Trash-Eating Gulls Cause Algae Blooms with Their Poop

Trash-Eating Birds Cause Algae Blooms with Their Poop
July 19, 2017

It is easy to recognize how closely tied the environment, the economy and wildlife are to our habits and to each other because we hear stories to that effect every day.

The amazing fact of that interconnectedness can sometimes lose its punch as we are repeatedly bombarded. It loses its punch that is, until a story like this pops up.

Gulls on a LandfillBirds surviving on a diet of human trash cause toxic algae blooms by pooping into lakes and rivers, costing the government millions of dollars per year in clean ups and pollution credits.

That is the finding of a new Duke University study, published in the journal, Water Research. Former postdoctoral researchers Scott Winton and doctoral candidate Mark River monitored gulls at local landfills, measured how much waste they introduced to local reservoirs and extrapolated those findings to the whole of North America.

Winton and River estimate that roughly 1.4 million gulls get their food from landfills across North America. More locally, landfills near Jordan Lake support a steady flock of roughly 49,000 seagulls.

Pictures of these flocks are striking but finding them at landfills makes biological sense. Gulls scavenge for food and you can see them in cities all over the country picking over dumpsters and other local food sources. Americans throw away about $165 billion worth of food every year so a natural scavenger can make a sweet living by dumpster-diving in vast landfills full of food.

This study, however, shows that lakes and rivers pick up the check for the gulls’ free lunch. Trash-birds drop 40-140 metric tons of phosphorus and 240-858 metric tons of nitrogen into North American bodies of water every year. That may not sound like much, spread over an entire continent, but those two nutrients, which are the active ingredients in fertilizer, can promote plant growth even in tiny amounts. Further, as these nutrients have been processed by the gulls, they are more bioavailable, or ready to be absorbed by living things, than other nutrient sources, so algae can do a lot with a little bird poop.

Gulls gathered at a park benchThe issue with the extra fertilizer is that when algae have enough food, they will rapidly reproduce in large blooms. Some species of algae release toxic chemicals, killing fish and/or making the water undrinkable by humans.

This nutrient pollution comes from other sources as well, like runoff from farms and local neighborhoods, but Winton and River found that in just the Jordan Lake area, the pollution from gulls alone costs about $2.2 million in government pollution credits. Nationwide, that total jumps to an estimated $100 million annually, Winton says. 

The trash-birds’ dinner is not all bad, however. Buried trash is slowly eaten by bacteria and other microorganisms which break it down into methane. Captured methane is useful as natural gas, but when it just escapes into the atmosphere, as it often does from landfills, it acts as a greenhouse gas more than 25 times as potent as carbon dioxide. Snacking gulls keep 1-4 million metric tons of methane out of the atmosphere per year. That only represents about a percent or two of total U.S. methane emissions, so not that much at the end of the day, but at least something positive comes out of the rain of bird poop and algal blooms.

Winton says, there are some strategies to curb this issue before it starts. Using smaller landfills and burying the trash faster could prevent too many gulls from gathering in one area. Reducing food waste could also help decrease the gulls’ food supply.

Thinking more outside the box, Canadian researchers experimented with using other animals to chase off the seagulls, and if that strategy were to take off, we could be shocked out of our human behavior/environment/economy connection apathy with this headline:

Patrolling falcons prevent toxic allergy by chasing away trash-munching, fertilizer-pooping gulls.”

—Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment in North Carolina.

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