Although fertilisers have revolutionized agriculture since the 19th century, its use for animal agriculture and growing feed has major implication on our environment, today. Fertilisers, both chemical or natural, are rich in nutrient, and particularly high in nitrogen and phosporus, that help increase crop yields. However, plants only intake less than 20% of nitrogen and phosphorus (Dybas 2005), the rest is washed away by rain and ends up directly in freshwater and groundwater ecosystems, riparian environments and oceans (See table). Nutrient encourage the growth of aquatic plants such as phytoplankton, just as they do on land. The lake is thus exposed to a process of eutrophication (excessive numbers of nutrient in an aquatic environment) and causes oxygen depletion a.k.a. hypoxia in which aquatic organisms are unable to survive due to the low oxygen concentrations. The remaining bodies of water are called dead zone.
Cattle manure is rich in nitrogen, phosphorus and potassium, which represent the major causes of eutrophication. One peer-reviewed study argued that Industrialised AnimalProduction were major sources of nutrients and therefore were contributing to the eutrophication of some environments in the United States (Mallin et al, 2003). Focusing onConcentrated Animal Feeding Operations (CAFOs), which represent big companies for intensive meat production, Mallin et al (2013) argue that the high concentration of CAFOs puts pressure on regional environment due to major imbalance in waste production and the capacity to effectively manage this waste. This ‘mismanaged’ waste is left to spread on fields and enters our environment through a process groundwater infiltration and overland flow(Edwards et al, 1992). In consequence, some of North Carolina’s major lakes have experienced large microbial contaminations and the presence of algal bloom that have caused major fish kills. These surface runoff not only affect large lakes and rivers but also heavily impact smaller temporal water bodies such as vernal pools, that are particularly important for containing endemic plant species but are often used for cattle grazing due it promoting the growth of native plant species (Brudvig et al, 2007). A recent 2011 study(Croel et al, 2011) showed that, though cattle grazing nearby vernal pools might increase some plant diversity on land, cattle manure was influencing vernal pool water quality. The presence of nutrient rich materials in pools, have caused increasing growth of algal blooms which have been detrimental for the already endangered plant communities that live amongst vernal environments (Croel et al, 2011).
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 theGulf 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: the case of a marine dead zone 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).
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 June2015 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 fromLouisiana shelf, Osterman et al (2005), recorded increasingly lower oxygen levels over a timespan 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 21st century.
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 Environment and Natural Resources2000 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 run off 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 highly oxygenated areas by traveling out of the hypoxia zones but often leads to overcrowding and density dependent grow reductions.
