As divers, we all too often witness the effects of marine pollution firsthand. Sadly, even the world’s most remote dive destinations usually bear some evidence of human contamination. Sometimes, this pollution appears as plastic bags strewn across a reef. Fuel leaks like rainbow-colored poison from an idling boat engine. Images of global environmental disasters, like the sea of plastic that extends for millions of miles across the North Pacific, or the birds left slicked with viscous black liquid after a major oil spill, have become synonymous with our perception of pollution.
Causes of marine pollution
However, much of the damage humans wreak on the world’s oceans is far less obvious. According to the National Oceanic and Atmospheric Administration (NOAA), 80 percent of marine pollution originates from land. This pollution is categorized into two distinct sources: point and non-point. Point-source pollution refers to those sources that are obvious, localized and identifiable. These include sewage pipelines or industrial waste outlets that pump directly into the sea. Non-point sources include all those small, unquantifiable pollutants, including car exhaust fumes and agricultural run-off.
Pollution enters our oceans in one of three main ways. It can happen via direct or intentional discharge, via run-off from the land through rivers and rainfall, and via pollutants released from the atmosphere.
Direct or intentional discharge
Direct discharge includes effluent from sewage and industrial plants, and trash intentionally discarded into the sea. Often, the discharge from manufacturing plants includes toxic waste, which enters the food chain at the lowest level. It subsequently transfers throughout the ecosystem, becoming more concentrated as it ascends the chain. Large predatory marine species like tuna, marlin, dolphins and sharks often contain high levels of mercury and other dangerous toxins. These often make them unfit for human consumption, particularly for pregnant women.
In communities that regularly eat shark meat, for example, studies show that individuals may have as much as 2,000 percent more methyl-mercury in their systems than is considered safe. As well as impacting human health, increased toxin levels can result in disease, mutation, behavior alteration, infertility, growth suppression and death in marine creatures. Because commercial animal feeds often contain fish derivatives, toxins can be transferred to terrestrial livestock and contaminate meat and dairy products as well.
Intentional discharge also includes discarded human trash, the vast majority of which is plastic. Scientists estimate that the total mass of plastic trash in the ocean could be as much as 100,000,000 metric tons. Plastic does not degrade the way that other materials do. It accumulates and poses a significant threat to marine life, either through entanglement or ingestion. Many creatures are vulnerable to entanglement in plastic waste, particularly discarded fishing nets. This fate can lead to to restricted movement, starvation, injury, and in those species that need to surface or maintain movement in order to breathe, suffocation.
Plastic ingestion is also a major concern, and occurs throughout the food chain. Animals cannot easily digest plastic, and eating it means the animal cannot take on further nutrition. This can mean starvation, infection and death. This is a widespread affliction seen globally in highly polluted areas. On Midway Atoll, one third of the resident Laysan albatross chicks die each year as a result of being fed plastic by their parents.
Runoff from land
The second major source of marine pollution is runoff from the land, from both agricultural and urban areas. Runoff can include soil, fertilizers, pesticides, particles rich in carbon, nitrogen, phosphorous, and chemical run-off from roads and highways. Inland mining is a major contributor to this kind of pollution. It causes soil and mineral deposits to flow into the sea via rivers and estuaries. An influx of soil into the ocean is a threat to marine ecosystems because it leads to sedimentation and the clouding of coastal water. This smothers coral reefs and blocks marine plants’ access to sunlight, affecting their ability to photosynthesize.
Most importantly, excess nutrients in estuaries and oceans can cause such large algae and plankton blooms that they use up all of the water’s oxygen. If the situation is dire enough, it can result in hypoxic conditions. These are environments with such a critical deficiency in oxygen that nothing can survive. This phenomenon is known as eutrophication. It’s so widespread as a result of agricultural and urban runoff that the World Resources Institute has recognized 375 hypoxic coastal zones across the planet.
Pollutants from the atmosphere
Finally, the release of pollutants from the atmosphere is a significant contributor to marine pollution. The ocean naturally absorbs carbon dioxide. But as global warming causes carbon dioxide in the atmosphere to increase, the oceans absorb even more, making them more acidic. This acidity compromises and degrades marine structures made from calcium carbonate, adversely affecting shallow and deep-water corals. The protective shells of certain calcifying shellfish become vulnerable to dissolution as a result of ocean acidification, which puts these species at risk and has ramifications throughout the food chain and for the sustainability of global fisheries.
Similarly, increased carbon dioxide levels negatively impact the ability of coral to regenerate and grow by reducing their skeleton-producing abilities. Acidification is a serious problem that could wreak destruction on reef ecosystems, and on the 1,000,000 species that rely on healthy coral habitats for their survival.
Addressing the problem
Governments have implemented countless regulations all over the world to limit marine pollution, to varying degrees of success. Environmental organizations are attempting to mitigate pollution, for example, by planting oyster beds in eutrophic estuaries to filter excess nutrients and eliminate hypoxic conditions. However, for every positive action there are countless examples of companies continuing to pump harmful effluent into the sea. Gratuitous littering along beaches and a general lack of awareness or concern about our collective carbon footprint continues. If we are to reduce marine population, we must address the problem at personal and governmental levels.
From supporting and lobbying for international environmental policies to the simple act of turning off the light when you leave a room, we are all responsible for the future health of our oceans, and for the survival of the species that inhabit them.