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Pollution
For all the "services" the ocean gives us, you'd think we'd take better care of it. However, because of human actions, the ocean is in trouble and, therefore, so are we. First, people put objects into the ocean that don't belong there. Pollution may be the most obvious threat. The dumping or discharge of oil, nuclear waste, plastics, debris, and many chemical contaminants causes a wide range of impacts. For example, contaminants poison or cause chronic disease, reproductive failure, or deformities in marine life. The sources of marine pollution include commercial, military, and recreational shipping and boating; runoff of pesticides, herbicides, and fertilizers from urban streets and agricultural fields; oil drilling; and industries and sewage treatment plants.
Much of the pollution is in the form of excess nutrients, for example, human wastes in sewage and animal wastes in fertilizers and farm runoff. Too many nutrients (such as fertilizers) cause depletion of oxygen, promotion of harmful algal blooms, and dramatic reductions in the richness of marine communities. Even tossed out litter creates problems. Sea birds and other ocean animals may mistake litter such as cigarette butts for food. Sea turtles and whales have been known to ingest floating plastic bags and balloons, while discarded fishing line or nets have entangled and killed many ocean animals.
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Taking from the ocean
Humans also damage the ocean by taking from it. Oversized, over-mechanized fleets of fishing vessels pull so many fish from the water that numerous fish populations are depleted, and some fisheries have collapsed. Many nations lack laws to protect fish populations, so the damage goes unchecked. Fishing practices can harm even non-target species because nets are indiscriminate. Nets set to catch fish or shrimp may snag and drown sea turtles and marine mammals.
Coastal ecosystems are being "taken away" too. Urbanization, road construction, port and marina activities, boating, dredging and dumping, mining, coastal agriculture, forestry, and aquaculture reduce, fragment, or degrade coastal habitats as well as plant and wildlife populations and they promote local and regional species extinctions. Because coastal areas often serve as "nurseries" for many ocean animals, the cost of beach development is high in marine lives. Moreover, people take water itself away from ocean systems when they dam or divert rivers for power generation, flood control, or irrigation. Such practices change the flow or timing of freshwater moving into the sea. Freshwater diversion drains wetlands, which are crucial fish-spawning habitats as well as giant "filters" for water entering the ocean. The water that does make it to the sea may be dirtier, and less freshwater flow affects ocean salinity.
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Aquaculture
Sometimes it isn't what people add to or take from the ocean that damages it, but simply moving species from native to foreign habitats. This practice is called the introduction of exotic species. Such introductions cause the disruption of natural systems on a global scale. The way most exotic species enter an ecosystem is through the transport and discharge of ships' ballast water. Aquaculture practices in which exotic species are either purposefully introduced or inadvertently escape into local waters can cause trouble. Introduced or exotic species may prey on or outcompete native species and have caused irreversible damage to natural communities of marine life.
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Air pollution
Finally, air pollution is taking a toll on the ocean. Loss of Earth's protective ozone layer has allowed increased ultraviolet radiation to reach the Earth's surface. This radiation can seriously affect human health and can damage or kill fish eggs, fish larvae, and plankton that live in the ocean's surface waters. Airborne pollutants from the burning of fossil fuels (coal, gas, and oil) may end up in ocean waters or add to human-induced global climate change. Warmer global temperatures lead to sea level rise, increased water temperatures, and changes in precipitation patterns. Global climate change and sea level rises are predicted to alter coastal and oceanic environments in ways we don't yet understand.
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