Data from the Nab show nitrogen levels are generally highest near the shores of Portland Harbor, suggesting land-based sources contribute to nitrogen pollution. Each dot on the map represents one water sample (click on the map to make it larger!). The dots are color coded based on the Maine Department of Environmental Protection’s nitrogen advisory thresholds. Green dots show healthy levels of nitrogen. Yellow dots show nitrogen levels that can harm eelgrass habitat. Red dots show nitrogen levels that can lead to low amounts of oxygen in the Bay.