A highly toxic form of mercury could jump by 300 to 600 percent in zooplankton – tiny animals at the base of the marine food chain – if land runoff increases by 15 to 30 percent, according to a new study. And such an increase is possible due to climate change, according to the pioneering study by Rutgers University and other scientists published in Science Advances.
"With climate change, we expect increased precipitation in many areas in the Northern Hemisphere, leading to more runoff," said Jeffra K. Schaefer, study coauthor and assistant research professor in Rutgers' Department of Environmental Sciences.
"That means a greater discharge of mercury and organic carbon to coastal ecosystems, which leads to higher levels of mercury in the small animals living there. These coastal regions are major feeding grounds for fish, and thus the organisms living there serve as an important source of mercury that accumulates to high levels in the fish people like to eat."
The study showed that an increase in natural organic matter entering coastal waters can boost the bioaccumulation of methylmercury – a highly toxic chemical found at elevated levels in many species of fish – in zooplankton by 200 to 700 percent. The huge increase in methylmercury shifts the food web from being autotrophic (largely microscopic plants and cyanobacteria that make food from inorganic matter) to heterotrophic (bacteria that eat organic matter produced by plants and cyanobacteria).
Natural organic matter from plants and animals in runoff also increased methylmercury levels in water by up to 200 percent, increasing exposure to the chemical in the food web, the study says.
Mercury is one of the top 10 chemicals of major public health concern, according to the World Health Organization (WHO), and the U.S. Environmental Protection Agency (EPA) says mercury is the main cause of fish consumption advisories aimed at protecting human health, the study notes.
Since the industrial era began, mercury that winds up in ecosystems is estimated to have increased by 200 to 500 percent, the study says. Mercury accumulates in fish and shellfish as methylmercury, which can affect the nervous, digestive and immune systems, as well as the lungs, kidneys, skin and eyes.
For the study, a group of scientists in Sweden tried to recreate the environmental conditions in a Bothnian Sea estuary off Sweden's east coast. They created simulated ecosystems that took up two floors of a building.
They collected intact sediment cores from the estuary, added water, nutrients and mercury, and studied what happened to the mercury, zooplankton and other organisms. Schaefer's role was to study microorganisms in the sediment that are responsible for producing the methylmercury which accumulates in the food web.
The scientists sought to understand, model and predict the impact of climate change on mercury accumulation and methylmercury production, said Schaefer, who specializes in methylmercury research and is trying to understand how bacteria transform mercury into methylmercury.
The results showed the importance of including the food web-related impacts of climate change on the bioaccumulation of methylmercury in future mercury models and risk assessments, the study says.
"We found that the increase in organic matter changed the food web structure in the simulated estuary and that had an impact on the mercury accumulation in zooplankton," Schaefer said. "That was the most dramatic effect."
"This is quite an important study," she added. "People haven't really considered the changes in food web structure at the bottom of the food chain and a link to mercury accumulation. I think these findings are quite surprising and, in hindsight, they make sense."
Efforts to reduce mercury emissions may be offset by the impacts of climate change, including increased precipitation and runoff, and we might not see an expected decrease of methylmercury in the food web, she said.
Climate changes may lead to more poisonous mercury in plankton
Umea, Sweden (SPX) Jan 31, 2017
Global warming is expected to increase runoff and input of organic matter to aquatic ecosystems in large regions of the Northern hemisphere including the Baltic Sea. Research performed in Sweden is now indicating a sevenfold increase in poisonous methylmercury in zooplankton as a consequence. This increase is due to an altered structure of the aquatic food web. The study has been published in the journal Science Advances.
"The study has revealed a phenomenon that has not been described before. The results are critical in the prediction of how global climate changes can affect the exposure of methylmercury to ecosystems, and humans," says Erik Bjorn, associate professor at Umea University and leader of the research project.
Mercury is regarded as one of the top ten chemicals of public health concern according to the World Health Organization, WHO. The problems with mercury are mostly caused by methylmercury, an organometallic mercury compound that acts as a strong neurotoxin and that can be accumulated in the food webs of seas and lakes.
The content of methylmercury in fish and other living organisms is controlled both by the total content of mercury in the ecosystems and by complex chemical and ecological processes in the environment. Climate changes and land use are expected to affect these processes in several ways, for instance by input of organic matter, humic substances, from land through watercourses out to lakes and seas.
Humic substances affect the aquatic environment in several ways – for instance by reducing the reach of sunlight into the water. That can lead to reductions in the production of phytoplankton via photosynthesis and instead favour growth of bacteria which can make use of humic substances for their growth. In turn, this can cause a trophic shift in the food web where it goes from being dominated by phytoplankton production (autotrophic) to being dominated by bacterial production (heterotrophic).
A heterotrophic food web generally has more levels of different organisms than an autotrophic food web, and the researchers' hypothesis was that this phenomenon would lead to an increased number of potential steps where methylmercury can be concentrated before reaching predators such as zooplankton and fish.
"Our study confirms this hypothesis and shows that an increase of 15-20 per cent of the content of organic matter in our waters can cause a shift from an autotrophic based to a heterotrophic based food web and lead to the content of methylmercury increasing two to sevenfold in zooplankton," says Erik Bjorn.
An increase in the content of organic matter by 15-20 per cent caused by increased precipitation and runoff is in accordance with climate change scenarios for large regions of the northern hemisphere, including the Baltic Sea region. The experiment also shows that the measured increase in methylmercury (two to sevenfold) is on a par with the estimated total increase (two to fivefold) of mercury in the ecosystems caused by human emissions during the entire industrial era from 1850 up until present time.
"The results emphasise the critical importance of including effects of changes to the food web in lakes and seas into models and risk assessments of mercury in a changing climate," says Erik Bjorn.
