9.3 Threats to ocean health and biodiversity

Anthropogenic threats to ecosystem health and biodiversity of oceans can modify the abiotic aquatic environment, for example, through water temperature and chemistry, by adding noise, and by changing atmospheric weather patterns. Threats can target specific biotic and abiotic natural resources for subsistence use or for economic gain, possibly affecting the aquatic environment at the same time. For much of human history, we lacked the capacity to cause harm to the oceans as a whole, however, as ships long ago became available that could cross oceans, and technology more recently developed that can seek out individual whales and schools of fish, human capacity for harm has advanced as well. And climate change has taken the concept of harm to oceans to new levels.

Among the UN Sustainable Development Goals, life in water is obviously relevant here, but so many human actions and benefits intersect with marine ecosystems and marine life that all of them are involved to some extent. Among the planetary boundaries, ocean acidification is directly tied to marine ecosystems, but it is caused by climate change and both of these threaten biosphere integrity. Modification of biogeochemical flows, especially of N and P causes the dead zones we first met in Chapter 2.

Pollution

Land-based pollution

In Chapter 8 we discussed threats to land-based ecosystems and biodiversity, including threats to rivers. Because most rivers discharge into the oceans, those threats are also marine threats. River pollution from industrial and municipal waste and runoff including plastic waste and the microplastics that derive from it, and run-off from agricultural lands all contribute to pollution of oceans. Coastal waters are most affected, due to their proximity to the land source. However, ocean currents carry pollution throughout the oceans.

Nutrient pollution from land activity is associated with coastal dead zones around the world (Fig 1). These are sometimes associated with agricultural lands but are often associated with high nutrient loads from municipal water treatment.

Land-based air pollution can also affect oceans. We saw, in chapter 2, that Saharan dust – particulate pollution – fertilizes the Atlantic, providing iron and other minerals that are in short supply in the surface waters of deep oceans. In the early 2000s, acid rain impacts on coastal waters were noted, but these have been reduced by reductions in sulfate air pollution. Now, ocean acidification occurs primarily as a result of climate change, as we will see shortly.

Pollution from sources on the oceans

Ocean-going ships contribute antifouling chemicals, sewage, leaking oil and gas, and air pollution to marine pollution. Antifouling paints are used on ships’ hulls to repel barnacles and other hitchhiking organisms that would slow the ship as it moves through the water. They are toxic and leach in small quantities. They also save fuel which reduces GHG emissions, and reduce chances of ships introducing hitchhikers into new waters where they could be invasive. Oil and gas leaks are generally in small quantities and are not considered major sources of water pollution for oceans.

GHG emissions and air pollution from ocean shipping (Fig 2) are a sufficient cause for concern that the International Maritime Organization, a rule-making body formed under UNCLOS, passed regulations in 2020 sharply reducing limits for sulfur in ship fuel. The resulting decrease in sulfate emissions reduced albedo and may have contributed slightly to warming over the Atlantic, ^[1] but did improve air quality and reduce associated acid rain. The IMO is also working to achieve zero emissions from the world shipping fleet by 2050; presently, the fleet is responsible for 2-3% of GHG emissions, globally.^[2]

Spills from ships carrying fossil fuels (Fig 3) and toxic chemicals cause local mortality of sea life and foul waters, in some cases for long periods. When the Exxon Valdez ran aground in 1989 in Prince William Sound, off the coast of Alaska, it dumped approximately 11 million gallons (41,640 cu m) of crude oil into coastal waters, much of which escaped initial clean-up efforts.

In the cold waters of the Alaskan coast, the oil that came onshore congealed into a mousse-like consistency that was largely undiminished on affected beaches in 2015.^[3] Wildlife deaths from acute effects included an estimated 250,000 seabirds, 302 harbor seals, and up to 2800 sea otters. Chronic effects included higher than usual mortality of salmon eggs in exposed streams at least 4 years later, prolonged recovery in sea otters (by 2014) and seabirds, some of which had not recovered as of 2014.^{[4]  [5]} In contrast, a 2007 spill of about one third the size of the Exxon Valdez spill occurred as a result of an accident involving the Hebei Spirit in the port of Daesan in South Korea was dealt with quickly, with extensive clean-up efforts. By 2013, intertidal species were considered “fairly recovered” and fish and plankton were fully recovered. Impacts to marine mammals and birds were not reported in Marron et al. (2020). ^[6] Such spills are uncommon and localized, but coastal habitats are biodiverse and spills can affect many species.

Ocean-going fishing fleets contribute substantial amounts of discarded fishing gear to the oceans each year. These are now the largest component of the various “garbage patches” that occupy large areas of the major ocean basins. The circulating currents in these basins – gyres – collect floating debris into a large, very loose masses. The material includes microplastics and much of it floats slightly below the surface, making it less visible. A research sampling mission into the North Pacific Garbage Patch in 2019 attributed 47% of the mass of plastic collected to fishing gear, with another 13% from plastic bottles from land. They traced most of the plastics in the gyre to the major industrial fishing nations of the region: Japan, China, Korea, USA, Taiwan and Russia.^[7] The materials in garbage patches leach toxins into the ocean and can entangle wildlife (see the discussion of ghost nets in Chapter 8). They degrade into microplastics, whose impacts we saw in section 3.4. The garbage patches become homes for marine organisms, and being novel habitats, they can support species that are out of place – such as crabs, which are not common in the surface waters of the deep oceans – and likely to become invasive. They can then transport these species around the oceans, increasing the odds that these potential invasive species will become truly invasive.

Fossil-fuel production at sea contributes GHG emissions and leaking oil and gas, as do ships. But failures at drilling rigs can be more substantial. In 2010, an explosion on the Deepwater Horizon resulted in continuous release of oil from the wellhead for more than 4 months. The US government estimated that 210 million gallons of oil were released (780,000 cu m). Clean-up efforts in these warmer waters (warmer at the surface – at the wellhead on the ocean floor, water temperatures were approximately 4°C (40°F)) included the use of chemicals to keep the oil from clumping, which were themselves potentially harmful to sea life, but reduced harm from the oil itself. Unlike the Exxon Valdez and Hebei Spirit spills, which occurred close to shore, the Deepwater Horizon wellhead was 41 miles (66 km) from shore; oil was released into the pelagic zone and oiled deep-sea corals in addition to fish, shellfish, marine mammals, and seabirds. The government agency tasked with the damage assessment estimated, among other losses, 4-8.3 billion harvestable oysters lost, 51-84,000 birds killed, 56,000 to 166,000 juvenile sea turtles killed, the local dolphin population decreased by 51%, and the local recreation industry lost over half a million dollars.^[8][9] Recovery times for longer-lived species such as sea turtles are estimated in decades. Recovery of the local dolphin population was estimated to need 39 years unless active restoration was undertaken.^[10]

Seabed mining is a proposed activity that has not yet commenced, owing to disagreements about how to proceed. Seabed mining is anticipated to cause substantial turbulence and turbidity and has the potential to be associated with the same kind of leakage and spills as accompanies shipping. In 2025, in the US, an executive order directed fast-tracking of extraction of critical minerals from the ocean floor, potentially in contravention of UNCLOS, which the US has not ratified, and by which it is therefore not bound.

Noise pollution from ocean-going ships, from sonar “thumping” used in oil exploration, and from in-ocean installations such as drilling rigs and wind turbines, disrupts navigation by and communication among cetaceans – whales and porpoises. The interference reduces feeding success and increases vulnerability to ship strikes, which are an important source of mortality.

Overfishing, including destructive fishing

Food and Agriculture Organization estimates show that the proportion of overfished stocks increased rather steadily from about 10% of stocks to about 30% of stocks from 1975 to 2021, while the proportion of stocks fished at the limit of sustainability increased from about 50% to about 60% (Fig 4).

In Section 8.4, we saw that many kinds of fishing gear catch many different species, including many non-target species, and may continue catching things even after they are discarded. Drift nets that hang in the surface waters are limited by the UN to 2.5 km (1.55 mi) in length, but illegal nets may be much longer – nets as long as 50 km (31 mi) have been reported.

Baited longlines can be set at any depth, depending on the main species targeted. Longlines in US waters average 28 miles (45 km long); in international waters they may extend over 100 miles (160 km). Longlines contain 25-50 hooks per mile.

Trawl nets can be large enough to enclose commercial aircraft, but the front-end mesh is usually quite large, to allow escape of some species such as smaller whales and dolphins. Slower swimming species are less likely to escape this way, as they must swim faster than the trawl is being dragged. Trawl nets can be dragged at any depth; they are particularly destructive when dragged along the bottom.

In addition to fishing gear with great potential to catch target and nontarget organisms, fishing ships also often employ a variety of sonar and other fish-finding technologies. The intensity of effort is often underwritten by governments supporting fishing fleets that have become too large to be efficient but that remain important to coastal economies. It’s not surprising, then, that overfishing results, even in managed fishing. But as we saw earlier, illegal take amounts to perhaps 20% of fishing harvest, overall.

One practice supporting illegal fishing practices is reflagging, in which ship owners register their ships under a nation other than their nation of citizenship or the nation of the port at which they do most of their business. Reflagging is (more or less) legal, and a few nations attract most of the ship owners who reflag. Often, regulations in the attractive nations allow owners to remain anonymous, set low taxes on ship profits, have few requirements associated with pay and treatment of ship crews, and have low enforcement of fishing regulations.^[11] The UN estimates that 73% of ships operate under so-called flags of convenience. The 4 largest registries are Liberia, Panama, the Marshall Islands, and Hong Kong.^[12]

As we have seen, legal fishing can have considerable bycatch of unwanted but still dead organisms. Illegal fishing is no different. Bycatch mortality is the leading threat listed for sea turtles, the tiny vaquita porpoise, most species of albatross and many other endangered marine species (Fig 5).

Destructive fishing destroys habitat in search of fish. Use of dynamite and poisons in reefs destroy reef structure or kill coral. Small doses of cyanide can be used to stun fish for the aquarium trade; the practice is considered a major threat to reefs where it is practiced.^[13] Dynamite is less target-specific and kills fish – it is used for subsistence fishing.

Invasive species

Estimates of economic harm from invasive species typically run into billions of dollars. As with terrestrial invasive species, marine invaders can disrupt food webs; destroy habitat; cause damage, disease and mortality; and modify entire ecosystems. They include algae that smother coral reefs; diseases that can extirpate native species; and toxic and poisonous species.

Marine invaders may travel on debris carried by wind and currents. Plastics and other anthropogenic debris are much more common than naturally occurring debris, and may last longer in the water, increasing their ability to disperse species around the world, thereby supporting invasions.

Ocean-going ships spread invasives in two major ways. Like pieces of debris, they can acquire hitchhikers that travel with them, attaching and dropping off as the ship travels. In addition, ocean-going ships take on ballast water to stabilize the ship and strengthen the hull. The less cargo the ship carries, and the more rambunctious the seas in which it travels, the more weight of water it must take on in order to sit properly in the water. With the water come larval fish and other marine life, along with plankton of all kinds. When a ship carrying ballast water receives more load, it needs less ballast water and discharges it, along with at least a sample of the organisms it scooped up with the water when it took on the ballast water. An American comb jellyfish introduced to the Black Sea, probably in ballast water, consumes such volumes of plankton that they severely reduce populations of native fish, including commercially important species.

The International Maritime Organization of UNCLOS has a Ballast Water Management Convention that provides guidance on ballast water practices that limit invasive species – ships take up coastal water in port, then exchange it, in the open ocean, where coastal organisms won’t likely survive, then dump the deep-ocean water when they get to port, into the coastal waters where the deep-ocean organisms are less likely to survive. Many nations have adopted the guidance into national law.

The ocean-bridging canals – Suez and Panama – are also potential avenues for invasion. Invasions of the Mediterranean from the Red Sea are fairly well documented and include 137 species of mollusks. Red Sea fish species have become so common in the Mediterranean that there are markets for some. Others, such as lionfish, are pest species that disrupt tourism and are harmful to human health.^[14]

Climate-change impacts on oceans

If climate-change impacts are increasingly bad on land, they are worse in the oceans. Because water is slow to change temperature, the marine environment is much less variable, thermally, than the terrestrial environment and marine species are less likely to tolerate temperature changes.

Impacts on ocean circulation

In Section 2.2, when we discussed tipping points, we briefly visited the possibility of a tipping point involving the major ocean circulatory pattern called the Atlantic Meridional Overturning Circulation (AMOC). This ocean conveyor belt is driven by sinking of cold salty water in the North Atlantic, together with the forces of a spinning planet, which turn the waters of the ocean in clockwise loops in the Northern Hemisphere and counterclockwise loops in the Southern Hemisphere. The Gulf Stream of the North Atlantic is an important part of AMOC that sends warm water from the Gulf of Mexico north and east across the Atlantic towards the United Kingdom and Europe, where it provides a warming effect. A collapse in the AMOC is predicted to have greatest impact on weather in this region.

Our understanding of the physics of the AMOC is still evolving. Very early climate-change literature suggested the AMOC might collapse under climate change, as melting freshwater from the Greenland icesheets diluted and floated over the Gulf Stream, reducing the rates of sinking salty water. Later but still early work suggested this was less likely. In 2020, a review of the earth system models used to study climate change showed that most of the models predicted a 34-45% weakening of the current.^[15] A more recent study concurs, but with a wider range of uncertainty: 18-43%, underscoring that we still have much to learn about this important aspect of planetary function.^[16] Note that, the article estimating a 34-45% decline uses the phrase “significant 21st century decline” while the other, indicating an 18-43% decline, describes it as “limited future … weakening!”

Impacts on marine ecosystems and species of warming waters

The oceans of the world have absorbed approximately 90% of the warming created by anthropogenic GHGs, saving the terrestrial world from warming far more than it has. The top 700 m (2300 ft) has already warmed approximately 1.5°F (0.8°C) since 1901. Because water takes so much energy to warm, the waters have warmed much less than the terrestrial world would have. But that energy is now bound in the ocean waters, and to return to pre-climate-change temperatures, that heat will need to dissipate.

Even more concerning is that ocean temperatures seem to have passed a tipping point with respect to extreme temperatures – marine heat waves – which have become the new normal. In 2019, 57% of the global ocean surface recorded extreme heat, which was rare before 1870-1919. Significant increases in the extent of extreme marine events over the past century resulted in many local climates shifting out of their historical sea-surface temperature bounds.

For the global ocean, 2014 was the first year to exceed the 50% threshold of extreme heat, with the South Atlantic (1998) and Indian (2007) basins crossing this barrier earlier. “Extreme” heat thus became the norm, rather than an extreme, in 2014.

Because of water’s thermal inertia – because it takes considerable energy to change the temperature of water – aquatic environments are thermally stable, relative to terrestrial environments. As a result, aquatic organisms tend to have narrower thermal tolerances than terrestrial organisms. Warming that might stress a terrestrial organism slightly is likely to stress an aquatic organism more. In addition to temperature stress, decreasing oxygen levels due to warming waters are also provoking range shifts. More mobile aquatic species can move poleward to cooler and more oxygen-rich waters, and they are moving, as we saw in Section 8.3.

Corals are not mobile. Under heat stress, reef-building corals expel their partner algae, called zooxanthellae, creating a condition called bleaching. Zooxanthellae are responsible for the colors in corals and also provide most of the food for coral, through photosynthesis. When they are gone, only the white skeleton remains. If heat stress continues for more than a few days, corals begin to starve. Some zooxanthellae and some corals are more resilient to heat than others, but most are already badly stressed by marine heat.

During 2014-2017, during a three-year heat event, more than 75% of all coral reefs bleached. Reefs can recover from bleaching over time, but repeated heat stress prevents recovery. In addition, coral diseases are increasing in frequency and severity with warming waters. In the 6th Assessment Report in 2023, the IPCC warned that a tipping point for coral extirpation was nearly reached. In 2025, researchers reported that it had passed. The world-wide 2023-2025 coral bleaching event affected 84% of world corals, and the warming point associated with the tipping point – 1.2 C above historical average temperature, was passed.^[17] Researchers predict that 95% of coral reefs will succumb to climate change by 2100. Even if all GHG emissions were to cease immediately, anticipated loss of coral reefs would still proceed, due to the momentum of existing GHG levels and because the ocean will be very slow to cool, once GHG emissions return to pre-industrial levels.

As we saw in Chapter 8, marine heat waves harm more than coral ecosystems. Extended heat waves can collapse ocean productivity and cause the collapse of entire food webs. As heat waves become the new normal, these impacts become more common.

Because species do not respond to climate change in the same way and at the same pace, phenological mismatches are increasingly being observed. Phenology is the study of life-history events – breeding, hatching, flowering, migration, leaf fall, seed set, etc. Phenological mismatches occur when coordinated life history events – like breeding by birds and hatching of insects in spring – become uncoordinated. For example, the timing of juvenile salmon migration from freshwater to the oceans is becoming misaligned to the availability of prey in the nearshore ocean waters.^[18]

Impacts of sea-level rise

As it warms above 4°C (40°F), water expands. More warmth, more volume of water. The IPCC Sixth Assessment Report indicated that thermal expansion of the global ocean accounted for 38% of sea-level rise from 1901 to 2018.^[19] For 1993–2003, thermal expansion was estimated to cause about half of the observed rate of global sea-level rise.

Melting of floating ice does not contribute to sea-level rise. When the ice in your soft drink melts, your glass does not overflow. But melting of glaciers and ice sheets on land does contribute to sea-level rise and it is increasing. Nevertheless, warmer, plumper water is still causing over one-third of sea-level rise. As ice sheets melt faster and provide a competing source of sea-level rise, that proportion will drop.

Sea-level rise is not constant around the planet, owing to wind activity that piles water up in some areas and blows it away from other areas. But, on average, experts predict that by the end of the century, sea level will have increased by 30 cm (1 ft) over levels in 2000, even for fairly mild increases in GHG.^[20]

Sea-level rise affects coastlines by increasing erosion, flooding coastal ecosystems such as salt marshes and mangroves forests, and overwashing into freshwater coastal wetlands during storms. Storm damage to ecosystems and property is greater because both property and ecosystems were established during times of lower sea levels. Carbon sequestered in coastal and marine ecosystems is at risk from erosion and ecosystem loss due to rising seas.

Rising seas also flood beaches of nesting sea turtles. In many places, inland migration of ecosystems and sea turtles is not possible due to coastal development.

Sea-level rise deepens waters over seagrass beds, kelp forests, and warm-water coral reefs. Deeper waters are colder, which can reduce heat damage, but they also limit light, reducing productivity. Increasing depth will exceed tolerances of shallow-water species.

Ocean acidification

As the amount of CO₂ in the atmosphere increases, the amount of CO₂ dissolved in the ocean increases. CO₂ dissolved in water creates carbonic acid, which lowers the pH of the ocean, increasing acidity, just as it makes rainwater acid. Ocean acidity has already increased nearly 30% compared to pre-industrial times and a doubling is predicted by 2100.

Carbonate chemistry is slightly complicated, because of buffering reactions but the simple version is that acid dissolves calcium carbonate. Calcium carbonate makes up coral reefs, mollusk and other marine shells, and shells of crabs and lobsters. Calcium carbonate occurs in three different crystalline forms, and they are differently susceptible to dissolution by acids. The form in coral and mollusk shells is more vulnerable than the form in crab and lobster shells.

Because gases dissolve better in cold water than in warm water, ocean acidification is proceeding more quickly in polar and deep waters. Thus, while warm-water corals are most vulnerable to warming oceans, cold-water corals are more vulnerable to ocean acidification. But both kinds of corals are exposed to both stressors; the deep oceans are warming significantly, and acidification is underway in warm waters, just slower than in cold waters.

Corals can still grow under acid conditions, but they grow longer and thinner and are more susceptible to damage. But the reef structure, which is purely mineral, is increasingly dissolved by the acid water, weakening the whole reef and potentially, over time, reducing it to rubble.

Shelled sea species are having increasing trouble building their shells against the dissolving forces of the acid ocean. Pteropods (also called sea butterflies) are shelled planktonic organisms important in ocean food webs (Fig 6). Because their shells sink to the seabed after death, pteropods also contribute to the ocean carbon pump. As early as 2012, researchers observed severely eroded shells in pteropods in the South Ocean.^[21]

In Washington State in the US, Puget Sound has some of the most acidic waters in the world, and oyster hatcheries in the region are unable to raise oysters in their earliest stages; they raise juveniles elsewhere and bring them back to Washington when they are big enough. ^[22] Native shellfish do not have the option of being reared elsewhere and will disappear from the region when increasing acidity causes similar problems with their larvae and young.

A 2025 report indicated that, like the coral warming threshold – a threshold for a group of species, rather than for the planet – the planetary boundary associated with ocean acidification is also in the process of being crossed.^[23] Although acidification, like warming and sea-level rise, is variable from place to place, all three are increasing everywhere. Range shifts poleward can alleviate warming impacts, but not acidification or sea-level rise.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.