What is a dead zone?, su oceanservice.noaa.gov, NOAA. URL consultato il 18 aprile 2015 (archiviato dall'url originale il 27 aprile 2015).
«The largest hypoxic zone in the United States, and the second largest hypoxic zone worldwide, forms in the northern Gulf of Mexico adjacent to the Mississippi River. This image from a NOAA animation shows how runoff from farms (green areas) and cities (red areas) drains into the Mississippi. This runoff contains an overabundance of nutrients from fertilizers, wastewater treatment plants, and other sources.»