due to climate change, and that these impacts may continue for a long time.

In particular, under the new nutrient management system, new special provisions have been established to allow for appropriate increases in nutrients in specific sea areas, such as through the direct release of nutrients and changes in wastewater treatment processes. Although the law makes it mandatory to ensure harmony and compatibility between nutrient management and conserving the surrounding environment, this change is a significant shift in the water environmental administration that is entirely unlike the previous regulations, which were based around one-way reductions.

However, the change manages nutrients essential for plant growth; excessive nutrients can lead to eutrophication problems such as red tides, while deficient nutrients can lead to reduced biological productivity. It can be said this regulation is unique in controlling substances that can have both positive and negative effects.

In the initial Law concerning Special Measures for Conservation of the Environment of the Seto Inland Sea, the entire Sea was treated as a single body of water. However, as water quality recovered, the Seto Inland Sea and Osaka Bay began to be treated as separate sea areas. In the previous revision in 2015, the Seto Inland Sea was managed according to the respective conditions of bay, and other open sea areas to achieve a more bountiful marine environment. In the 2021 revision, it can be said that the sea areas under the management were more precisely defined.

Areas covered by the Law concerning Special Measures for Conservation of the Environment of the Seto Inland Sea (Source: Ministry of Land, Infrastructure, Transport and Tourism)

Aiming for a Newly Restored Seto Inland Sea

In this way, instead of managing the Seto Inland Sea as a homogeneous area, it has become possible to manage bays and even some specific areas of the Sea independently with targets that differ from those of the surrounding areas, enabling the restoration of the highly diverse Seto Inland Sea. To achieve this, biodiversity and biological productivity targets will need to be set for each marine area, and management methods will have to be extremely diverse.

However, each area of the Seto Inland Sea, its bays and specific marine areas are also interconnected. Therefore, target setting and nutrient management in one area should be consistent with other areas and the Sea as a whole. As the unique role of each area become even more critical, ensuring consistency in how the Seto Inland Sea as a whole is managed without conflictions will pose a significant challenge in the future.

It should be noted that nutrient management is only one aspect of ensuring biological productivity and diversity. There is no guarantee that nutrient management alone will achieve these goals. We must not forget that the conservation, restoration, and creation of SAV and tidal flats, as well as improvements to bottom sediment quality, must be implemented simultaneously. (End)