Myriad Seabirds Help Track Mercury Pollution

Mercury affects marine life from phytoplankton to seabirds. Its most toxic form, methylmercury, is a neurotoxin that can cause organ damage and death in wildlife and in humans. Scientists have for years kept tabs on marine mercury pollution by monitoring levels in seabirds—and a recent study in the Gulf of Maine suggests that factoring in the lifestyles of several species can help us see patterns of contamination in detail.

Most environmental mercury comes from industrial sources such as coal-fueled power-plants and travels widely on the winds. Eventually it falls to land, and into rivers, lakes, or the sea, where it is taken up by bacteria, phytoplankton, and other organisms. Studies of lake sediments show that, today, methylmercury is accumulating two to five times faster than in preindustrial times.

Like many pollutants, methylmercury becomes more concentrated as it travels up the food chain from plants to grazers to predatory animals. Mercury in top ocean predators can reach levels a million times that of the surrounding water. That’s why even low levels of mercury in seawater can make large, long-lived fish such as swordfish risky for humans to eat in large quantities.

Though mercury can harm fish and birds, their ability to gather it from the environment can also help us monitor the problem. From 2001 to 2006, a team of biologists led by Wing Goodale and David Evers, of the BioDiversity Institute in Gorham, Maine, analyzed seabird eggs and blood on 35 islands in the Gulf of Maine; the results were published in EcoHealth in 2008.

Fortunately, average mercury levels in all 17 species were below what’s thought to be toxic in birds (though a few individual eggs did show higher amounts). On average, Leach’s Storm-Petrels, Razorbills, and Black Guillemots had the most mercury.

The researchers learned that relatively small numbers of eggs sampled sparingly from seabird colonies provided excellent data with less disturbance than sampling adult blood levels. Mercury in eggs is directly related to the mother bird’s recent dietary uptake, and because different species have different foraging patterns, their eggs yield information about different parts of the ocean environment.

Leach’s Storm-Petrels, for example, feed 100 miles or more offshore, eating fish that rise to the surface each night. Their eggs may represent global levels of mercury pollution, the researchers suggested. Black Guillemots feed close to shore and within 2.5 miles of their nesting colonies, eating fish and invertebrates from the bottom. Their mercury levels apparently reflect local pollution, which is influenced by such variables as nearby coal plants, municipal waste combustors, or medical waste incinerators.

Double-crested Cormorants catch fast-swimming fish and forage in an area roughly 25 miles in radius around their nesting colonies. Mercury levels of cormorant eggs were less variable than those of guillemots, apparently influenced less by site-specific factors. Common Eiders also provide a useful perspective, as they eat almost exclusively invertebrate prey.

Though these four species live beside one another across the Gulf of Maine, a thorough understanding of the way their foraging strategies differ provides important insights about how mercury moves through the environment—insights that may ultimately help us to develop laws and policies that will protect us all.

Originally published in the Summer 2010 issue of BirdScope.