Why is there Heavy Metal in Our Oceans?

Covering 71% of the earth’s surface, the oceans are a major component of our ecosystem. And, while it’s thought that the very beginning of life on Earth happened in hydrothermal vents deep below the ocean’s surface, they remain an essential element in sustaining life on earth:

• The oceans produce more oxygen than the world’s rainforests.
• Oceans absorb almost half of the carbon dioxide generated by humans.
• 86% of drinking water comes from the oceans.

Overfishing is a well-known human impact on the ocean, but there is another human impact that you might not be aware of — heavy metal poisoning.

What is a heavy metal?

There is no firm definition of what a heavy metal is, nor is there a definitive list of which metals are considered heavy metals — even in regulations. Some definitions are based on density, with elements having a density greater than 3.5 g/cm³ classed as heavy metals. Most definitions include some of following metals: copper, nickel, cadmium, iron, lead, mercury and zinc.

When considering metals in the oceans however: lead, mercury and cadmium give most cause for concern — but how do they get into the oceans?

Why is there heavy metal in our oceans?

Heavy metals are a natural component of the Earth’s crust — hence natural processes like weathering and erosion cause metals to be washed from the land and transported to the oceans. However, increased industrial activity means that human activity is now the largest contributor to heavy metal concentration in our oceans.

The majority of the metal in the oceans comes from the atmosphere — with an increase in the atmospheric concentration over the past few hundred years subsequently leading to an increase in ocean metal concentration. Which leads to a slow poisoning of sea-life. In the vast oceans, the concentration of metal is usually insufficient to kill — but it weakens animals and reduces the amount of young born — increasing the stress on the population. With additional stresses like climate change, this can lead to an irrecoverable population drop, changing the balance of the ocean’s ecosystem.

Localised increases in metal concentration in coastal waters are the most likely changes to have a direct impact on humans. Between 1932 and 1968, Minimata Bay in Japan was used to dispose of wastewater from a factory. The water contained mercury, which found its way into the food chain of the local fish, and by a process of bioaccumulation, the concentration of mercury in the fish increased to toxic levels — the fish were subsequently eaten by the residents of Minimata. Over 1700 people died of mercury poisoning and the effects are still being felt today.

In contrast to this, there is evidence that the lead concentration in surface waters is falling as lead fuel in vehicles has been phased out. This demonstrates that the oceans can naturally reverse the damage done by past human activity. Monitoring of the change in lead concentration is discussed in Corals as Natural Archive of Environmental and Climate Changes.