No other North American river has a drainage basin as large as that of the Mississippi. The amount of nutrients it discharges into the Gulf of Mexico is correspondingly large. Because freshwater from the river is lighter than the salty seawater, it settles as a distinct layer above the seawater. This phenomenon is called stratification. The freshwater layer acts like a blanket to prevent the exchange of gases, oxygen for example, between the seawater and the atmosphere. This kind of stratification is also observed in other coastal areas, such as in the Baltic Sea between Denmark and Sweden, and in the Norwegian fjords. In the case of the Mississippi River, however, the situation is exacerbated by the especially high levels of nutrients it contains. The presence of the nutrients leads to profuse algal growth. When the algae die their remains sink into the lower water layer. There they are broken down by bacterial activity, a process that consumes oxygen. This causes the oxygen levels in the deep-lying saltwater layer to drop drastically. Free-swimming organisms flee the area due to the oxygen deficiency. Less mobile animals such as mussels die. For this reason, the low-oxygen areas off the coasts of Louisiana and Texas are called “dead zones“. In 2002 an oxygen-deprived area of more than 20,000 square kilometres was observed. This is equal to half the area of Germany. There is considerable evidence that the oxygen problem associated with stratification has only begun to occur more frequently since the middle of last century. The increase is probably due to the rising nutrient concentrations, especially nitrogen, which has trebled since the 1950s. Stratification in the northern Gulf of Mexico is, in fact, a natural phenomenon that is especially pronounced during years with high rainfall. Storm events like hurricanes can cause effective mixing of the water, and even counteract the effects of stratification. But the nutrient transport of the Mississippi River is still too great. A management plan has now been adopted to attempt to reduce the nutrient input, with an aim of limiting the maximum area of the dead zone to around 5000 square kilometres. The measures being applied include improved wastewater treatment, optimized fertilization practices, and the creation of flood-plain areas along the rivers, which would absorb significant amounts of the nutrients.

4.5 > The Mississippi River carries vast amounts of sediments (yellow-brown) and nutrients into the Gulf of Mexico, which are then transported westward along the coast by the wind. The nutrients cause a strong growth of algae (green). Oxygen is consumed in the deep water as bacteria break down the algae. This results in a completely oxygen-depleted dead zone along a broad strip of the US coast.