Bays offer a diverse range of geographic formations. These range from Australia’s secluded Botany Bay, which is 1 mile wide at its mouth, to Canada’s vast Hudson Bay, spanning 316,000 square miles. The term “bay” describes a semi-enclosed ecosystem situated where rivers and lakes intersect with the ocean, creating a blend of fresh and saltwater environments that nurture an array of unique species—conditions that also render them particularly vulnerable to the severe impacts of ocean pollution.

While pollutants in the wider ocean are quickly swept away by currents before being diluted, the calmer waters and sheltered location of bays lead to higher concentrations of toxic substances. “Plastic gets trapped in the bay and slowly accumulates in the sediments,” says Helen Bostock Lyman, associate professor and deputy head of the School of the Environment at the University of Queensland. “Nutrients and chemicals aren’t quickly washed away by ocean currents, so they are there long enough to change the biology of the water.”

This is compounded by the fact that bays worldwide are important economic and population centres. For instance, the Bay of Bengal houses major industries such as oil refineries, thermal power facilities, aluminium plants, paper mills, and fertiliser plants in an area populated by over a billion people. Smaller bays are equally crucial for local industries and services, such as the UK’s Liverpool Bay, which hosts two offshore wind farms as well as a major commercial port. Yet the pollution that contaminates these kinds of crucial areas is often chronic and persistent, coming from overlooked sources. “When it comes to acute incidents like oil spills, public money and coast guards are often immediately deployed, but chronic problems need attention too,” says Jesse Ausubel, director of the programme for the human environment at Rockefeller University.

One of the primary challenges in addressing chronic ocean pollution is the complexity of attributing responsibility or assigning liability. “It’s very difficult to show where water pollution is coming from or pin it on a particular polluter because it’s so diffuse,” says Sandy Luk, CEO of the Marine Conservation Society. “Even if companies are found liable for chemical pollution, the question is whether remediation of the pollution is even possible.”

Moreton Bay, near Brisbane, Australia, has experienced chronic pollution since the start of European settlement and agricultural activities, says Professor Bostock, who worked on a study of plastic pollution in the bay alongside Dr Elvis Okoffo at the University of Queensland. Clearing the land for agriculture increased sediment runoff into the bay. The situation further deteriorated as industrial development intensified, leading to the discharge of chemicals from sewerage systems, fertilisers, and wastewater treatment plants into the adjoining rivers, ultimately finding their way into the bay. “Up until the 1980s, people thought the ocean was too large to be influenced by added chemicals until we started seeing toxic algal blooms from the increased concentration of nutrients that were completely changing the chemistry of the bay,” says Professor Bostock.

Algal blooms are fed by nutrients such as nitrogen and phosphorus, commonly found in agricultural runoff. They cause anoxia by depleting the water of oxygen as the algae die and break down, creating “dead zones” where no marine life can exist. A huge dead zone in the Bay of Bengal is nearing a tipping point where a further reduction of oxygen could strip the water of nitrogen entirely, which scientists warn will have consequences for the global nutrient cycle as well as millions of people’s livelihoods. Toxic algal blooms are triggered by warm water and sunlight, so they will worsen alongside climate change.

“Bays are where most of the pollution gets stored, giving us the best idea of what’s happening,” says Professor Bostock. “But they’re also home to so many important ecosystems, as well as fishing, aquaculture and recreational activities, so they’re where people are most likely to feel the impacts of pollution.”

Many of the most at-risk bays are popular recreation sites cherished for their natural beauty. In fact, 90% of coastal bays in US tourist hotspot Cape Cod have “unacceptable” water quality due to algal blooms from nitrogen and phosphorus pollution. Guanabara Bay, Brazil, is a UNESCO World Heritage Site home to the world-famous Christ the Redeemer statue. It is also one of the world’s most polluted coastal ecosystems. The canals that feed into it have thousands of litres of sewage discharged into them every second. As a result, the beaches are unsafe for swimming and its native population of bottlenose dolphins has nearly disappeared.

Guanabara Bay has seen some improvement through mangrove reforestation initiatives coupled with upgrades to outdated sewage infrastructure. Mangrove trees sequester carbon dioxide, filter toxins from soil and water, and prevent erosion with their intricate root systems, reducing the influx of soil pollutants. Consequently, mangrove forests have facilitated the return of fish and marine life to lagoons along the bay. Conservation efforts involving mangrove preservation have also been implemented in the Bay of Bengal. Other nature-based solutions, like planting seagrass and nurturing mussel populations, have similar effects on reducing the impacts of chemical and nutrient pollution through biological processes that clean and filter the water.

Chesapeake Bay provides hope for future restoration initiatives. This bay was home to some of the first perceived dead zones from nitrogen and phosphorus pollution in the US. With decades of attention, state laws requiring wastewater filtration helped reduce last year’s dead zone to the smallest on record since 1985. Mussel restoration initiatives also contributed. Still, the battle is far from over, as other contaminations, including per- and polyfluoroalkyl substances (PFAS), microplastics, and legacy pollutants like mercury, remain a looming threat to the ecosystem.

“Removing legacy chemical pollutants, heavy metals, microplastics and PFAS from the ocean is extremely difficult, as they’re present even at the bottom of the Mariana Trench and in every drop of water on Earth,” says Ms Luk. “How can you possibly clean all that up?”

Microplastics are particularly troubling, due to the impossible task of removing them and the difficulty of identifying their sources. An estimated 4 million tonnes of microplastics are discharged annually into the Bay of Bengal region; microplastics threaten mangrove forests, jeopardising the environmental benefits of reforestation initiatives. Microplastics were frequently found trapped in mangrove and seagrass areas of Moreton Bay, noted Dr Okoffo.

Many community-led bay clean-up initiatives focus on removing plastic. In the Philippines, hundreds of volunteers collected non-biodegradable waste from Manila Bay to mark International Coastal Cleanup Day in 2022. Others are led by companies deploying innovative technology. The Ocean Cleanup, a non-profit, has removed ten million kilograms of trash from oceans and rivers around the world, using boats to pull a long barrier similar to a net through polluted areas to pick up plastic waste. “Large-scale plastic debris is relatively easy to address through citizen science efforts or clean-ups,” says Professor Bostock. “After flooding in Moreton Bay in 2022, there was a big clean-up to remove larger items from the bay area, but the big problem is when the plastic breaks down into smaller pieces that cannot be easily cleaned up.”

To better understand the microplastic problem, Professor Bostock and Dr Okoffo conducted a study of plastic pollution in Moreton Bay, examining sediments to observe where plastic was entering the water. Although they found plastics in every sediment sample, calculating a total of around 7,000 tonnes of plastic, concentrations varied in different areas. Most were at sites fed by the Brisbane River, which passes through the city and is a discharge point for many wastewater treatment plants. “Once you identify the plastic types present, you can look at the potential contributors—like polyethylene entering from wastewater—and target source control measures accordingly,” says Dr Okoffo. “Tracking sources and implementing measures to control plastics at the source will be the best way to go about correcting the microplastics issue.”

Examining sediments also helps document the history of plastic pollution and what landmarks in plastic production have caused the problem. “There was no plastic in the sediment record before 1970, and concentrations then grew exponentially over time as single-use plastics came into use,” says Professor Bostock. Other innovative research methods are helping to better understand the pollution landscape. One of these is environmental DNA (eDNA) monitoring, which allows researchers to use fish DNA to observe how marine life has been impacted by pollution events and clean-up initiatives. “With eDNA, we can see whether clean-ups of plastic or oil, or other measures like restricting shipping activity, have impacted the biology of the species in an area, which is a level of monitoring that hasn’t been possible in the past,” says Mr Ausubel.

As pollution levels have escalated, bays have emerged as crucial sites for scientific research, providing invaluable insights to guide effective clean-up strategies. For instance, if a clean-up group discovers a high concentration of sanitary items and wet wipes on a beach, it indicates that sewage is a significant source of waste, pointing to the need for targeted interventions in sewage infrastructure. However, when it comes to legacy pollutants like persistent chemicals and microplastics, it is crucial to exercise caution and not overly rely on clean-ups, as they may prove ineffective or even counterproductive by dredging up and re-releasing stored pollutants into the environment.

“We need to stop it at the source so it doesn’t get worse, but once it’s trapped in the sediment, it’s stored there and we don’t want to release it,” says Professor Bostock. “When going about bay clean-ups, it’s important not to disturb the sediments, because this can actually re-release the heavy metals and plastics back into the water column and the ecosystem. It’s better to focus on minimising future inputs.”

Clean-up initiatives can be a means of getting people involved in citizen science, educating them on the importance of the ocean for planetary and human health, says Ms Luk. “But the most crucial step is stopping additional pollutants from entering the ecosystem and natural world,” she says. “Clean-up efforts will always be limited in scope compared to preventing further pollution in the first place.”