Bald Eagle Chicks as Pollution Sentinels: DDT and PCBs in the Upper Midwest

Legacy contaminants are chemicals once used in the United States and later discontinued or banned — but which linger in soil and water, breaking down slowly and harming wildlife long after they were released. Two of the best known are still showing up in the bald eagles of the upper Midwest.

After blood and feather samples are taken, a nestling waits to go back to its nest. Photo by T. Gostomski, National Park Service

The two chemicals

Eagles take these in through their diet, mostly fish. Concentrations rise at each step up the food web — biomagnification — until they reach levels known or suspected to harm wildlife. People who eat fish face the same risk.

The U.S. National Malaria Eradication Program (1947–1952) sprayed DDT on waters where mosquitoes bred. Image from the CDC Public Health Image Library

Why eaglets

Bald eagles sit at the top of the aquatic food web, which makes them good sentinels. Nestlings are especially useful: their food comes from a known local area.

• Blood shows current exposure.
• Feathers, grown fresh each year, hold contaminants that build up over time and can cause health and reproductive problems.

Great Lakes eaglets have been monitored for more than 30 years: by the Wisconsin Department of Natural Resources along Lake Superior's south shore and in the Apostle Islands (1989–2002), then by the NPS Great Lakes Inventory and Monitoring Network (2006–2015).

The study sites. Image from the National Park Service

DDE: falling steadily

DDE, a common breakdown product of DDT, was highest in Apostle Islands eaglets — likely because contaminants break down slowly in Lake Superior's deep, cold water — and lowest on the upper St. Croix.

From 2006 to 2015, DDE fell 5.8% a year across the region, in line with the 5.5% a year decline over 26 years in the two Lake Superior study areas.

Figure 1. DDE in nestling blood plasma, 2006–2015. Image from the National Park Service

PCBs: slower to change

Total PCBs — here, the sum of 75 PCB types (congeners) — were:

• highest at Pools 3 and 4, the lower St. Croix and MISS;
• moderately high on Lake Superior (APIS and LSSS);
• lowest on the upper St. Croix.

Since 2006, the average level at every study area has stayed below 190 µg/L, the estimated threshold for harm to eagle reproduction. There was no significant decline in total PCBs from 2006 to 2011 — but six dioxin-like PCBs, among the most toxic, fell 2.7% at APIS and LSSS over 16 years.

Figure 2. Maximum total PCBs (micrograms per liter) in nestling plasma, 2006–2011; each dot is a territory, and an asterisk marks a possible threshold for neurological effects. Image from the National Park Service

What next

• DDE should keep declining slowly or level off low. Periodic monitoring is still worthwhile — it's cheap, and the test also measures DDT itself — especially because DDT use may expand in the tropics against mosquitoes as the climate warms, and could reach the region on the wind and in migratory birds.
• PCBs should also keep declining slowly. Managers can speed that by collecting old PCB-containing equipment and by banning, or enforcing bans on, low-temperature burning of plastics and Styrofoam. Burn barrels used for garbage are a major source of PCBs and dioxins; several Great Lakes states and provinces have banned them, and monitoring will show whether education and enforcement are working.

The bald eagle is now an established bioindicator for many agencies in the Great Lakes region — thanks to its place atop the food web, decades of data, and regulatory requirements.

Sources

Based on "A Legacy of Contamination," Great Lakes Inventory and Monitoring Network, National Park Service; a work of the United States government in the public domain.