Oceans provide a home for hundreds of thousands of species on Earth, and it is essential for human life. Unfortunately, while many species depend on the ocean for its ability to create food and oxygen, human activities can negatively impact the ocean and its wildlife. In the United States alone, about one in six jobs has something to do with the ocean and many of them come with dire consequences for the ecosystem.

A common method of gathering food, fishing impacts the oceans in drastic ways. An increasing demand for protein has led to an increase in large-scale fishing operations, and throughout the 20th century, many countries failed to put safeguards into place to prevent overfishing. As a result, the populations of a number of large fish species have dropped by as much as 90 percent from their preindustrial populations. This depletion disrupts ocean food chains: it removes predators and allows their prey populations to grow unchecked. As the populations of targeted fish decline, many operations move down the food chain to other species, and over time this can cause significant alterations to marine ecosystems.

Human pollution also significantly affects the oceans. In the 1980s, travelers passing through the Pacific Ocean began to notice areas containing a high concentration of plastic trash, apparently collected by the ocean's natural currents into one area. The so-called Pacific Trash Vortex may contain up to 1.9 million pieces of trash per square mile, and a similar patch of garbage exists in the northern Atlantic. In addition, oil spills such as the one resulting from the Deepwater Horizon fire in 2010 can contaminate large stretches of the ocean, wiping out entire populations of fish and other species, and affecting the regional ecosystem for decades.

As the carbon dioxide —a common greenhouse gas — found in the atmosphere increases, the ocean absorbs some of the excesses. The gas reacts with seawater and reduces its pH, increasing the acidity of the water. Since the industrial revolution, the pH of the ocean has decreased by 0.1 pH, representing a 30 percent increase in the acidity of seawater. This affects the growth of animals and plants in the ocean, weakening coral and shellfish.

Organic waste dumped into the oceans can have a devastating effect on ecosystems. Excess nutrients from fertilizers and sewage runoff flow into the ocean via rivers. This sudden abundance of organic material can disrupt the balance of life in affected areas. Organic pollution can cause algae blooms, a rapid increase in certain species of microorganisms that may produce toxins or consume the free oxygen in the region, killing off or driving away other species.