There have long been dead zones – water too low in oxygen to sustain most forms of life – in the Gulf of Mexico, which receives the waters of the Mississippi River. Scientists studied sediment cores from areas where the gulf's most recent dead zone occurred. The scientists dated the sediment and counted species of foraminifera (marine protozoans) in the sediment; these species thrive in low-oxygen waters. As far back as 1823, the foraminifera thrived especially during Mississippi River flood years (during which nutrients levels increase), suggesting that nutrients in floodwaters can trigger low-oxygen water. The foraminifera in the core samples were most abundant after 1950, when farmers began using some fertilizer, which is rich in nutrients. Researchers believe that increased use of fertilizer leads to more-extreme dead zones.