Ocean dead zones, the case of the Gulf of Mexico

In focus today…. The Gulf of Mexico Dead Zone.

In continuation to my post on eutrophication and dead zones, I will investigate how livestock farming has caused major impacts on marine environments by taking a closer look at the Gulf of Mexico (GoM) dead zone. I thought focusing on a case study would be a better way to illustrate and understand an issue that is happening today in the United States.

The Gulf of Mexico Dead Zone is a temporal dead zone characterized by seasonal periods of hypoxia due to rich nutrient discharges arriving from the Atchfalaya and Mississippi River that cross Louisiana State in the United States. Levels of hypoxia decrease in October and continue to do so throughout most of the winter until the warmer season where hypoxic regions expand over most of the summer (NOAA, 2015).

 Figure 1: 2015 map area of Gulf of Mexico Dead Zone - NOAA 2015

Recently, in August 2015, the National Oceanic and AtmosphericAdministration (NOAA, 2015) reported the dead zone to be ‘above average size’ due to high precipitation in June 2015 and increasing nutrient presence in Louisiana Rivers.  Trends have shown that the GoM hypoxic zones have slowly been increasing over the past 180 years covering most northern waters of the Gulf (Osterman et al, 2005). Through four sediment cores extracted from Louisiana shelf, Osterman et al (2005), recorded increasingly lower oxygen levels over a time span of 180 years.  Indeed, hypoxic periods were measured according to the abundance of three benthic foraminifers species, here called PEB (Pseudomonion atlanticum, Epistominella vitrea and Buliminella morgana), that live in nutrient rich habitats with low oxygen levels such as dead zones. According to the plots, PEB percentages start increasing in the 1950s and peak at the beginning of the 21^st century. 

Figure 2: Plots of PEB percentage and trends over the course of 180 years - (Osterman et al, 2005)

Turner et al (2003) attributes these changes in PEB numbers partially to natural factors but essentially to human induced factors such as land clearing for agriculture.

Indeed, The National Science and Council Committee on Environmentand Natural Resources 2000 assessment on hypoxia in the Northern Gulf of Mexico reported that landscape alteration for agriculture, manifested through deforestation were causing greater numbers of nutrients from entering aquatic environments. Harmful nutrients are no longer filtered by soil due to the lack of plant coverage and soil destruction caused by deforestation (NSCCENR, 2000). The report places agricultural fertilisers and particularly nitrogen (fertilisers composed mainly of nitrogen)  as the main factor contributing to the eutrophication of GoM waters. Both the Mississippi and Atchfalaya Rivers, collect runoff from Midwestern farmer’s fertilizing practices, that end up in the GoM and heavily impacts unique species in the region (NSCCENR, 2000). Some species are more affected than others. For instance, in extreme hypoxia cases, longer living species died with low levels of oxygen and shorter living species tend to survive and adapt to conditions (NSCCENR, 2000). The Gulf of Mexico has experienced biodiversity imbalances with increased numbers of more resilient species such as jellyfish (OECD, 2010). Furthermore, Eby et al (2004) explain that some surviving species try to find refuge in more highy oxygenated areas by traveling out of the hypoxia zones but often leads to overcrowding and density dependent grow reductions.