Mercury rising

Two striped, iridescent green fish swim close together in dark water.

Mercury rising

When atmospheric mercury falls back to Earth in raindrops, it rises up the food web, and is now found in increasing levels in fish
June 23, 2015
A single green and yellow fish with red fins lies on a wide ruler.
Mercury levels in yellow perch, or Perca flavescens,
have increased in some lakes in Nova Scotia.
Credit: Jennifer Thera, University of New Brunswick

Mercury was once most familiar as the friendly silvery liquid inside thermometers. Then scientists discovered that the metal causes deformities in fish and birds and can inflict irreversible damage to human fetuses, infants, children and women of child-bearing age.

Today, scientists estimate that the amount of atmospheric mercury is three times the naturally occurring level. The excess comes from industrial effluents, coal-fired electricity plants, artisanal gold mining and improper disposal of consumer products, such as batteries and light bulbs. Rainfall returns the freed mercury to Earth — and directly into water bodies around the globe.

A swampy lake surrounded by trees. The sun is shining and the sky is blue.
Lake Hilchemakaar is one of several lakes in Nova Scotia’s
Kejimikujik National Park where methylmercury con-
centrations were measured in fish in summer 2013.
Credit: Jennifer Thera
, University of New Brunswick

These include freshwater lakes in Nova Scotia’s Kejimkujik National Park, which in the 1990s were found to contain fish and loons with some of the highest mercury levels in North America. Recent follow-up work by Karen Kidd, a biologist at the University of New Brunswick, has revealed that mercury levels in the tissues of yellow perch, which are common in Kejimkujik, have increased in some lakes. Interestingly, mercury is damaging the perch at the cellular level, especially in the kidneys and spleen, but the fish still seem to reproduce and grow normally.

Kidd has also shown that it’s not an unusually rapid movement of the mercury up the food web that’s causing the high mercury levels in the fish. Rather, more mercury is entering the base of the food web, which is driving the high mercury concentrations in perch.

Kidd’s data will inform a national mercury assessment sponsored by Environment Canada and will help spur international efforts to curtail releases of mercury into the atmosphere, which is a priority of the United Nations Environment Programme.

“What we really need to do is focus on what’s coming in from the atmosphere and try to reduce it,” says Kidd. “That’s how we can benefit the fish, the fish-eating wildlife and humans.”

Main photo credit: United States Department of Agriculture

Originally posted June 2014