The ocean covers over 70% of Earth’s surface and plays a fundamental role in sustaining life. It provides a significant portion of the world’s oxygen, regulates global climate patterns, and serves as a major food source for billions. This vast system also harbors diverse marine life, from microscopic plankton to the largest whales. Despite its scale, human activities are increasingly altering marine environments, placing considerable strain on this interconnected ecosystem.
Ocean Pollution
Ocean pollution, originating from various land-based and sea-based activities, is a widespread challenge facing marine environments. Plastic debris, from large macroplastics to tiny microplastics, poses a pervasive threat. Macroplastics like abandoned fishing nets, termed “ghost gear,” can entangle marine animals such as seals, turtles, and seabirds, leading to injury or death. Microplastics, less than five millimeters in size, are ingested by marine organisms, from zooplankton to fish, potentially transferring harmful chemicals through the food web.
Chemical and nutrient pollution also impacts marine ecosystems. Agricultural runoff, with fertilizers and pesticides, introduces excess nutrients like nitrogen and phosphorus into coastal waters. This nutrient overload can cause eutrophication, leading to algal blooms that deplete oxygen upon decomposition, creating vast “dead zones” where marine life cannot survive. Industrial waste and pharmaceutical residues contaminate marine habitats, introducing toxic substances that impair reproduction, development, and survival.
Oil spills, while often localized, inflict immediate damage on marine life and coastal habitats. Large spills can coat seabirds, mammals, and fish, impairing their movement, foraging, and temperature regulation, often leading to mortality. Crude oil chemicals are also toxic to marine organisms, causing internal damage and disrupting ecosystem functions. Beyond chemical contaminants, noise pollution disrupts marine environments. Shipping traffic, seismic surveys for oil and gas, and military sonar systems generate loud, continuous sounds that interfere with the communication, navigation, and foraging behaviors of marine mammals like whales and dolphins, potentially leading to strandings or habitat abandonment.
Overfishing and Marine Life Exploitation
Unsustainable harvesting of marine species represents another human impact on ocean health, disrupting the balance of marine ecosystems. Overfishing occurs when fish are caught faster than their populations can replenish, leading to declining stock sizes and, in some cases, fishery collapse. This depletion of target species can have cascading effects throughout the food web, altering predator-prey relationships and impacting dependent species. For instance, removing large predatory fish can lead to an increase in their prey, which might overgraze other organisms, changing the ecosystem structure.
Bycatch, the accidental capture of non-target species during fishing operations, is a related concern. This unintended catch often includes vulnerable marine animals such as sea turtles, dolphins, sharks, and seabirds, many already endangered or threatened. Bycatch contributes to the decline of these populations, undermining conservation efforts despite regulations. For example, longline fishing targeting tuna or swordfish can accidentally hook thousands of sharks annually.
Illegal, Unreported, and Unregulated (IUU) fishing exacerbates the problem, operating outside established management frameworks. IUU fishing undermines conservation measures, distorts markets, and depletes fish stocks without accountability. These activities make it challenging to assess fish populations and implement management plans, contributing to global overfishing and hindering species recovery. Exploitation extends beyond commercial fisheries to include the targeted removal of apex predators like sharks for their fins, which plays a disproportionate role in maintaining marine food web health and stability.
Physical Destruction of Coastal and Marine Ecosystems
Human activities directly alter and destroy marine habitats, leading to biodiversity loss and reduced ecosystem services. Coastal development (urbanization, tourism, ports, marinas) encroaches upon and eliminates valuable shoreline habitats. Mangrove forests, salt marshes, and coral reefs, which serve as natural storm barriers and nurseries, are cleared or degraded for human expansion. This loss removes natural coastal protection and diminishes biodiversity.
Seafloor habitats are altered by practices like dredging and destructive fishing methods. Dredging, often conducted to deepen shipping channels or extract sand for construction, removes entire sections of the seabed, destroying benthic communities and altering sediment composition. Bottom trawling, with heavy nets dragged across the seafloor, causes extensive physical damage, pulverizing coral, sponges, and other slow-growing organisms that form complex habitats. Destructive fishing practices such as blast fishing (using explosives to stun or kill fish) devastate coral reefs and sensitive seafloor structures, leaving behind rubble and barren landscapes.
Accumulated pollution, particularly plastics, can inflict direct physical damage on marine habitats. Large plastic debris aggregations can smother coral reefs and seagrass beds, blocking sunlight and impeding nutrient exchange. These disturbances eliminate breeding grounds and nurseries for marine species, reducing populations and ecosystem resilience. Habitat destruction also removes their capacity to provide ecosystem services, such as filtering water, stabilizing coastlines, and supporting fisheries.
Climate Change Effects on Ocean Systems
Global climate change, driven by human greenhouse gas emissions, causes systemic alterations across ocean systems. Ocean warming results from the atmosphere absorbing excess heat, with the ocean absorbing over 90% of this thermal energy. Rising ocean temperatures lead to widespread coral bleaching, where corals expel symbiotic algae, turning white and often dying if temperatures remain elevated. This warming also forces marine species to shift distributions toward cooler waters, disrupting ecosystems and food webs. Increased frequency and intensity of marine heatwaves further stress marine life and contribute to mass mortality events.
Excess carbon dioxide absorption from the atmosphere also leads to ocean acidification. As more CO2 dissolves in seawater, it forms carbonic acid, reducing the ocean’s pH. This chemical change diminishes carbonate ion availability, which are building blocks for shells and skeletons of many marine organisms. Shell-forming creatures like corals, oysters, clams, and pteropods (sea snails) find it harder to build and maintain calcium carbonate structures, impacting their survival and the marine food web. This process can weaken existing shells and skeletons, making organisms more vulnerable.
Sea level rise, from thermal expansion and melting glaciers, poses a direct threat to coastal communities and ecosystems. Higher sea levels inundate low-lying coastal areas, increasing coastal flooding and erosion. Coastal habitats like mangrove forests and salt marshes, which provide storm protection and carbon sequestration, face submergence or being squeezed between rising seas and human development, leading to their decline.
Warmer waters also hold less dissolved oxygen, known as deoxygenation. This oxygen reduction creates expanding “dead zones” where levels are too low to support most marine life, forcing mobile species to flee and leaving sessile organisms to perish. Combined warming and deoxygenation stress marine organisms, making them more susceptible to disease and reducing growth rates. A warmer ocean also fuels more powerful tropical storms and hurricanes, as higher sea surface temperatures provide more energy for intensification, leading to increased coastal damage and disruption.