How Does Mercury Enter the Ocean?

Mercury, a potent neurotoxin, finds its way into our oceans through a complex web of natural and human-driven processes, ultimately impacting marine ecosystems and human health. Atmospheric deposition from industrial emissions, artisanal gold mining, and volcanic activity stands out as the primary pathway, delivering mercury both directly onto the ocean surface and indirectly via terrestrial runoff.

Understanding Mercury’s Oceanic Journey

Mercury’s journey to the ocean is multifaceted, involving atmospheric transport, riverine discharge, and direct releases. While natural sources contribute, anthropogenic activities significantly amplify the influx of mercury, disrupting natural biogeochemical cycles and posing a serious environmental threat.

Natural Sources: A Baseline of Mercury

Even without human intervention, mercury exists in the environment.

• Volcanic eruptions: Volcanoes release mercury stored deep within the Earth’s crust, injecting it directly into the atmosphere where it can travel globally.
• Weathering of rocks: As rocks containing mercury-bearing minerals erode, they release mercury into the soil and waterways, eventually making its way to the ocean.
• Geothermal activity: Geothermal vents and hot springs can leach mercury from the surrounding geology, contributing to localized increases in mercury concentrations.

Anthropogenic Sources: A Surge in Mercury Input

Human activities are the dominant force driving increased mercury levels in the ocean.

• Coal-fired power plants: Burning coal releases mercury trapped within the coal seams. This mercury is then emitted into the atmosphere and subsequently deposited onto land and water. This is perhaps the largest single anthropogenic source.
• Artisanal and small-scale gold mining (ASGM): ASGM practices often use mercury to extract gold from ore. Significant amounts of mercury are released directly into waterways and the atmosphere during this process. This is particularly problematic in developing countries.
• Industrial processes: Various industrial processes, including chlorine alkali production and the manufacturing of certain chemicals, can release mercury as a byproduct.
• Waste incineration: Incinerating mercury-containing products, such as batteries and fluorescent lights, releases mercury into the atmosphere.
• Cement production: Limestone, a key ingredient in cement, can contain trace amounts of mercury that are released during the cement manufacturing process.

Atmospheric Deposition: Mercury’s Aerial Route

Atmospheric deposition is arguably the most significant pathway for mercury entering the ocean. Mercury emitted into the atmosphere can travel long distances before being deposited via:

• Wet deposition: Rain, snow, and fog scrub mercury from the atmosphere and deposit it onto the ocean surface.
• Dry deposition: Mercury in gaseous form or attached to particulate matter settles directly onto the ocean surface.

Riverine Transport: Mercury’s Terrestrial Route

Rivers act as conduits, transporting mercury from terrestrial sources to the ocean.

• Runoff from contaminated sites: Mercury deposited on land or released from industrial sites can be washed into rivers during rainfall.
• Erosion of mercury-rich soils: Erosion can mobilize mercury stored in soils and transport it to rivers.
• Discharge from wastewater treatment plants: Wastewater treatment plants may not effectively remove all mercury, leading to its discharge into rivers.

FAQs: Delving Deeper into Ocean Mercury

Here are frequently asked questions that provide further insight into the complexities of mercury contamination in the ocean.

FAQ 1: What is the difference between inorganic and organic mercury?

Inorganic mercury refers to mercury in its elemental form (Hg0) or as inorganic salts (HgCl2, HgS). Organic mercury, specifically methylmercury (CH3Hg+), is formed when inorganic mercury is converted by microorganisms in aquatic environments. Methylmercury is far more toxic than inorganic mercury and readily bioaccumulates in the food chain.

FAQ 2: Why is methylmercury so dangerous?

Methylmercury is highly toxic because it readily crosses the blood-brain barrier and the placental barrier, causing neurological damage. It interferes with brain development in fetuses and young children and can cause cognitive impairment, motor dysfunction, and sensory deficits in adults. Its high affinity for proteins allows it to accumulate in living tissues.

FAQ 3: How does mercury bioaccumulate in the marine food web?

Bioaccumulation occurs when organisms absorb mercury faster than they can eliminate it. Small organisms, like plankton, absorb methylmercury from the water. Larger organisms, such as fish, consume these smaller organisms, accumulating even higher concentrations of mercury. This process is amplified as you move up the food chain, leading to very high mercury levels in top predators like tuna and sharks.

FAQ 4: What are the health risks associated with consuming seafood contaminated with mercury?

Consuming seafood contaminated with mercury can lead to neurological problems, kidney damage, and developmental problems in children. Pregnant women, nursing mothers, and young children are particularly vulnerable. The severity of the health effects depends on the amount and frequency of mercury exposure.

FAQ 5: Which types of seafood typically have the highest mercury levels?

Generally, larger, longer-lived predatory fish tend to have the highest mercury levels. Examples include swordfish, shark, king mackerel, and tilefish. Smaller fish like salmon, sardines, and herring typically have lower mercury levels.

FAQ 6: Are there any steps I can take to reduce my mercury exposure from seafood?

Yes. The US EPA and FDA provide guidelines on safe seafood consumption. They recommend choosing fish with lower mercury levels, limiting consumption of high-mercury fish, and being mindful of serving sizes, especially for pregnant women and children.

FAQ 7: How does mercury impact marine ecosystems beyond human health?

Mercury can disrupt marine ecosystems by impairing the health and reproductive success of marine organisms. It can affect their behavior, growth, and survival. High mercury levels can also lead to declines in fish populations and alterations in food web structure.

FAQ 8: Can mercury pollution in the ocean be reversed?

While completely reversing mercury pollution is extremely difficult, mitigation efforts can reduce future inputs and promote natural attenuation. This includes reducing mercury emissions from industrial sources, phasing out mercury in products, and restoring contaminated sites. Natural attenuation processes include sedimentation, where mercury is buried in sediments, and methylation/demethylation cycles.

FAQ 9: What is being done at the international level to address mercury pollution?

The Minamata Convention on Mercury is an international treaty aimed at protecting human health and the environment from the adverse effects of mercury. It covers the entire lifecycle of mercury, from mining to disposal, and includes provisions for reducing mercury emissions and releases, phasing out mercury-containing products, and remediating contaminated sites.

FAQ 10: How do climate change and ocean acidification affect mercury cycling in the ocean?

Climate change and ocean acidification can alter mercury cycling in complex ways. Rising sea temperatures can increase methylation rates, leading to higher methylmercury concentrations. Changes in ocean currents and stratification can also affect the distribution and fate of mercury. Ocean acidification may alter the bioavailability of mercury, making it more or less accessible to organisms.

FAQ 11: What role does sediment play in mercury contamination of the ocean?

Sediment acts as a both a sink and a source of mercury. Mercury deposited on the ocean floor can be buried in sediments, effectively removing it from the water column. However, sediments can also release mercury back into the water column through processes like resuspension and microbial activity. Anoxic sediments can promote methylation.

FAQ 12: How can I contribute to reducing mercury pollution in the ocean?

You can contribute by reducing your consumption of electricity generated from coal, supporting policies that promote clean energy, properly disposing of mercury-containing products like batteries and fluorescent lights, and choosing seafood responsibly. Educating yourself and others about the dangers of mercury pollution is also crucial.