At least 1.4 million gulls feed at landfills across North America, which aside from the nuisance it might pose, is also a threat to the health of nearby waters, a Duke University study finds.
“We estimate these gulls transport and deposit an extra 240 tons of nitrogen and 39 tons of phosphorus into nearby lakes or reservoirs in North America each year through their feces,” says lead author Scott Winton, a 2016 doctoral graduate at the Nicholas School.
The added nutrients contained in the birds’ droppings can contribute to extensive algal blooms that rob surface waters of much of the oxygen needed to sustain healthy aquatic animal life—a process known as eutrophication.
Oxygen depletion and algal toxins that result from the blooms can have far-reaching ecological and economic impacts, including fish kills, increased costs for local governments, and reduced recreational or fishing values in affected waters.
“It costs local U.S. governments an estimated $100 million a year in nutrient offset credits to address or prevent the problem and maintain nutrient levels at or below the total maximum daily load threshold for water quality,” says Mark River, a doctoral student at the Nicholas School, who conducted the research with Winton.
The scale of the problem and the cumulative cost of dealing with it may be even larger than the new study suggests, says Winton, who is now a visiting postdoctoral fellow at ETHZurich, a science and technology university in Switzerland.
“We estimated and mapped a landfill-gull population of 1.4 million based on documented sightings reported in the eBird Citizen Science database. But the actual population is probably greater than 5 million,” Winton says. “That means the amount of nutrients deposited in the lakes, and the costs of preventing or remediating the problem, could be substantially higher.”
Winton and River published their study, which is the first to look at the transport of nutrients into surface waters from gulls in the journal Water Research (June 15).
They conducted the research at landfills near two major drinking water reservoirs—Jordan Lake and Falls Lake—that serve the Raleigh-Durham region of North Carolina. Nitrogen and phosphorus data from these two lakes were then scaled up to estimate total loading at water bodies near landfills across North America using a well-established model for measuring the nutrient transport of carnivorous birds.
The findings are applicable to lakes and reservoirs in other parts of the world as well.