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Human activities affectmarine life andmarine habitats throughoverfishing,habitat loss, the introduction ofinvasive species,ocean pollution,ocean acidification andocean warming. These impactmarine ecosystems andfood webs and may result in consequences as yet unrecognised for thebiodiversity and continuation of marine life forms.^[3]

The ocean can be described as the world's largest ecosystem and it is home for many species of marine life. Different activities carried out and caused by human beings such as global warming, ocean acidification, and pollution affect marine life and its habitats. For the past 50 years, more than 90 percent ofglobal warming resulting from human activity has been absorbed into the ocean. This results in the rise of ocean temperatures and ocean acidification which is harmful to many fish species and causes damage to habitats such ascoral.^[4] With coral producing materials such as carbonate rock and calcareous sediment, this creates a unique and valuable ecosystem not only providing food/homes for marine creatures but also having many benefits for humans too. Ocean acidification caused by rising levels ofcarbon dioxide leads to coral bleaching where the rates of calcification is lowered affecting coral growth.^[5] Additionally, another issue caused by humans which impacts marine life ismarine plastic pollution, which poses a threat to marine life.^[6] According to theIPCC (2019), since 1950 "many marine species across various groups have undergone shifts in geographical range and seasonal activities in response to ocean warming, sea ice change and biogeochemical changes, such as oxygen loss, to their habitats."^[7]

It has been estimated only 13% of the ocean area remains aswilderness, mostly in open ocean areas rather than along the coast.^[8]

Overfishing is occurring in one third of world fish stocks, according to a 2018 report by theFood and Agriculture Organization of the United Nations.^[9] In addition, industry observers believeillegal, unreported and unregulated fishing occurs in most fisheries, and accounts for up to 30% of total catches in some important fisheries.^[10] In a phenomenon calledfishing down the foodweb, themean trophic level of world fisheries has declined because ofoverfishing hightrophic level fish.^[11]

Coastal ecosystems are being particularly damaged by humans.^[13] Significanthabitat loss is occurring particularly in seagrass meadows, mangrove forests and coral reefs, all of which are in global decline due to human disturbances.

Coral reefs are among the more productive and diverse ecosystems on the planet, but one-fifth of them have been lost in recent years due to anthropogenic disturbances.^[14][15] Coral reefs aremicrobially driven ecosystems that rely onmarine microorganisms to retain and recycle nutrients in order to thrive inoligotrophic waters. However, these same microorganisms can also trigger feedback loops that intensify declines in coral reefs, with cascading effects acrossbiogeochemical cycles andmarine food webs. A better understanding is needed of the complex microbial interactions within coral reefs if reef conservation has a chance of success in the future.^[16]

Seagrass meadows have lost 30,000 km² (12,000 sq mi) during recent decades. Seagrassecosystem services, currently worth about $US1.9 trillion per year, includenutrient cycling, the provision of food and habitats for many marine animals, including the endangereddugongs,manatee andgreen turtles, and major facilitations forcoral reef fish.^[13]

One-fifth of the world'smangrove forests have also been lost since 1980.^[17] The most pressing threat tokelp forests may be the overfishing of coastal ecosystems, which by removing higher trophic levels facilitates their shift to depauperateurchin barrens.^[18]

Aninvasive species is a species not native to a particular location which can spread to a degree that causes damage to the environment, human economy or human health.^[19] In 2008, Molnar et al. documented the pathways of hundreds of marine invasive species and found shipping was the dominant mechanism for the transfer of invasive species in the ocean. The two main maritime mechanisms of transporting marine organisms to other ocean environments are viahull fouling and the transfer ofballast water.^[20]

Ballast water taken up at sea and released in port is a major source of unwanted exotic marine life. Theinvasive freshwater zebra mussels, native to the Black, Caspian, and Azov seas, were probably transported to the Great Lakes via ballast water from a transoceanic vessel.^[21] Meinesz believes that one of the worst cases of a single invasive species causing harm to an ecosystem can be attributed to a seemingly harmlessjellyfish.Mnemiopsis leidyi, a species of comb jellyfish that spread so it now inhabits estuaries in many parts of the world, was first introduced in 1982, and thought to have been transported to theBlack Sea in a ship's ballast water. The population of the jellyfish grew exponentially and, by 1988, it was wreaking havoc upon the localfishing industry. "Theanchovy catch fell from 204,000 tons in 1984 to 200 tons in 1993;sprat from 24,600 tons in 1984 to 12,000 tons in 1993; horsemackerel from 4,000 tons in 1984 to zero in 1993."^[22] Now that the jellyfish have exhausted thezooplankton, including fish larvae, their numbers have fallen dramatically, yet they continue to maintain a stranglehold on theecosystem.

Invasive species can take over once occupied areas, facilitate the spread of new diseases, introduce newgenetic material, alter underwater seascapes, and jeopardize the ability ofnative species to obtain food. Invasive species are responsible for about $138 billion annually in lost revenue and management costs in the US alone.^[23]

Nutrient pollution is a primary cause ofeutrophication of surface waters, in which excess nutrients, usuallynitrates orphosphates, stimulate algae growth. This algae then dies, sinks, and is decomposed by bacteria in the water. This decomposition process consumes oxygen, depleting the supply for other marine life and creating what is referred to as a "dead zone." Dead zones are hypoxic, meaning the water has very low levels of dissolved oxygen. This kills off marine life or forces it to leave the area, removing life from the area and giving it the name dead zone. Hypoxic zones or dead zones can occur naturally, but nutrient pollution from human activity has turned this natural process into an environmental problem.^[29]

There are five main sources of nutrient pollution. The most common source of nutrient runoff is municipal sewage. This sewage can reach waterways through storm water, leaks, or direct dumping of human sewage into bodies of water. The next biggest sources come from agricultural practices. Chemical fertilizers used in farming can seep into ground water or be washed away in rainwater, entering water ways and introducing excess nitrogen and phosphorus to these environments. Livestock waste can also enter waterways and introduce excess nutrients. Nutrient pollution from animal manure is most intense from industrial animal agriculture operations, in which hundreds or thousands of animals are raised in one concentrated area. Stormwater drainage is another source of nutrient pollution. Nutrients and fertilizers from residential properties and impervious surfaces can be picked up in stormwater, which then runs into nearby rivers and streams that eventually lead to the ocean. The fifth main source of nutrient runoff is aquaculture, in which aquatic organisms are cultivated under controlled conditions. The excrement, excess food, and other organic wastes created by these operations introduces excess nutrients into the surrounding water.^[30]

Toxic chemicals can adhere to tiny particles which are then taken up byplankton andbenthic animals, most of which are eitherdeposit feeders orfilter feeders. In this way, toxins areconcentrated upward within oceanfood chains. Many particles combine chemically in a manner which depletes oxygen, causingestuaries to becomeanoxic.Pesticides andtoxic metals are similarly incorporated into marine food webs, harming the biological health of marine life. Manyanimal feeds have a highfish meal orfish hydrolysate content. In this way, marine toxins are transferred back to farmed land animals, and then to humans.

Phytoplankton concentrations have increased over the last century in coastal waters, and more recently have declined in the open ocean. Increases in nutrient runoff from land may explain the rise in coastal phytoplankton, while warming surface temperatures in the open ocean may have strengthened stratification in the water column, reducing the flow of nutrients from the deep that open ocean phytoplankton find useful.^[31]

Over 300 million tons of plastic are produced every year, half of which are used insingle-use products like cups, bags, and packaging. At least 14 million^[32] tons of plastic enter the oceans every year. It is impossible to know for sure, but it is estimated that about 150 million metric tons of plastic exists in our oceans. Plastic pollution makes up 80% of all marine debris from surface waters to deep-sea sediments. Because plastics are light, much of this pollution is seen in and around the ocean surface, but plastic trash and particles are now found in most marine and terrestrial habitats, including thedeep sea, Great Lakes, coral reefs, beaches, rivers, and estuaries. The most eye-catching evidence of the ocean plastic problem are thegarbage patches that accumulate ingyre regions. A gyre is a circular ocean current formed by the Earth's wind patterns and the forces created by the rotation of the planet.^[33] There are five main ocean gyres: theNorth andSouth Pacific Subtropical Gyres, theNorth andSouth Atlantic Subtropical Gyres, and theIndian Ocean Subtropical Gyre. There are significant garbage patches in each of these.^[34]

Largerplastic waste can be ingested by marine species, filling their stomachs and leading them to believe they are full when in fact they have taken in nothing of nutritional value. This can bringseabirds,whales,fish, andturtles to die of starvation with plastic-filled stomachs. Marine species can also be suffocated or entangled in plastic garbage.^[35]

The biggest threat of ocean plastic pollution comes frommicroplastics. These are small fragments of plastic debris, some of which were produced to be this small such as microbeads. Other microplastics come from the weathering of largerplastic waste. Once larger pieces of plastic waste enter the ocean, or any waterway, the sunlight exposure, temperature, humidity, waves, and wind begin to break the plastic down into pieces smaller than five millimeters long. Plastics can also be broken down by smaller organisms who will eat plastic debris, breaking it down into small pieces, and either excrete these microplastics or spit them out. In lab tests, it was found that amphipods of the speciesOrchestia gammarellus could quickly devour pieces of plastic bags, shredding a single bag into 1.75 million microscopic fragments.^[36] Although the plastic is broken down, it is still an artificial material that does not biodegrade. It is estimated that approximately 90% of the plastics in the pelagic marine environment are microplastics.^[33] These microplastics are frequently consumed by marine organisms at the base of the food chain, like plankton and fish larvae, which leads to a concentration of ingested plastic up thefood chain. Plastics are produced with toxic chemicals which then enter the marine food chain, including the fish that some humans eat.^[37]

• 
  Microplastics among sand and glass spheres in sediment from theRhine. The white bar represents 1 mm.
• 
  Vast plastic garbage patches like theGreat Pacific Garbage Patch have accumulated at the centre ofocean gyres.^[38]
• 
  Model results for the count density of small planktonic plastic particles^[39]
  

  Red: more dense

  Green: less dense
• 
  Interactions betweenmarine microorganisms and microplastics^[40]

There is a natural soundscape to the ocean that organisms have evolved around for tens of thousands of years. However, human activity has disrupted this soundscape, largely drowning out sounds organisms depend on for mating, warding off predators, and travel. Ship and boat propellers and engines, industrial fishing, coastal construction, oil drilling, seismic surveys, warfare, sea-bed mining and sonar-based navigation have all introducednoise pollution to ocean environments. Shipping alone has contributed an estimated 32-fold increase of low-frequency noise along major shipping routes in the past 50 years, driving marine animals away from vital breeding and feeding grounds.^[41] Sound is the sensory cue that travels the farthest through the ocean, and anthropogenic noise pollution disrupts organisms' ability to utilize sound. This creates stress for the organisms that can affect their overall health, disrupting their behavior, physiology, and reproduction, and even causing mortality.^[42] Sound blasts from seismic surveys can damage the ears of marine animals and cause serious injury. Noise pollution is especially damaging for marine mammals that rely on echolocation, such as whales and dolphins. These animals use echolocation to communicate, navigate, feed, and find mates, but excess sound interferes with their ability to use echolocation and, therefore, perform these vital tasks.^[43]

The prospect ofdeep sea mining has led to concerns from scientists and environmental groups over the impacts on fragiledeep sea ecosystems and wider impacts on the ocean'sbiological pump.^[44][45]

Rapid change to ocean environments allows disease to flourish. Disease-causing microbes can change and adapt to new ocean conditions much more quickly than other marine life, giving them an advantage in ocean ecosystems. This group of organisms includes viruses, bacteria, fungi, and protozoans. While these pathogenic organisms can quickly adapt, other marine life is weakened by rapid changes to their environment. In addition, microbes are becoming more abundant due to aquaculture, the farming of aquatic life, and human waste polluting the ocean. These practices introduce new pathogens and excess nutrients into the ocean, further encouraging the survival of microbes.^[46]

Some of these microbes have wide host ranges and are referred to as multi-host pathogens. This means that the pathogen can infect, multiply, and be transmitted from different, unrelated species. Multi-host pathogens are especially dangerous because they can infect many organisms, but may not be deadly to all them. This means the microbes can exist in species that are more resistant and use these organisms as vessels for continuously infecting a susceptible species. In this case, the pathogen can completely wipe out the susceptible species while maintaining a supply of host organisms.^[46]

In marine environments, microbialprimary production contributes substantially toCO₂ sequestration.Marine microorganisms also recycle nutrients for use in themarine food web and in the process release CO₂ to the atmosphere. Microbial biomass and other organic matter (remnants of plants and animals) are converted tofossil fuels over millions of years. By contrast, burning offossil fuels liberates greenhouse gases in a small fraction of that time. As a result, thecarbon cycle is out of balance, and atmospheric CO₂ levels will continue to rise as long as fossil fuels continue to be burnt.^[47]

Most heat energy from global warming goes into the ocean, and not into the atmosphere or warming up the land.^[49][50] Scientists realized over 30 years ago the ocean was a key fingerprint of human impact onclimate change and "the best opportunity for major improvement in our understanding of climate sensitivity is probably monitoring of internal ocean temperature".^[51]

Marine organisms are moving to cooler parts of the ocean as global warming proceeds. For example, a group of 105 marine fish and invertebrate species were monitored along the US Northeast coast and in the eastern Bering Sea. During the period from 1982 to 2015, the average center of biomass for the group shifted northward about 10 miles as well moving about 20 feet deeper.^[52][53]

There is evidence increasing ocean temperatures are taking a toll on marine ecosystem. For example, a study onphytoplankton changes in theIndian Ocean indicates a decline of up to 20% in marine phytoplankton during the past six decades.^[55] During summer, the western Indian Ocean is home to one of the largest concentrations of marine phytoplankton blooms in the world. Increased warming in the Indian Ocean enhances ocean stratification, which prevents nutrient mixing in theeuphotic zone where ample light is available for photosynthesis. Thus, primary production is constrained and the region's entire food web is disrupted. If rapid warming continues, the Indian Ocean could transform into an ecological desert and cease being productive.^[55]

• 
  Ocean warming is changing the distribution of the keystone speciesAntarctic krill.^[56][57]
• 
  In the Antarctic, limited-range shallow-water species, like thisemerald rockcod, are under threat.^[56][57]

TheAntarctic oscillation (also called theSouthern Annular Mode) is a belt ofwesterly winds or low pressure surroundingAntarctica which moves north or south according to which phase it is in.^[58] In its positive phase, the westerly wind belt that drives theAntarctic Circumpolar Current intensifies and contracts towardsAntarctica,^[59] while its negative phase the belt moves towards the Equator. Winds associated with the Antarctic oscillation cause oceanicupwelling of warm circumpolar deep water along the Antarctic continental shelf.^[60][61] This has been linked toice shelfbasal melt,^[62] representing a possible wind-driven mechanism that could destabilize large portions of the Antarctic Ice Sheet.^[63] The Antarctic oscillation is currently in the most extreme positive phase that has occurred for over a thousand years. Recently this positive phase has been further intensifying, and this has been attributed to increasinggreenhouse gas levels and later stratospheric ozone depletion.^[64][65] These large-scale alterations in the physical environment are "driving change through all levels of Antarctic marine food webs".^[56][57] Ocean warming is also changing the distribution ofAntarctic krill.^[56][57] Antarctic krill is thekeystone species of theAntarctic ecosystem beyond the coastal shelf, and is an important food source formarine mammals andbirds.^[66]

TheIPCC (2019) says marine organisms are being affected globally by ocean warming with direct impacts on human communities, fisheries, and food production.^[67] It is likely there will be a 15% decrease in the number of marine animals and a decrease of 21% to 24% in fisheries catches by the end of the 21st century because of climate change.^[68]

A 2020 study reports that by 2050 global warming could be spreading in the deep ocean seven times faster than it is now, even if emissions of greenhouse gases are cut. Warming inmesopelagic and deeper layers could have major consequences for thedeep ocean food web, since ocean species will need to move to stay at survival temperatures.^[69][70]

Coastal ecosystems are facing further changes because ofrising sea levels. Some ecosystems can move inland with the high-water mark, but others are prevented from migrating due to natural or artificial barriers. This coastal narrowing, calledcoastal squeeze if human-made barriers are involved, can result in theloss of habitats such asmudflats andmarshes.^[72][73]Mangroves andtidal marshes adjust to rising sea levels by building vertically using accumulatedsediment andorganic matter. Ifsea level rise is too rapid, they will not be able to keep up and will instead be submerged.^[74]

• 
  Sea-level change, 1880 to 2015^[75][76]
• 
  As the sea level rises it moves further inland.

Coral, important for bird and fish life, also needs to grow vertically to remain close to the sea surface in order to get enough energy from sunlight. So far it has been able to keep up, but might not be able to do so in the future.^[77] These ecosystems protect against storm surges, waves and tsunamis. Losing them makes the effects of sea level rise worse.^[78][79] Human activities, such as dam building, can prevent natural adaptation processes by restricting sediment supplies to wetlands, resulting in the loss oftidal marshes.^[80] When seawater moves inland, thecoastal flooding can cause problems with existing terrestrial ecosystems, such as contaminating their soils.^[81] TheBramble Cay melomys is the first known land mammal to go extinct as a result of sea level rise.^[82][83]

Ocean salinity is a measure of how much dissolvedsalt is in the ocean. The salts come from erosion and transport of dissolved salts from the land. The surface salinity of the ocean is a key variable in the climate system when studying the globalwater cycle,ocean–atmosphere exchanges andocean circulation, all vital components transporting heat, momentum, carbon and nutrients around the world.^[84] Cold water is more dense than warm water and salty water is more dense than freshwater. This means the density of ocean water changes as its temperature and salinity changes. These changes in density are the main source of the power that drives the ocean circulation.^[84]

Surface ocean salinity measurements taken since the 1950s indicate an intensification of the global water cycle with high saline areas becoming more saline and low saline areas becoming more less saline.^[85][86]

Ocean acidification is the increasing acidification of the oceans, caused mainly by the uptake ofcarbon dioxide from theatmosphere.^[88] The rise in atmospheric carbon dioxide due to the burning of fossil fuels leads to more carbon dioxide dissolving in the ocean. When carbon dioxide dissolves in water it forms hydrogen and carbonate ions. This in turn increases theacidity of the ocean and makes survival increasingly harder for microorganisms, shellfish and other marine organisms that depend oncalcium carbonate to form their shells.^[89]

Increasing acidity also has potential for other harm to marine organisms, such as depressing metabolic rates and immune responses in some organisms, and causingcoral bleaching.^[90] Ocean acidification has increased 26% since the beginning of the industrial era.^[91] It has been compared toanthropogenic climate change and called the "evil twin ofglobal warming"^[92] and "the other CO₂ problem".^[93]

Ocean deoxygenation is an additional stressor on marine life. Ocean deoxygenation is the expansion ofoxygen minimum zones in the oceans as a consequence ofburning fossil fuels. The change has been fairly rapid and poses a threat to fish and other types of marine life, as well as to people who depend on marine life for nutrition or livelihood.^{[94][95][96][97]} Ocean deoxygenation poses implications forocean productivity, nutrient cycling,carbon cycling, andmarine habitats.^[98][99]

Ocean warming exacerbates ocean deoxygenation and further stresses marine organisms, limiting nutrient availability by increasingocean stratification through density and solubility effects while at the same time increasing metabolic demand.^[100][101] According to the IPCC 2019Special Report on the Ocean and Cryosphere in a Changing Climate, the viability of species is being disrupted throughout theocean food web due to changes inocean chemistry. As the ocean warms,mixing between water layers decreases, resulting in less oxygen and nutrients being available formarine life.^[102]

Until recently,ice sheets^[104] were viewed as inert components of the carbon cycle and largely disregarded in global models. Research in the past decade has transformed this view, demonstrating the existence of uniquely adapted microbial communities, high rates of biogeochemical/physical weathering in ice sheets and storage and cycling of organic carbon in excess of 100 billion tonnes, as well as nutrients.^[105]

The diagram on the right shows some human impacts on themarine nitrogen cycle. Bioavailable nitrogen (Nb) is introduced into marine ecosystems by runoff or atmospheric deposition, causingeutrophication, the formation ofdead zones and the expansion of theoxygen minimum zones (OMZs). The release of nitrogen oxides (N₂O, NO) from anthropogenic activities and oxygen-depleted zones causes stratosphericozone depletion leading to higherUVB exposition, which produces the damage of marine life,acid rain andocean warming. Ocean warming causes water stratification, deoxygenation, and the formation of dead zones. Dead zones and OMZs are hotspots foranammox anddenitrification, causing nitrogen loss (N₂ and N₂O). Elevated atmospheric carbon dioxide acidifies seawater, decreasing pH-dependent N-cycling processes such as nitrification, and enhancing N₂fixation.^[106]

Coccolithophore

Sea urchin

Increases in acidity makes it difficult for microorganisms like coccolithophores, andshellfish like sea urchins, to build their carbonate shells.

Aragonite is a form ofcalcium carbonate many marine animals use to build carbonate skeletons and shells. The lower the aragonitesaturation level, the more difficult it is for the organisms to build and maintain their skeletons and shells. The map below shows changes in the aragonite saturation level of ocean surface waters between 1880 and 2012.^[107]

To pick one example,pteropods are a group of widely distributed swimmingsea snails. For pteropods to create shells they requirearagonite which is produced through carbonate ions and dissolved calcium. Pteropods are severely affected because increasing acidification levels have steadily decreased the amount of water supersaturated with carbonate which is needed for the aragonite creation.^[108]

When the shell of a pteropod was immersed in water with a pH level the ocean is projected to reach by the year 2100, the shell almost completely dissolved within six weeks.^[109] Likewisecorals,^[110]coralline algae,^[111] coccolithophores,^[112]foraminifera,^[113] as well asshellfish generally,^[114] all experience reduced calcification or enhanced dissolution as an effect of ocean acidification.

• 
  Decreases in aragonite saturation make it more difficult for marine organisms like pteropods to buildcalcium shells.
• 
  Shells of pteropods dissolve in increasingly acidic conditions caused by increased amounts of atmospheric CO₂.

Unhealthy pteropod showing effects of ocean acidification

Ocean acidification causes brittle stars to lose muscle mass

Pteropods andbrittle stars form the base of Arctic food webs

Pteropods andbrittle stars together form the base of the Arcticfood webs and both are seriously damaged by acidification. Pteropods shells dissolve with increasing acidification and brittle stars lose muscle mass when re-growing appendages.^[115] Additionally the brittle star's eggs die within a few days when exposed to expected conditions resulting from Arctic acidification.^[116] Acidification threatens to destroy Arctic food webs from the base up. Arctic waters are changing rapidly and are advanced in the process of becoming undersaturated with aragonite.^[108] Arctic food webs are considered simple, meaning there are few steps in the food chain from small organisms to larger predators. For example, pteropods are "a key prey item of a number of higher predators – larger plankton, fish, seabirds, whales".^[117]

The rise in agriculture of the past 400 years has increased the exposure rocks and soils, which has resulted in increased rates of silicate weathering. In turn, the leaching ofamorphous silica stocks from soils has also increased, delivering higher concentrations of dissolved silica in rivers.^[118] Conversely, increased damming has led to a reduction in silica supply to the ocean due to uptake by freshwater diatoms behind dams. The dominance of non-siliceousphytoplankton due to anthropogenic nitrogen and phosphorus loading and enhanced silicadissolution in warmer waters has the potential to limit siliconocean sediment export in the future.^[118]

In 2019 a group of scientists suggested acidification is reducingdiatom silica production in theSouthern Ocean.^[119][120]

Diatom

Radiolarian

Changes inocean silicic acid can make it difficult for themarine microorganisms that construct silica shells.

• 
  Concentration ofsilicic acid in the upperpelagic zone,^[121] showing high levels in the Southern Ocean
• 
  Generalizedmarine silica cycle, adapted fromTreguer et al., 1995^[122]

As the technical and political challenges of land-based carbon dioxide removal approaches become more apparent, the oceans may be the new "blue" frontier for carbon drawdown strategies in climate governance.^[128] Marine environments are the blue frontier of a strategy for novel carbon sinks in post-Paris climate governance, from nature-based ecosystem management to industrial-scale technological interventions in the Earth system. Marine carbon dioxide removal approaches are diverse ^[129][130] — although several resemble key terrestrial carbon dioxide removal proposals.^[128] Ocean alkalinisation (adding silicate mineral such asolivine to coastal seawater, to increase CO₂ uptake through chemical reactions) is enhanced weathering,blue carbon (enhancing natural biological CO₂ drawdown from coastal vegetation) is marine reforestation, and cultivation of marine biomass (i.e., seaweed) for coupling with consequent carbon capture and storage is the marine variant of bioenergy and carbon capture and storage.Wetlands,coasts, and theopen ocean are being conceived of and developed as managed carbon removal-and-storage sites, with practices expanded from the use of soils and forests.^[128]

If more than one stressor is present the effects can be amplified.^[133][134] For example, the combination of ocean acidification and an elevation of ocean temperature can have a compounded effect on marine life far exceeding the individual harmful impact of either.^{[135][136][137]}

While the full implications of elevated CO₂ on marine ecosystems are still being documented, there is a substantial body of research showing that a combination of ocean acidification and elevated ocean temperature, driven mainly by CO₂ and othergreenhouse gas emissions, have a compounded effect on marine life and the ocean environment. This effect far exceeds the individual harmful impact of either.^{[135][138][137]} In addition, ocean warming exacerbatesocean deoxygenation, which is an additional stressor on marine organisms, by increasing ocean stratification, through density and solubility effects, thus limiting nutrients,^[139][140] while at the same time increasing metabolic demand.

The direction and magnitude of the effects of ocean acidification, warming and deoxygenation on the ocean has been quantified bymeta-analyses,^{[136][142][143]} and has been further tested bymesocosm studies. The mesocosm studies simulated the interaction of these stressors and found a catastrophic effect on the marine food web, namely, that the increases in consumption from thermal stress more than negates any primary producer to herbivore increase from more available carbon dioxide.^[144][145]

Changes in marine ecosystem dynamics are influenced by socioeconomic activities (for example, fishing, pollution) and human-induced biophysical change (for example, temperature, ocean acidification) and can interact and severely impact marine ecosystem dynamics and theecosystem services they generate to society. Understanding these direct—or proximate—interactions is an important step towards sustainable use of marine ecosystems. However, proximate interactions are embedded in a much broader socioeconomic context where, for example, economy through trade and finance, human migration and technological advances, operate and interact at a global scale, influencing proximate relationships.^[146]

In 2024 a study^[147] was released, dedicated to the impact of fishing and non fishing ships on the coastal waters of the ocean when 75% of the industrial activity occur. According to the study: "A third offish stocks are operated beyond biologically sustainable levels and an estimated 30–50% of criticalmarine habitats have been lost owing to human industrialization". It mentions that except traditional impacts likefishing,maritime trade andoil extraction there are new emerging likemining,aquaculture andoffshore wind turbines. It used satellite data to monitor the vessels. It found that 72% - 76% offishing ships and 21%-30% of energy and transport ships are "missing frompublic tracking systems". When the data was added to previously existing information about ships that were publicly tracked, this led to several discoveries including:

The study discovered a significant increase inoffshore wind turbins which had overpassedoil platforms by number already in 2021. Fishing increased only a little in the latest years and may begin to decline because fisheries are exhausted. It concluded that "transport and energy vessel traffic may continue to expand, following trends in global trade and the rapid development of renewable energy infrastructure. In this scenario, changes to marine ecosystems brought by infrastructure and vessel traffic may rival fishing in impact".

Shifting baselines arise in research on marine ecosystems because changes must be measured against some previous reference point (baseline), which in turn may represent significant changes from an even earlier state of the ecosystem.^[149] For example, radically depleted fisheries have been evaluated by researchers who used the state of the fishery at the start of their careers as the baseline, rather than the fishery in its unexploited or untouched state. Areas that swarmed with a particular species hundreds of years ago may have experienced long-term decline, but it is the level a few decades previously that is used as the reference point for current populations. In this way large declines in ecosystems or species over long periods of time were, and are, masked. There is a loss of perception of change that occurs when each generation redefines what is natural or untouched.^[149]

• Human impact on the environment
• Marine biology

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