The biogeochemical implications of massive gull flocks at landfills.

Gulls have long been observed concentrating in flocks of tens to hundreds of thousands at the anthropogenic food sources provided by landfills. Yet, the biogeochemical implications of the landfill gull phenomenon have been largely ignored. This study has two goals: 1) to understand the magnitude and geographic extent of landfill gulls in North America, and 2) to quantify the amount of carbon (C), nitrogen (N), and phosphorus (P) transported from landfills to gull roosting sites in order to understand their potential impacts on water quality and methane (CH4) emission. We synthesized and mapped data from the eBird Citizen Science database and found that 1.4 million gulls have been documented at landfills throughout North America, though the actual population is probably greater than 5 million. Using a carnivorous bird transport model we estimate that these gulls transport 39,000 to 139,000 kg of P and 240,000 to 858,000 kg of N y-1 to neighboring water bodies and avoid roughly 1.1 to 3.9 Tg y-1 of landfill CH4 emissions. The avoided CH4 emission mitigation is insignificant in the context of gross landfill emissions, but the transported nutrients may be relevant to water quality management at local and continental scales. For example, at the Jordan Lake reservoir in North Carolina, a flock of 49,000 Ring-billed Gulls (Larus delawarensis) annually deposits landfill feces containing 1070 kg P, an amount equivalent to approximately half of total maximum daily load reduction targets and worth roughly $2.2 million USD in nutrient credits. We estimate that continent-wide gull impacts are worth at least $100 million in nutrient offset credits. We conclude that mega-flocks of landfill gulls are common and widespread, and that their capacity to transport nutrients may be contributing to the eutrophication of aquatic ecosystems and water supplies.