Mercury is a naturally occurring element found throughout Earth’s systems, including the ocean. While its presence in marine environments is a natural phenomenon, the quantities and chemical forms of mercury found in the ocean have become a growing concern. Certain forms of mercury can pose risks to marine life and human health through seafood consumption.
Natural Origins of Ocean Mercury
Mercury has always circulated within Earth’s environment due to natural processes. Volcanic eruptions, both on land and underwater, release mercury into the atmosphere and directly into water bodies. These emissions contribute to the global mercury cycle.
The slow breakdown of rocks containing mercury adds to its presence in water. This process, known as weathering, leaches mercury from the Earth’s crust into soils and rivers, which then carry it to the ocean. Forest fires similarly release mercury stored in biomass into the atmosphere, allowing it to settle into marine waters. Geothermal vents on the seafloor represent another natural pathway, discharging mercury-rich fluids directly into the ocean.
Human Activities and Ocean Mercury
Human activities have dramatically increased the amount of mercury entering the global environment, far surpassing natural releases. Since industrialization, atmospheric mercury has risen by an estimated 300% to 500% above natural levels, finding its way into the ocean.
The combustion of fossil fuels, particularly coal in power plants, is a major human-caused source. Coal contains trace mercury, released as vapor when burned for electricity. This airborne mercury travels long distances before depositing onto land or directly into the ocean.
Artisanal and small-scale gold mining (ASGM) is another significant contributor, often releasing more mercury pollution than combined emissions from coal combustion and large-scale industrial activities. Miners use elemental mercury to extract gold from ore by forming an amalgam, then heating it to evaporate the mercury and isolate the gold. This process directly releases mercury vapor into the air and liquid mercury into surrounding water bodies, contaminating local ecosystems that drain into larger river systems and the ocean.
Industrial processes, including metal smelting, cement production, and chlor-alkali production, also contribute to mercury emissions. These operations can release mercury as a byproduct or through mercury-containing materials. Waste incineration, especially the burning of products that contain mercury like old fluorescent lamps or thermometers, disperses mercury into the atmosphere.
Mercury’s Journey and Forms in the Ocean
Once released, mercury undertakes a complex journey through the environment and can exist in different chemical forms. Elemental mercury (Hg0) is a volatile vapor that can remain in the atmosphere for months to a year, enabling its global transport via air currents. Other forms, such as reactive gaseous mercury and particulate-bound mercury, have shorter atmospheric lifetimes and tend to deposit closer to their origin.
Mercury enters the ocean primarily through atmospheric deposition, where it settles onto the water surface via both wet deposition (rain and snow) and dry deposition (particles settling from the air). Direct discharge and runoff from land-based sources, like rivers carrying industrial waste or agricultural runoff, also deliver mercury into coastal waters and eventually the open ocean. Once in the marine system, ocean currents play a role in distributing mercury globally, allowing it to reach even remote oceanic regions.
A significant transformation occurs when inorganic mercury enters oxygen-limited environments within the ocean, such as sediments or deeper water columns. Here, certain types of anaerobic bacteria, which thrive without oxygen, convert inorganic mercury into methylmercury (MeHg). This biotransformation is important because methylmercury is the most toxic form of mercury and is readily absorbed by marine organisms. The relative efficiency of this methylation process, along with demethylation, determines the concentration of methylmercury in the water and subsequently in marine life.
Why Mercury Builds Up in Seafood
The presence of methylmercury in the ocean leads to its accumulation in marine organisms through two interconnected processes: bioaccumulation and biomagnification. Bioaccumulation describes how methylmercury, once ingested by small marine organisms like phytoplankton and zooplankton, accumulates in their tissues. These organisms absorb methylmercury from the water or their diet faster than they can excrete it, leading to increasing concentrations within their bodies over time.
As methylmercury moves up the marine food web, its concentration intensifies through a process called biomagnification. When smaller organisms containing accumulated methylmercury are consumed by larger predators, the mercury transfers to the predator’s tissues. Since predators consume many prey items over their lifespan, the mercury concentration becomes progressively higher at each successive trophic level. Top predatory fish, such as tuna, swordfish, and sharks, can therefore have significantly elevated levels of methylmercury compared to smaller fish or primary producers. Consuming these larger, longer-lived fish is the primary way people are exposed to higher levels of mercury from oceanic sources.