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Humans depend on the ocean for many reasons including food, transportation, energy sources, and recreation. The use of resources from the world’s oceans has had a major impact on the biodiversity in these ecosystems. Human activities play an important role in changing the environment of the oceans. Runoff is the flow of water that occurs when the soil is saturated and excess water from rain, or other sources, flows over the land and can accumulate in larger reservoirs of water including oceans. When runoff flows along the ground, it can pick up contaminants, such as toxic chemicals or pollutants. Human urbanization increases surface runoff by creating more impermeable surfaces such as concrete and pavement that do not allow percolation of water down through the soil and into the aquifer. Instead, water is forced directly into the water source such as a river, lake, or ocean. When contaminants are dissolved in runoff, the human impact creates water pollution that can have a detrimental effect on water quality and the living organisms dependent on that water source. Artificial reefs are man-made underwater structures built to promote marine life in areas that typically have a flat, featureless bottom. They can be intentional by placing unused or non-working structures that were once used for other purposes such as old automobiles, buses, train cars, and oil rigs down to the ocean floor. Artificial reefs can also occur unintentionally such as a ship that sinks during a shipwreck. These artificial reefs provide hard surfaces where algae and invertebrates such as coral, oysters, and barnacles can attach. As they grow and accumulate, these reefs then provide a structure for habitats and food for other marine life including many types of fish. Although they may be first used as benefits for marine life, artificial reefs may have some environmental concerns. These concerns include toxicity of some man-made structures, damage to natural ecosystems, and even worsening the effects of overfishing by accumulating fish into one area. Current research shows the impacts of human activity on ocean ecosystems can be seen worldwide and in every type of ecosystem found in the oceans. As humans depend on resources from oceans on an increasing basis, the effects raise environmental concerns. Activities such as overfishing, pollution from dumping, pollution from shipping transportation, recreation use, runoff, and coastal development all have had a major impact on some parts of the world’s oceans. These impacts affect most marine life, coral and rocky reefs, seamounts, estuaries, open waters, as well as shallow and deep ocean ecosystems. Most human activities including using resources from the oceans have a major impact on fragile marine ecosystems. Oceans are Earth’s largest ecosystem. Ocean systems circulate energy and ocean temperatures regulate Earth’s climate and weather. Marine algae produce oxygen and consume atmospheric carbon and play an important role in the cycling of matter for life on Earth. Human activity such as runoff pollution can originate from small or large sources on land and water, including motorized vehicles, oil spills, agricultural chemicals, and recreation. Runoff pollution negatively affects beaches and ocean habitats. Overharvesting food from the ocean creates an imbalance in existing ocean food webs. Other examples of the effects of human activity on oceans include climate change, spread of disease, and introduction of exotic species. Runoff and the Ocean For many years, scientists believed that runoff from commercial farms can cause excessive phytoplankton blooms. In 2005, Stanford University scientists were able to finally prove that theory. Now we have verified the information we need to better protect oceans from human pollution. How does this affect the ocean ecosystem and the human food chain? To answer that, we need to understand how humans interact with the ocean. The ocean ecosystem is more sensitive than it looks. We can see with the naked eye that plants and fish live in a delicate balance. Schools of fish move through the ocean in ways that do not put too much pressure on any one source of food. Different populations are kept in check by both the amount of food available and their predators. We must use scientific methods, however, to see that amounts of tiny molecules like nitrogen and oxygen also have a profound affect on ocean ecosystems. The ocean drives Earth’s water cycle. All but 3% of the water on Earth is in the ocean. It also strongly influences Earth’s climate because much of the Sun’s heat is absorbed by the ocean. We get many resources from the ocean, such as tuna, shrimp, fish, crab, kelp, and blubber, among many others. The ocean influences all life on Earth. That is why it is very important that we are aware of how we affect ocean systems. Water that travels over the ground and drains into a stream, lake, river, or ocean is called runoff. Most runoff comes from rain, but pollution in the runoff often comes from water that has been used for agricultural purposes. This is water that has been used to irrigate the plants that farmers grow. Sometimes this water can collect contaminants from the ground as it rolls downhill. These contaminants can include lead, nitrogen, and phosphorus. Nitrogen is a chemical element that many farmers use to increase crop yields. It is the “active” ingredient in manure and many other plant fertilizers. Unfortunately, it has the same affect on ocean plants. This introduction of additional plant fertilizer into the ocean changes the balance of ocean plant growth. This can have serious consequences for ocean wildlife. Phytoplankton are microscopic algae that live in the ocean. Phytoplankton form the base of the aquatic food chain. Thousands of marine organisms rely on phytoplankton for food. Individually they are too small to be seen. They can be easily seen as a green color in the water when millions of phytoplankton are together. The chlorophyll in their bodies causes the green color. When phytoplankton quickly increase in numbers, these large concentrations of them are called “algal blooms.” Algal blooms have been happening naturally since man has been recording observations about the ocean. However, we have now begun to understand the relationship between contaminated runoff and ocean algal blooms. Algal blooms can cause hypoxic (low-oxygen) areas in waters. As the short-lived algae die, oxygen is taken from the water to help bacteria decompose their bodies. Hypoxic areas, or dead zones, are places where most oxygen dependent life forms cannot live. Dead zones can cause fish kills and other events where large amounts of marine organisms die. The scientists at Stanford University were the first to prove the connection between nitrogen in runoff water and algal blooms. They studied aerial photos of the Gulf of California. The land surrounding the Gulf of California is used heavily for agriculture. Hundreds of thousands of acres are used as farm land. Farmers fertilize these farms four times per year. They spray their crops with a nitrogenbased fertilizer. The fertilizer collects in the runoff and drains into the ocean. The researchers discovered that there was always an algal bloom within days of this fertilization cycle. Algal blooms can cause major problems for both aquatic life and commercial fisheries. It is important to understand how humans can cause — and prevent — abnormally concentrated algal blooms.