Marine debris pollution Plastic pollution Ocean acidification in the
Maldives.
Coral reefs are dying around the world.
Nutrient pollution Eutrophication is an increase in chemical
nutrients, typically compounds containing
nitrogen or
phosphorus, in an
ecosystem. It can result in an increase in the ecosystem's
primary productivity (excessive plant growth and decay), and further effects including lack of oxygen and severe reductions in water quality, fish, and other animal populations.
Nutrient pollution, a form of
water pollution, refers to contamination by excessive inputs of nutrients. It is a primary cause of eutrophication of surface waters, in which excess nutrients, usually
nitrates or
phosphates, stimulate algae growth. Such blooms are naturally occurring but may be increasing as a result of anthropogenic inputs or alternatively may be something that is now more closely monitored and so more frequently reported. The biggest culprit are rivers that empty into the ocean, and with it the many chemicals used as
fertilizers in agriculture as well as waste from
livestock and
humans. An excess of oxygen-depleting chemicals in the water can lead to
hypoxia and the creation of a
dead zone.
Estuaries tend to be naturally eutrophic because land-derived nutrients are concentrated where
runoff enters the marine environment in a confined channel. The
World Resources Institute has identified 375 hypoxic coastal zones around the world, concentrated in coastal areas in Western Europe, the Eastern and Southern coasts of the US, and East Asia, particularly in Japan. In the ocean, there are frequent
red tide algae blooms that kill fish and marine mammals and cause respiratory problems in humans and some domestic animals when the blooms reach close to shore. In addition to
land runoff, atmospheric
anthropogenic fixed nitrogen can enter the open ocean. A study in 2008 found that this could account for around one third of the ocean's external (non-recycled) nitrogen supply and up to three per cent of the annual new marine biological production. It has been suggested that accumulating reactive nitrogen in the environment may have consequences as serious as putting carbon dioxide in the atmosphere. One proposed solution to eutrophication in estuaries is to restore shellfish populations, such as oysters.
Oyster reefs remove nitrogen from the water column and filter out suspended solids, subsequently reducing the likelihood or extent of
harmful algal blooms or anoxic conditions. Filter feeding activity is considered beneficial to water quality by controlling phytoplankton density and sequestering nutrients, which can be removed from the system through shellfish harvest, buried in the sediments, or lost through
denitrification. Foundational work toward the idea of improving marine water quality through shellfish cultivation to was conducted by Odd Lindahl et al., using
mussels in Sweden.
Industrial pollution and toxic chemicals Apart from plastics, there are particular problems with other toxic pollutants that either do not break down or only very slowly in the marine environment. Examples of
persistent toxicants are
PCBs,
DDT,
TBT,
pesticides,
furans,
dioxins,
phenols,
radioactive waste, and
PFAS.
Heavy metals are metallic chemical elements that have a relatively high density and are toxic or poisonous at low concentrations. Examples are
mercury,
lead,
copper and
cadmium. Some
toxicants can accumulate in the tissues of many species of aquatic life in a process called
bioaccumulation. They are also known to accumulate in
benthic environments, such as
estuaries and
bay muds: a geological record of human activities of the last century. DDT is a very toxic chemical that was used as a pesticide in mass quantities throughout the United States and is known to be neurotoxic, a reproductive toxin, an
endocrine disruptor, and a carcinogen. DDT is a major focus of the book
Silent Spring published by
Rachel Carson in 1962. This is often attributed to launching the
modern environmental movement and setting the stage for the creation of the
EPA in 1970. DDT was banned in the U.S. two years later in 1972. Unfortunately, large quantities had already entered the ocean through runoff and had been dumped directly into the ocean. This toxin impacts marine ecosystems by accumulating from
lower trophic levels and up the food chain into higher trophic levels such as from arctic cod into seals, from fish then eaten by dolphins, and from cod and eels into seals. Shortly after Rachel Carson's publication of
Silent Spring, PCBs were identified as another persistent, toxic chemical that has been released in extensive quantities to the environment. PCBs are a very well-studied class of chemicals that are manufactured from oil. These chemicals are banned in the United States under the
Toxic Substance Control Act, but are still found in the soil, air, sediments, and biota. These chemicals cause reproductive issues for many species. PFAS are an important emerging class of man-made persistent toxicants that contain extremely strong
carbon-fluorine bonds which make these chemicals extremely difficult to break down. They have
unique properties that make them useful for manufacturing a wide variety of products such as
firefighting foams, clothing, carpets, and fast food wrappers. These useful properties in manufacturing unfortunately translate to problematic properties in the environment and organisms from plants to people. Because PFAS are not broken down in the environment, they have been circulated through the air and water to essentially all regions of the atmosphere, land, and ocean. These chemicals have many negative effects on marine life, such as significantly inhibited growth of
phytoplankton over time and accumulation in seals, polar bears, and dolphins. Current research is underway investigating the full extent of the harm to marine ecosystems caused by PFAS. ;Specific examples: • Chinese and Russian industrial pollution such as
phenols and heavy metals in the
Amur River have devastated fish stocks and damaged its
estuary soil. • Acute and chronic
pollution events have been shown to impact southern California
kelp forests, though the intensity of the impact seems to depend on both the nature of the contaminants and duration of exposure. • Due to their high position in the
food chain and the subsequent
accumulation of
heavy metals from their diet,
mercury levels can be high in larger species such as bluefin and
albacore. As a result, in March 2004 the
United States FDA issued guidelines recommending that pregnant women, nursing mothers and children limit their intake of tuna and other types of predatory fish. • Some shellfish and crabs can survive polluted environments, accumulating heavy metals or toxins in their tissues. For example,
mitten crabs have a remarkable ability to survive in highly modified
aquatic habitats, including polluted waters. The farming and harvesting of such species needs careful management if they are to be used as a food. • Surface runoff of pesticides can alter the gender of fish species genetically, transforming male into female fish. • Heavy metals enter the environment through
oil spills – such as the
Prestige oil spill on the
Galician coast and Gulf of Mexico which unleashed an estimated 3.19 million barrels of oil – or from other natural or
anthropogenic sources. • In 2005, the
'Ndrangheta, an
Italian mafia syndicate, was accused of
sinking at least 30 ships loaded with toxic waste, much of it radioactive. This has led to widespread investigations into
radioactive waste disposal rackets. • Since the end of World War II, various nations, including the Soviet Union, the United Kingdom, the United States, and Germany, have disposed of chemical weapons in the
Baltic Sea, raising concerns of environmental contamination. • The
Fukushima Daiichi nuclear disaster in 2011 caused radioactive toxins from the damaged power plant to leak into the air and ocean. There are still many isotopes in the ocean, which directly affects the benthic food web and also affects the whole food chain. The concentration of 137Cs in the bottom sediment that was contaminated by water with high concentrations in April–May 2011 remains quite high and is showing signs of very slow decrease with time. • During the 20th century, large amounts of DDT, petroleum products, radioactive materials, sulphuric acid, and other toxins
were dumped in the Pacific Ocean off the coast of Southern California.
Underwater noise Marine life can be susceptible to noise or the sound pollution from sources such as passing ships,
oil exploration seismic surveys, and naval low-frequency active
sonar. Sound travels more rapidly and over larger distances in the sea than in the atmosphere. Between 1950 and 1975, ambient noise at one location in the Pacific Ocean increased by about ten
decibels (that is a tenfold increase in intensity). Underwater noise pollution is unevenly distributed across marine environments, with the highest con-centrations occurring in shipping lanes, port areas, and densely trafficked ocean routes. These areas experience sustained high ambient noise levels due to the dominance of older and larger vessels, which emit significant low-frequency noise (10 to 500 Hz) caused by engine vibrations, propeller cavitation, and hull turbulence. While advancements in ship design have shown potential to reduce noise emissions, older, noisier vessels remain prevalent in major shipping routes, largely due to economic and logistical constraints. Additionally, the overall increase in global shipping activity in the 20th century contributed to a rise of approximately 12 decibels in ambient noise levels in the
Northern Hemisphere, particularly in the low-frequency range, which propagates over long distances with minimal attenuation. Studies in the
Southern Hemisphere do not show a continuation of this trend in the 21st century. The cumulative effects of concentrated noise pollution pose a unique risk to localised ecosystems, particularly for species with limited mobility or specific habitat requirements, as they are unable to escape these high-noise regions. Research also highlights variations in noise behaviour across marine environments, with factors such as water depth, salinity, and seabed composition influencing how noise propagates in coastal areas versus open seas. The localised nature of underwater noise pollution amplifies its ecological consequences, particularly for species that rely on sound for survival. The ecological impacts of underwater noise are most prevalent for marine mammals like whales and dolphins, which rely heavily on sound for communication, navigation, and foraging. Cetaceans, such as whales and dolphins, are especially vulnerable because they rely on echolocation and acoustic signals for communication and navigation. They experience disrupted communication patterns, altered migration routes, and stress-related behavioural changes as some of the consequences of chronic exposure to ship noise. For example, endangered whale populations in the Saguenay–St. Lawrence Marine Park experience considerable acoustic space reduction, limiting their communication ranges and altering their natural behaviours. Studies have shown that underwater noise can reduce communication ranges, impairing essential behaviours such as mating and social cohesion. Beyond marine mammals, fish and invertebrates are also affected, though they are less frequently studied. Fish use acoustic signals for mating, predator avoidance, and territory defence. Noise interference can cause habitat avoidance, reduced reproductive success, and disrupted predator-prey relationships, destabilising local food webs. These cumulative effects of URN contribute to the destabilisation of nutrient cycling and broader eco-system processes. Noise also makes species communicate louder, which is called the Lombard vocal response.
Whale songs are longer when submarine-detectors are on. If creatures don't "speak" loud enough, their voice can be masked by
anthropogenic sounds. These unheard voices might be warnings, finding of prey, or preparations of net-bubbling. When one species begins speaking louder, it will mask other species voices, causing the whole ecosystem to eventually speak louder. Noise from ships and human activity can damage Cnidarians and Ctenophora, which are very important organisms in the marine ecosystem. They promote high diversity and they are used as models for ecology and biology because of their simple structures. When there is underwater noise, the vibrations in the water damage the cilia hairs in the Coelenterates. In a study, the organisms were exposed to sound waves for different numbers of times and the results showed that damaged hair cells were extruded or missing or presented bent, flaccid or missed kinocilia and stereocilia. Ships can be certified to meet certain noise criteria. According to the oceanographer
Sylvia Earle, "Undersea noise pollution is like the death of a thousand cuts. Each sound in itself may not be a matter of critical concern, but taken all together, the noise from shipping, seismic surveys, and military activity is creating a totally different environment than existed even 50 years ago. That high level of noise is bound to have a hard, sweeping impact on life in the sea." Sources of noise below 100 Hz in the ocean include ships and airgun arrays. However, the non-mandatory nature of these guidelines has resulted in inconsistent adoption across the shipping industry. In contrast, the European Union's Marine Strategy Framework Directive (MSFD) mandates the management of underwater noise levels as part of achieving or maintaining Good Environmental Status (GES). Scholars argue that a combination of technical and economic measures is needed to tackle the issue effectively. These include mandatory noise limits, subsidies for retrofitting ships with quieter technologies, and spatially informed policies, such as the creation of quiet zones or Marine Protected Areas (MPAs), to safeguard sensitive ecosystems.
Other There are a variety of secondary effects stemming not from the original pollutant, but a derivative condition. An example is
silt-bearing
surface runoff, which can inhibit the penetration of sunlight through the water column, hampering
photosynthesis in aquatic plants.
Dredge plumes can contain silt and thus have similar effects on aquatic life. ==Mitigation==