The ocean has always been vital to human communities. From the moment organized society formed, the water around humankind has provided nourishment, allowed for travel and trade, and even inspired some of the most dreaded nautical tales across literary traditions. Despite our intimate relation with the sea, the extent of our maritime knowledge pales in comparison to humanity’s other scientific achievements: while space exploration has become a staple of 20^th and 21^st century science, only 5% of the Earth’s oceans has been explored to date.

Unfortunately, this doesn’t mean that the sea suffers the impact of human activity any less than the rest of the Earth’s biosphere. Time and again, plastic has been spotted floating along the depths of the Marianas Trench, the deepest explored part of the ocean. In the surface, closer to home, plastic pollution and hazardous wastewaters proliferate with catastrophic effects on marine ecosystems. An armada of human activities is responsible for water pollution – from manufacturing to large-scale fishing to cruise ships, the hydrosphere can hardly catch a break. Ironically, one of the most damaging industries for the environment involves an activity humans have depended on for thousands of years: shipping.

While maritime shipping is a lot more carbon efficient than land or air routes, ships are complex structures that produce air and water pollution in a number of different ways. Bigger vessels can even jeopardize marine animal populations that stand in their path. As with all environmental matters, the effects of shipping on ecosystems are complicated, so let’s take a better look.

Wastewater

Direct water pollution is perhaps the most complex aspect of the relationship between shipping and the environment. Maritime vessels produce massive amounts of wastewater, which can generally be split into two concise categories depending on its source: blackwater and greywater.

Blackwater refers to the sewage discharged from a ship. It can come from toilets or on-board medical facilities and is likely to include various pathogens as well as dangerous chemicals if left untreated. Bacteria, intestinal parasites, and viruses comprise the bulk of the microbial load in blackwater, and can contaminate marine ecosystems – especially shellfish beds and fisheries – with broader implications on public health via the food chain. Blackwater also contains nutrients such as nitrogen and phosphorus which, when introduced to bodies of water, can promote algal blooms and deplete the oxygen in the adjacent ecosystem.

Greywater, on the other hand, originates in sinks, showers, and washing machines; if left untreated, it typically contains a diversity of harmful materials, including fecal coliforms, oils, detergent, metallic and organic compounds, food waste, etc. The host of substances found in greywater comes with a variety of adverse environmental consequences, one of which is – much like with blackwater – the high oxygen demand some of these substances have, which can lead to eutrophication and fish kills.

That being said, wastewater isn’t the only liquid-form waste produced by shipping. Oil leaks from ship engines and machinery can blend with the water concentrated in the bilge of the ship – the lowest part of the hull – and damage the marine ecosystems in which this bilgewater is later released. These materials also have high oxygen demand, and can kill fish or cause sub-lethal effects when discharged without proper treatment. Likewise, solid waste produced by the ship and its crew can accumulate in the form of marine waste, compromising the integrity of the ocean ecosystems in the vessel’s path.

Effects on Wildlife and Ocean Biodiversity

Larger marine vessels can damage marine ecosystems by colliding with oceanic wildlife as they make their way through the sea. Marine mammals – particularly whales and manatees – are at significant risk of such collisions, with a 2007 study indicating that a ship traveling at only 15 knots has a 79% chance of crashing onto and subsequently killing a whale.

There is also the issue of noise pollution: while we might consider the open sea to be a vast, silence expanse, marine traffic and the activities on board ships can create loud noises that travel far enough to disturb the feeding, mating, and migration of many marine species. This is best exemplified by the changing migratory roads of multiple whale packs in order to circumvent the routes of cargo ships.

Lastly, the ballast water of tankers and cargo ships often contains life forms from faraway ecosystems, including animals, plants, and microbial agents. When those organisms are introduced to foreign ecosystems, they act as invasive species: they can proliferate, antagonize local flora and fauna, and ultimately compromise the balance of the ecosystem they have been brought into by competing with local life forms over an unchanged, limited amount of vital resources.