Brian Barton
Tokyo Bay, a semi-enclosed bay in central Japan, has been facing water pollution since the late 1950s due to high organic matter input from human activities. Eutrophication, or excessive nutrient enrichment, has been a significant issue, leading to hypoxia and anoxia, which in turn cause high carbon dioxide emissions. Additionally, Tokyo Bay faces microplastics pollution, with 77% of Japanese anchovy in the bay found to have ingested microplastics. While Tokyo's air pollution is moderate compared to other large cities, the bay's pollution levels are concerning, and understanding the carbon cycle in such urbanized coastal areas is essential for managing and reducing pollution.
| Characteristics | Values |
|---|---|
| Annual mean of dangerous PM2.5 particles | 15 micrograms/m3 |
| Definition of pollution | "the presence in or introduction into the environment of a substance which has harmful or poisonous effects" |
| Population of Tokyo | above a million |
| Eutrophication in Tokyo Bay | severe since the late 1950s |
| Chemical oxygen demand loading in Tokyo Bay | decreased from 477 to 183 t per day between 1980 and 2010 |
| Organic carbon concentrations in Tokyo Bay | decreased from the late 1970s to the 2010s |
| Microplastics pollution level in Tokyo Bay | 3.98 pcs/m3 in May 2019 |
| Microplastics size in Tokyo Bay | 50–5000 μm |
| Areal percentage of hypoxic bottom water in inner Tokyo Bay | < 15% around 2015 |
| Partial pressure of carbon dioxide in surface water | ranged from 29 to 1476 μatm |
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What You'll Learn
Eutrophication and hypoxia
The semi-enclosed Tokyo Bay has an average depth of 19 meters, with estuarine circulation driven by freshwater discharge from rivers and the inflow of offshore waters. The bay's environment has significantly changed over the last century, particularly since the 1950s and 1960s, when nutrient loading from the surrounding metropolitan areas rapidly increased. This rapid industrialization and urbanization resulted in various environmental issues, including air pollution, offensive odors, and water pollution.
The excessive nutrient loading in Tokyo Bay has led to eutrophication, causing severe hypoxic conditions in the inner part of the bay. Despite recent decreases in nutrient inputs, hypoxia has been increasing in duration and spatial extent. This suggests that the loss of tidal flats from reclamation is reducing the bay's ability to recycle nutrients. Hypoxia in Tokyo Bay typically occurs from spring to autumn, causing defaunation and larval settlement failure of benthic organisms.
The adverse effects of hypoxia on benthic organisms in Tokyo Bay have been well documented. Benthic organisms, including megabenthic species, experience mortality, growth impairment, reproductive issues, and spatial distribution changes due to hypoxic conditions. The duration and extent of hypoxia in the bay have hindered the recovery of benthic organism populations. Additionally, hypoxia has been linked to ocean acidification in the bottom waters of Tokyo Bay.
De-eutrophication, achieved through pollution control, is essential to address the issues of hypoxia and ocean acidification in Tokyo Bay. By reducing nutrient loading and improving oxygen conditions, de-eutrophication can decelerate ocean acidification and suppress anoxic conditions. However, if atmospheric CO2 levels continue to rise, the summer bottom waters of Tokyo Bay may still experience prolonged aragonite undersaturation, even with successful de-eutrophication efforts.
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Microplastics pollution
Tokyo Bay, a semi-enclosed bay in central Japan, has faced significant environmental challenges due to its proximity to the Tokyo metropolitan area, the world's largest megacity. While various forms of pollution affect the bay, microplastics pollution stands out as a pressing issue.
Microplastics (MPs) are tiny plastic particles that have become a significant pollutant in Tokyo Bay. Surveys conducted in May 2019 and January 2020 revealed a high level of microplastics pollution in the bay, with an average of 3000 pieces per cubic meter in its inner part. The pollution was dominated by polyethylene fragments, with a mode size of 800 micrometres. MPs were found to be more abundant in rivers flowing into the bay, indicating that these rivers are a primary source of the pollution.
The presence of microplastics in Tokyo Bay has severe ecological consequences. Studies have shown that 77% of Japanese anchovy (Engraulis japonicus) caught in the bay had ingested MPs, highlighting the extent to which this pollution has infiltrated the food chain. Microbeads, a type of microplastic, were found to account for 6% of the total MPs pollution load.
The abundance of MPs in Tokyo Bay can be attributed to several factors. The bay's location in a highly urbanized area, with a population of approximately 31 million people, contributes to the high volume of waste and pollution. Additionally, the bay's semi-enclosed nature may trap microplastics, preventing them from being carried out to sea.
Addressing the issue of microplastics pollution in Tokyo Bay requires a comprehensive understanding of the problem. While the role of rivers as a source of MPs has been recognized, further research is needed to fully grasp the complexity of the pollution. By studying the seasonality and distribution patterns of MPs, scientists can develop effective strategies to mitigate the pollution and protect the delicate marine ecosystem of Tokyo Bay.
In conclusion, microplastics pollution in Tokyo Bay is a critical environmental concern. The high levels of MPs, particularly polyethylene fragments, have far-reaching ecological impacts, including the contamination of marine life. The metropolitan area's influence and the bay's geographical characteristics contribute to the severity of the problem. To safeguard the health of the bay and the surrounding ecosystem, it is imperative to continue research and implement measures that address microplastics pollution in Tokyo Bay.
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Carbon cycling
The carbon budget of Tokyo Bay was estimated using a box model that incorporated inorganic and organic carbon data over an annual cycle (2011-2012). The surface water represented a net autotrophic system in which the annual net community production (NCP) was 19 × 10¹⁰ gC year⁻¹. The annual loading of dissolved inorganic carbon and total organic carbon (TOC) from freshwater inputs was 11.2 × 10¹⁰ and 4.9 × 10¹⁰ gC year⁻¹, respectively. The annual TOC sedimentation rate was 3.1 × 10¹⁰ gC year⁻¹, similar to the annual air-sea CO2 uptake (5.0 × 10¹⁰ gC year⁻¹).
The changes in carbon flow between the 1970s and 2011-2012 resulted from improved water quality due to increased sewage treatment facilities and improved sewage treatment efficiency in the catchment, which decreased the amount of labile organic carbon flowing into the bay. The NCP and TOC loading from freshwater inputs were 3.0 and 2.7 times lower in 2011-2012 than in the 1970s, respectively. However, the TOC sedimentation rate remained similar. Therefore, a relatively high carbon efflux from Tokyo Bay likely occurred in the 1970s, including CO2 efflux to the atmosphere and/or the export of labile organic carbon to the open ocean.
The nutrient concentration in the bay increased markedly with the urbanization of the watershed between the 1950s and 1980s but gradually decreased thereafter due to advances in effluent treatment and sewage treatment efficiency. The bay has been severely eutrophicated since the late 1950s due to high organic matter input via human activities during the 1950s and 1960s. A large amount of terrestrial organic carbon flowed into Tokyo Bay (4.9 × 10¹⁰ gC year⁻¹). The bioavailable DOC (BDOC), recalcitrant DOC (RDOC), and POC fluxes flowing into the bay were 1.0, 2.0, and 1.9 × 10¹⁰ gC year⁻¹, respectively.
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Nitrogen pollution
The high levels of nitrogen in the water are due to freshwater discharge and inflow loads from rivers, such as the Tamagawa and the river Tamagawa. This has resulted in a negative impact on the bay's ecosystem, with effects on phytoplankton and benthic organisms.
To combat this issue, methods such as the travel cost method (TCM) and the contingent valuation method (CVM) have been proposed and implemented to reduce the nitrogen load in the bay. The installation of denitrification systems in sewage plants and industrial facilities has been suggested as a way to improve water quality and reduce nitrogen pollution.
The benefits of reducing the nitrogen load in Tokyo Bay have been estimated to be substantial, with a net benefit of 458.3 billion yen in consumers' surplus before and after improving the chemical oxygen demand. This improvement in water quality would also positively impact recreational activities in the bay, such as clam-digging, paddling, and shore fishing.
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Sewage treatment
In 1970, a pivotal law was enacted to restrict the discharge of organic pollutants, and advancements like phosphorus-free detergents and enhanced sewage treatment methods were introduced. These measures led to a notable reduction in chemical oxygen demand loading and organic carbon concentrations in the bay.
Microplastics (MPs) pollution, however, remains a pressing issue in Tokyo Bay. Surveys conducted in 2019 and 2020 revealed high levels of MPs, with polyethylene fragments being the most prevalent. The abundance of MPs in rivers mirrored that in the inner bay, indicating rivers as the primary source. The metropolitan area during the wet season is also suspected to contribute to MPs pollution.
The high population density and industrialization of the surrounding area have resulted in a significant nutrient load, leading to high CO2 absorption and issues of hypoxia and anoxia. This has resulted in the emission of high levels of CO2, with bottom hypoxic and anoxic waters developing from early summer to autumn in the central bay and enclosed areas.
To address these challenges, integrated management schemes have proven effective, demonstrating greater nitrogen pollution reduction in bay and river systems. Additionally, the dissolved inorganic phosphorus (DIP) concentration in the eutrophic Tokyo Bay has decreased in recent decades, indicating further progress in mitigating water pollution in this critical urban coastal ecosystem.
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Frequently asked questions
The pollution level in Tokyo Bay is high, with a mean water depth of 19 meters and an area of approximately 920 square kilometers.
Tokyo Bay has been affected by eutrophication, hypoxia, and anoxia, resulting in high carbon dioxide emissions. The bay also faces issues with microplastics, with 77% of Japanese anchovy in the bay found to have ingested microplastics.
The main sources of pollution in Tokyo Bay are the four major rivers that discharge into the western side of the bay: the Edogawa River, Arakawa River, Tamagawa River, and Tsurumigawa River.
While the pollution level in Tokyo Bay is high, it is lower than other semi-closed bays due to differences in the Enclosed Index.
