The ocean, a vast and interconnected body of water, plays a foundational role in sustaining life on Earth. It regulates global climate patterns, produces a significant portion of the oxygen we breathe, and provides sustenance for billions of people worldwide. Human activities, however, are profoundly altering marine environments, leading to widespread consequences that affect both ocean health and the well-being of human societies. These impacts stem from various sources, ranging from the introduction of foreign substances to modifications of the ocean’s physical characteristics.
Contamination from Land and Sea
Harmful substances enter the ocean from both terrestrial and marine sources, leading to pervasive contamination. Plastic pollution represents a significant concern, with an estimated 8 million metric tons of plastic entering oceans annually. This plastic waste breaks down into smaller fragments, known as microplastics, which are easily ingested by a wide array of marine organisms, including corals, plankton, and fish. Ingested plastic can cause physical harm, interfere with feeding and reproductive behaviors, and transfer adsorbed toxic chemicals up the food chain.
Chemical runoff originating from agricultural and industrial activities introduces excess nutrients and pollutants into coastal waters. Fertilizers and pesticides from farmlands, along with industrial discharges and sewage, deliver nitrogen and phosphorus into rivers that flow to the sea. This nutrient overloading triggers explosive growths of microscopic algae, known as algal blooms. When these large algal populations die and decompose, oxygen levels in the water plummet, creating vast hypoxic “dead zones” where marine life struggles to survive. One of the largest such dead zones forms annually in the Gulf of Mexico, spanning approximately 8,000 square miles.
Beyond chemical and plastic contamination, noise pollution also affects marine ecosystems. Noise from commercial shipping, oil and gas exploration, and military sonar constantly permeates the underwater environment. This incessant din interferes with marine animals’ ability to communicate, navigate, find food, and avoid predators, impacting species like whales and dolphins that rely on sound for their survival.
Impacts of Climate Change
The ocean absorbs over 90% of the excess heat generated by human activities since 1950. This leads to rising ocean temperatures, which directly affect marine life and habitats. Increased thermal stress causes mass coral bleaching events, where corals expel their symbiotic algae and become vulnerable to disease or death. Warmer waters also force many marine species to migrate towards cooler polar regions, disrupting established ecosystems. Rising temperatures can also disrupt the synchronized life cycles of marine plants and animals.
Alongside warming, the ocean absorbs atmospheric carbon dioxide (CO2), altering its chemistry in a process called ocean acidification. As CO2 dissolves in seawater, it forms carbonic acid, which lowers the ocean’s pH. This change reduces the availability of carbonate ions, building blocks calcifying organisms use to construct their shells and skeletons. Species such as corals, oysters, mussels, sea urchins, and pteropods face difficulties forming or maintaining their calcium carbonate structures, impairing their growth and survival. Pteropod shells, for example, are already dissolving in acidic waters of the Southern Ocean, threatening the food webs that depend on them.
Overexploitation of Marine Resources
The removal of marine resources at unsustainable rates threatens ocean health. Overfishing occurs when fish populations are harvested faster than they can naturally replenish, leading to declines in stock abundance. Many of the world’s fish stocks are currently overfished or fished at their maximum sustainable limits. This unsustainable practice can disrupt entire marine food webs, causing a domino effect where the depletion of one species impacts predators, prey, and other organisms within the ecosystem.
Certain fishing methods inflict damage on marine environments. Bottom trawling involves dragging heavy nets along the seafloor, destroying delicate seabed habitats like coral reefs and seagrass beds. Bycatch, the unintentional capture of non-target species such as sea turtles, seabirds, marine mammals, and juvenile fish, is another widespread problem. Bycatch can account for a significant portion of a net’s total catch.
Beyond living resources, the extraction of non-living resources from the seabed is expanding. Offshore oil and gas drilling is one example. Deep-sea mining, a more recent development, targets valuable minerals like cobalt, copper, and rare earth elements. Concerns exist about the potential for disturbing the seabed, creating vast sediment plumes, and releasing toxic substances, which could harm deep-sea ecosystems.
Physical Alteration of Marine Habitats
Physical alteration of marine environments results from human activities along coastlines and on the seafloor. Coastal development leads to the permanent loss of habitats like mangroves, seagrass beds, and estuaries. These coastal ecosystems serve as nurseries for marine species, providing sheltered areas for juvenile fish and invertebrates to grow. The destruction of these areas can significantly reduce fish populations and broader marine biodiversity.
Seafloor modification occurs through activities like dredging and sand mining. Dredging is performed to deepen shipping lanes and harbors. This process removes and reshapes the seabed, disrupting the habitats of organisms living on or within the seafloor. Sediment plumes generated by dredging can also reduce water clarity, affecting filter-feeding organisms and potentially releasing trapped pollutants.
Human activities facilitate the introduction of species to new marine environments. Global shipping, particularly through the discharge of ballast water, is a primary pathway. Ballast water can carry microscopic organisms, larvae, and even adult marine life. These non-native species can become invasive, outcompeting native organisms for resources, altering food webs, and changing the structure of local ecosystems, threatening biodiversity.