Sources and discharge of nitrogen pollution from agriculture and wastewater in the Mesoamerican Reef region

Land-based nitrogen pollution is a major threat to coastal ecosystems, especially in tropical regions home to high biodiversity habitats such as coral reefs and seagrass beds. The sustained addition of excess nutrients (in the form of nitrates) to these ecosystems, which are adapted to oligotrophic environments, disrupts ecosystem function and the ability to provide services that support livelihoods and benefit human well-being. Nitrogen (N) primarily originates from agricultural crop production, livestock waste, and human sewage, as well as excretion from seabird and feral ungulates for some small atolls and cayes. Determining the most effective mitigation strategies to reduce N pollution in a given location begins with identifying and quantifying input from the source. Here we model four major sources of N pollution – crop production, livestock production, wastewater generated from permanent residents and wastewater generated from seasonal populations – at the regional scale, measuring inputs and impacts from 430 watersheds that drain into the Mesoamerican Reef region. We find that leaching of fertilizer from crop production and runoff associated with livestock contribute the majority of N pollution (92%), while contributions from residential and tourism wastewater sources were lower (8%). A north-south divide in N pollution was observed, with watersheds from the south, Guatemala and Honduras, associated with the highest levels of N pollution. The top 20 most polluting watersheds contributed nearly 90% percent of N pollution from all sources analyzed to the MAR, with the majority (11) located in the southern part of the MAR. The top two most polluting watersheds on their own, the Rio Ulua, Honduras, and Rio Motagua, Guatemala, contribute approximately 50% percent of the total N input to the MAR. However, high biodiversity habitats were more exposed to plumes originating from smaller, coastal watersheds. Our approach provides a map of the N pollution in the region highlighting hotspots for managers to target for mitigation, as well as the main sectors and industries to engage when designing solutions.