Mercury levels in the upper layers of the ocean are up 3.4x since the beginning of the industrial revolution, according to the first study to have done truly global measurements of marine mercury levels by taking thousands of samples around the world over half a decade. Unsurprisingly, humans are to blame for this alarming rise in neurotoxic pollution, with the highest concentrations to be found in the Arctic and North Atlantic oceans.
The primary culprits are the usual suspects: Burning fossil fuels, especially coal, and the mining industry.
We've been lucky so far because marine currents have been moving a lot of that mercury from the upper layers of the ocean, where there's more life, to the deeper layers that are more sparsely populated. But we might be getting closer to the point where we exhaust the ocean's ability to do that and mercury levels closer to the surface start rising faster. This is especially worrisome because humanity is on track to release into the environment as much mercury in the next 50 years as in the past 150... Not exactly a healthy trend.
Between 5–10% of US women of childbearing age already have blood mercury levels that that increase the risk of neurodevelopmental problems in their children, and an estimated 1.5 million–2 million children are born in the European Union each year with mercury exposure levels associated with IQ deficits. Wildlife and marine life is not spared either. Studies have found that mercury levels compromise the reproductive health and fertility of some fish and birds. (source)
What can we do? Moving away from coal for power generation would help tremendously, and the mining industry needs to clean up its act. We know what we need to do to reduce mercury emissions - it's not about inventing new techniques and technologies - we just need to enforce and implement what already exists.
There's reason for hope. With the rapidly falling cost of solar and wind power, as well as energy storage technologies that make renewables more practical on a large scale, coal could lose its primacy faster than most expect.