The global ocean is an immense, interconnected system that governs the habitability of the planet. This vast body of water generates over half the world’s breathable oxygen, primarily through the photosynthesis of microscopic organisms called phytoplankton. It also plays a profound role in regulating global climate by absorbing about 30 percent of atmospheric carbon dioxide and capturing over 90 percent of the excess heat generated by greenhouse gas emissions. The health of this marine environment directly influences weather patterns, food security, and the stability of coastal regions worldwide. Human activity is now driving systemic changes that threaten the fundamental chemical and physical processes that make the ocean function.
Altering Ocean Chemistry and Temperature
The burning of fossil fuels creates a dual burden on the marine environment by altering both its temperature and its chemistry on a global scale. As greenhouse gases trap heat, the ocean absorbs the vast majority of this energy, leading to a measurable rise in water temperature. This ocean warming directly impacts species sensitive to thermal stress, such as reef-building corals.
Elevated sea temperatures cause mass coral bleaching events, where corals expel the symbiotic algae (zooxanthellae) living within their tissues. These algae provide the coral host with up to 90 percent of its energy, and without them, the coral turns white and is susceptible to mortality. Warming waters also force many marine species to shift their geographical distribution toward cooler poles, disrupting established ecosystems and affecting prey availability.
A parallel threat is ocean acidification, which results from the ocean absorbing atmospheric carbon dioxide. When $\text{CO}_2$ dissolves into seawater, it forms carbonic acid, releasing hydrogen ions. This increase in hydrogen ions lowers the water’s $\text{pH}$, making it more acidic; the average $\text{pH}$ of the surface ocean has already dropped by about 0.1 units since the Industrial Revolution.
This change in chemistry reduces the availability of carbonate ions, a necessary building block for organisms that create shells or skeletons from calcium carbonate. Calcifying species like oysters, mussels, pteropods, and corals struggle to form and maintain their structures in these less alkaline conditions. The reduced ability to calcify impacts the base of the marine food web, as pteropods, often called “sea butterflies,” are a common food source for many fish and seabirds.
Contamination by Plastics and Chemical Runoff
The direct introduction of human-made materials and substances creates widespread contamination problems in marine environments. Plastic pollution is a pervasive issue, ranging from large macroplastics to microplastics. Macroplastics, such as abandoned fishing gear and larger debris, cause physical harm through entanglement, which can lead to injury and drowning in marine mammals, turtles, and seabirds.
Microplastics are ingested by a vast range of marine life, from zooplankton to whales. These particles can block digestive tracts, leading to starvation, and they concentrate persistent organic pollutants ($\text{POPs}$) and heavy metals from the surrounding water. The transfer of these concentrated toxins into the tissues of marine organisms is a concern for the entire food web.
Chemical and nutrient runoff from land-based activities severely compromises water quality through a process known as eutrophication. Agricultural fertilizers containing nitrogen and phosphorus are washed into rivers and coastal waters, triggering explosive growth of algae known as algal blooms. When these blooms die, their decomposition by bacteria consumes vast amounts of dissolved oxygen.
This severe oxygen depletion creates hypoxic zones, commonly referred to as “dead zones,” where most marine life cannot survive. The Gulf of Mexico, for instance, develops one of the largest dead zones globally each year, fueled by nutrient runoff from the Mississippi River basin. Other forms of chemical contamination include industrial heavy metals and persistent chemicals that bioaccumulate in organisms, posing long-term health risks.
Unsustainable Removal of Marine Life
The extraction of marine animals for commercial purposes profoundly destabilizes ocean ecosystems. Overfishing occurs when target species are harvested faster than their populations can naturally replenish through reproduction. This unsustainable pressure leads to the decline of commercially exploited fish stocks, such as the historic collapse of the Atlantic cod fisheries.
A consequence of fishing practices is bycatch, the unintentional capture and death of non-target species. This includes vulnerable animals like sea turtles, dolphins, and seabirds trapped in nets or on longlines. The high mortality from bycatch depletes non-harvested populations and disrupts the natural balance of the ecosystem.
The removal of marine life, particularly apex predators, can trigger destructive trophic cascades. When animals at the top of the food chain, such as large sharks, are drastically reduced, the populations of their prey species can increase unchecked. This overabundance of mid-level predators can then decimate species lower on the food chain, altering the entire food web structure and leading to ecosystem instability.
Physical Destruction of Coastal Ecosystems
Human activities are causing the physical destruction and modification of the ocean’s most productive habitats. Coastal development, including the construction of ports, marinas, and beachfront infrastructure, often requires dredging and land reclamation. This modification directly removes and fragments crucial nursery grounds that support a vast array of marine life.
Key habitats like mangrove forests, seagrass meadows, and coral reefs are lost to these development pressures. Mangroves and seagrass beds serve as nurseries for many commercial fish species, and their loss reduces the reproductive capacity of entire fisheries. The construction of hard coastal defenses and the filling of natural wetlands also remove the natural buffers that protect shorelines from storms and erosion.
Destructive fishing practices inflict physical damage on the seafloor environment. Bottom trawling, for example, involves dragging heavy nets and weighted doors across the seabed. This practice scrapes and levels the ocean floor, destroying complex habitats like deep-sea coral formations and sponge gardens that can take centuries to recover.