During the last 50 years, oxygen minimum zones (OMZs) have been shown to be expanding both vertically and horizontally (Stramma et al., 2008, Fuenzalida et al., 2009). Although there is no exact threshold at which an OMZ can be defined, regions of the ocean where the intermediate depth level waters have an approximate dissolved oxygen concentration of less than 20µM of O2 can be classified as some of the most intense OMZs (Paulmier and Ruiz-Pino, 2009). These areas of the ocean are essential for the balance of the global carbon and nitrogen cycles via marine biogeochemical processes (Stramma et al., 2008, Paulmier and Ruiz-Pino, 2009). This balance is ultimately responsible for the maintenance of biological ecosystems, therefore despite the OMZs only occupying 0.1% of the global ocean, even the smallest increase in this percentage could result in dire consequences for both the planets biogeochemical cycles and the biological life inhabiting it. (Ulloa and Pantoja., 2009). The focus of this essay, (concerning the biological consequences of the expansion of the zones), will be the adaption of species and community structure in response to the low oxygen conditions (Levin et al., 2000, Levin, 2003), the extinction of species that cannot adapt quick enough to cope with the new environment (Diaz and Rosenburg, 2008) and the blockade of benthos migration patterns due to the oxygen deprived zone (Quiñones et al., 2006). The other aspect of this essay will concentrate on the major biogeochemical consequence of the expansion, which is in effect the of the amount of fixed nitrogen lost from the global oceans as a result of the processes of denitrification and anammox occurring within the OMZs (Ward et al., 2009, Deutsch et al., 2007, Kuypers et al., 2003), and how this goes onto to affect the carbon cycle (Falkowski, 1997), and subsequently their impact on climate change (Paulmier et al., 2008).
What are Oxygen Minimum Zones?
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Within the oceans, the majority of oxygen-rich waters remain near the surface of the water as it continually exchanges with the atmosphere. A substantial amount of the biological life, especially that which requires oxygen for respiration, therefore, stays near the surface (Fuenzalida et al., 2009). Even so, intermediate depth level waters below, also contain biological life as they normally have oxygen transported to them via the cold and denser surface waters. The denser waters are due to the oxygen dissolved within them and this thereby facilitates the descent of oxygen into the waters below (Coghlan, 2008). However, as you descend into the depths of some areas of the oceans, a rapid reduction in the amount of dissolved oxygen in the water results; this therefore creates an area within the waters which is uninhabitable by organisms only capable of aerobic respiration (Diaz and Rosenburg, 2008). This effect is most substantial in the core of these regions and these regions of water (occurring at depths of roughly 200 to 1000metres), where the oxygen saturation is the lowest, are called oxygen minimum zones (OMZs) (Diaz and Rosenburg, 2008). The water beneath the OMZ, thereby of depths greater than 1000metres, is not oxygen deprived and retains its normal dissolved oxygen concentrations.
Where and why do OMZs occur?
The distribution and degree to which OMZ waters are oxygen deprived vary around the globe. This is because the intensity of the OMZs is largely dependent upon changes in the climate, and natural or anthropogenic fertilisation that is occurring in the area (Stramma et al., 2008, Diaz and Rosenburg, 2009). Paulmier and Ruiz-Pino have identified some of the largest and most prominent OMZs in the world in the Eastern Tropical/Subtropical North Pacific Ocean, the Eastern South Pacific Ocean, the Arabian Sea and the Bay of Bengal (Paulmier and Ruiz-Pino, 2009).
As sea temperatures rise in response to global warming, primarily, as a result of increased carbon emissions caused by excessive burning of fossil fuels, the more rapidly these oxygen-deprived regions of water will expand. This is due to the warmer surface waters holding less oxygen and being less dense; thus preventing oxygen being carried to the deeper waters below (Coghlan, 2008). It may even be for this reason that the three major OMZs, in the Eastern Tropical South Pacific Ocean, the Eastern Tropical North Pacific Ocean and the Arabian Sea (Dalsgaard et al., 2006), are located in equatorial waters as the water is warmer (Levin, 2003). It may also be that these are the most severe OMZs due to the age of the deeper layers of these oceans. The older waters would mean that the OMZs are more established, and would therefore have been depleting oxygen from the waters over a longer duration of time.
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Human activities which relate to agricultural methods can also lead to the creation of OMZs as they often result in nutrient enrichment of the waters via run off from sewage or changes in wind patterns. This causes increased eutrophication by fertilisation of marine ecosystems, which goes on to affect the global nitrogen cycle. However, this effect is more profound in coastal waters such as that of the Bay of Bengal, the Black Sea, and the Gulf of Mexico (Diaz and Rosenburg, 2009). This nutrient enrichment can also occur naturally and ultimately creates waters where the dissolved oxygen concentrations fall to such a low level that the water becomes hypoxic due to increased oxygen consumption that is caused by increased primary production. Hypoxic is the term used to define the water in the occurrence of the oxygen concentrations dropping below approximately 60-120 µmol kg-1. When the waters have become completely depleted of oxygen, they are referred to as anoxic (Stramma et al., 2008).
What happens if the OMZs expand?
Even the smallest expansion of the OMZs, therefore, has immense consequences on both marine ecosystems and the biogeochemical processes of the global nitrogen and carbon cycles (Stramma et al., 2008, Ulloa and Pantoja, 2009). The two major consequences are, however, intertwined as they ultimately affect each other to almost form an ongoing cycle in itself. The carbon and nitrogen cycles maintain the marine ecosystem, so the marine ecosystem and its inhabitants can live and thrive, which ultimately keep the biogeochemical cycles going through the various processes the organisms use to facilitate life.
Biological Consequences of OMZ Expansion
OMZs play a key influential role in several aspects concerning the marine life that inhabits them. The main limitation of primary production within the oceans is due to insufficient nutrients, which sometimes creates areas of water called nitrate deficit maximum zones (NMZs), and these are common occurrences in OMZs (Paulmier and Ruiz-Pino, 2009). As populations of marine organisms increase, consumption of the nutrients increases, and when this positive feedback loop reaches a threshold as the nutrients become limiting, the greatly increased population can longer be sustained.
This results in a population decrease as the organisms die, and the metabolic processes carried out by certain marine organisms, namely bacteria, act to remove utilisable forms of the nutrients. As these bacteria carry out these metabolic processes, they remineralise the dead organisms and restore the nutrients to the water in a useable form. They do, however, consume oxygen in the process and thereby enhance the oxygen deficit in the OMZs (Stramma et al., 2010, Ulloa and Pantoja., 2009). Nevertheless, despite marine organisms imposing a key limitation on primary production, they are still also responsible for around half of the earth's primary production, which shows just how important marine organisms in OMZs are, in terms of the productivity of the world's oceans (Arrigo, 2005).
These organisms, therefore, also play a crucial role in the global cycling of nutrients. Their importance in the nutrient cycle was first discovered by Alfred Redfield, who noticed that the stoichiometric composition of the plankton inhabiting the waters was almost equivalent to the waters major dissolved nutrients. He observed a ratio of approximately 16:1 of nitrate to phosphate both in the phytoplankton and in the ocean; however this has been expanded upon to include carbon. The magnitude of the ratio of carbon, nitrogen and phosphorous is 106:16:1 respectively (Arrigo, 2005). Therefore the occurrence of a lack of even one of these nutrients, which is very likely to occur in the event of an expansion of OMZs, will result in a profound effect on plankton population.
Effects on Microbial Organisms
The smallest types of organism living within the OMZs are the bacterioplankton. These microorganisms are very diverse and include bacteria and protists. As mentioned before, removal of fixed nitrogen is one of the factors limiting primary production. This is because it is only the fixed form of nitrogen which serves as the readily useable nutrient for a considerable amount of the marine life living in the ocean. It removal occurs via the denitrification process in the nitrogen cycle and this process was once thought to be the main loss term for fixed nitrogen availability within the oceans (Ward et al., 2009, Francis et al., 2007). Autotrophic denitrifying bacteria, such as Thiobacillus denitrificans, are responsible for carrying out this process and they are mainly located in the core of OMZs, where the oxygen concentrations are the lowest (Paulmier and Ruiz-Pino, 2009). An expansion of OMZs will result in more low-oxygen regions where denitrifying bacteria can thrive and ultimately lead to an increased amount of fixed nitrogen loss from the oceans.
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Although denitrification was considered to be the key factor in the loss of nitrogen from the oceans, Kuypers et al have provided evidence for the removal of fixed nitrogen via another process called the anammox reaction. This process involves the anaerobic oxidation of ammonium (NH4+) using nitrite (NO2-) as the electron carrier, to dinitrogen gas (N2) and it is carried out by certain bacteria (Kuypers et al., 2003, Francis et al., 2007). The special group of bacteria attributed to this process are biologically classified under the Plancomycytes phylum. It has been suggested by Galan et al that the anammox bacterial community is one of the most diverse systems within OMZs as they found several different members of the anammox group within the Eastern Tropical South Pacific OMZ (Galan et al., 2009). If the OMZs were to expand, an even greater diversity of the entire bacterial community within these zones could be anticipated, as they are capable of thriving and adapting at an enhanced rate to ensure their survival in the regions of water they inhabit.
Other bacterial organisms which have been found in significant numbers within OMZs are sulphide-oxidising bacteria, such as Thioploca and Beggiatoa (Levin, 2003). These types of bacteria are important in OMZs because they oxidise dissolved sulphide which has been produced by sulphide-reducing bacteria under aerobic conditions. The oxidation reaction thereby creates an oxygen-sink in the oceans which can eventually lead to complete anoxia within the waters and hence a positive feedback loop is generated, in which larger OMZs are created as more and more oxygen is consumed (Levin, 2003, Brüchert et al., 2003). The completely anoxic waters would be unable to support any organisms incapable of anaerobic respiration, causing mass mortality of these organisms and possibly even extinction of species which cannot evolve quickly enough to retain their place in the ecosystem.
Effects on whole organisms
Adaptational responses to the OMZ environment
The OMZs in the oceans are, however, also considered to be gene flow barriers as the oxygen-deprived waters have been shown to give rise to the development of specific adaptations within organisms in order to make them capable of exploiting the conditions of the environment surrounding them (Molina et al., 2007). Levin has stated that protozoan and metazoan assemblages seem to thrive in oxygen-depleted waters therefore their populations are generally elevated in the OMZs (Levin, 2003).
Some of the adaptations which the metazoans, in particular, have developed to survive in these conditions include small thin bodies and blood pigments such as haemoglobin, for greatened respiratory surface area and to enable them to breathe more efficiently in the poorly oxygenated waters. Another feature they have developed is an increased number of pyruvate oxidoreductases, which are enzymes that allow them to have pyruvate-dependent nitrogenase activity in order to facilitate biological nitrogen fixation (Levin, 2003). Expanding OMZs could thereby lead to a greater diversity of organisms which are more specifically adapted to hypoxic conditions, as OMZs can quite possibly influence genetic diversity, and may even affect the evolution of species at the depths of the oceans where OMZs occur (Wishner et al., 2000, Levin, 2003).
Fish mortality and disruption of ecosystems in OMZs
As the bacterial populations increase within the OMZs as they expand, due to a greater tolerance to the hypoxic waters, an abundant food supply from the sinking organic matter and as they are released from predation, more and more areas of the ocean become both nutrient and oxygen depleted (Levin, 2003, Cuevas and Morales, 2006). The resulting inhospitable deoxygenated waters would have a profound effect on the marine ecosystem in terms of both the environmental habitat, and the organisms that constitute the ecosystem. The severely reduced oxygen levels within the OMZs would give rise to fish mortality as the aerobic aquatic life is put under stress and cannot survive in or adapt quick enough to the hypoxic waters. This will lead to substantially reduced secondary production as predator-prey relationships collapse and thereby disrupt the energy flow of the ecosystem (Diaz and Rosenburg, 2008). Megafauna and macrofauna are therefore typically very rare or absent within the OMZs (Levin, 2003, Levin and Gage, 1998).
An expansion of the OMZs will therefore result in more resource depletion and create more areas of the oceans where microbial remineralisation of virtually all organic matter occurs.
Consequently, this will generate more regions of the ocean where little or no marine life can survive, which will diminish biodiversity within the waters and eventually lead to completely barren waters.
Blockage of fish migration patterns
OMZs can not only act as gene flow barriers, but can also act as physical barriers which block the patterns of fish migration. Blockage of fish migration patterns is another consequence that can occur as a result of the low oxygen and nutrient environment created within OMZs. In fish, migration patterns are generally associated with large-scale movement between areas which serve the purpose of feeding grounds and the areas in which they can breed most successfully. Some fish use habitat shifts and migration as an adaptation to avoid the low-oxygen regions (Wannamaker and Rice, 2000). However, it has also been found that some zooplankton species, such as the euphausiids migrate into OMZs cores via vertical diel migration. Antezana showed a vertical zonation of the euphausiids in relation to OMZs that corresponded to a number of factors that include, more physiological adaptations to the low-oxygen waters, and migration to areas where there is more food available and where they will incur less predation, such as in the OMZ core (Antezana, 2009). This shows that whilst the environment of the OMZ can be lethal to some species, it can also be a refuge to others. The OMZs thereby go on to further affect the dynamics of ecosystems as certain species will move in and out of their normal habitats in response to the OMZs expansion, which can even lead to either a missing or extra link in the food chain if the migration is permanent.
Biogeochemical Consequences of OMZ Expansion
Although the OMZs occupy approximately only 0.1% of the world's oceans, they are responsible for the loss of roughly 20-50% of oceanic nitrogen to the atmosphere (Lam et al., 2009). Even the slightest expansion of the OMZs will have an immense effect on the climate of our planet as the amount of nitrogen being removed from the oceans and released into the atmosphere increases substantially. Therefore, the most profound effect of OMZs on biogeochemical cycles is on the nitrogen cycle, however there are also four others are under potential threat of being thrown out of balance as a result of the processes occurring within the OMZs. For example, the carbon cycle has been thought to be affected as processes within the nitrogen cycle have been shown to play a role in carbon dioxide (CO2) sequestration (Falkowski, 1997), and the sulphur cycle has been shown to be affected by the of sulphide oxidising bacteria in OMZs (Brüchert et al., 2003, Paulmier and Ruiz-Pino, 2009). The other two biochemical cycles are the global oxygen and phosphorous cycles. The oxygen cycle is also affected as microbial species consume oxygen in the process of remineralisation (Arrigo, 2005). It is the phosphorous cycle which is least affected as although phosphate (PO43-) serves as a major algal nutrient in the oceans along with nitrate, nitrate is always depleted first (Deutsch et al., 2007).
Effect on nitrogen cycle
Inert diatomic nitrogen (N2) is the most abundant gas in the earth's atmosphere; however nitrogen can exist in several oxidation states depending on the chemical species in question. Since many organisms do not have the capability to utilize the inert N2, some species have evolved to convert the inert form into more easily exploitable nitrogen forms, such as ammonia (NH3) via nitrogen fixation, which can then be oxidised to the other useable form, nitrate (NO3-), via nitrification (Francis et al., 2007).
However, one of the final steps that occurred in the evolution of the nitrogen cycle involved denitrification, and it is this step which is the most important when concerning the impact of expanding OMZs on the climate.
Denitrification in the OMZs involves the removal of fixed nitrogen by the conversion of the utilisable nitrate (NO3-) to the gaseous N2, via a series of intermediates; nitrite (NO2-), nitric oxide (NO), and the greenhouse gas nitrous oxide (N2O) (Ward et al., 2009). Although denitrification was thought to be the major microbial process that acts as a means of global fixed nitrogen loss in OMZs, Kuypers et al provided evidence for the removal of fixed nitrogen via the anammox reaction, which involves the anaerobic oxidation of ammonium by nitrite to dinitrogen gas (Kuypers et al., 2003, Francis et al., 2007). Expanding OMZs will thus further increase the oceanic nitrate deficit as more and more nitrite and nitrous oxide is released into the atmosphere as a result of this process. As OMZs have been shown an intense source of the greenhouse gas, N2O, via their effect on the global nitrogen cycle, the expansion of these oxygen-deficient zones in the oceans will amplify the current effect on they have on the climate, therein fuelling the rate of warming of our planet (Paulmier et al., 2008).
However, the N2O by product of denitrification not only adds to global warming because it is a greenhouse gas, but also because it decomposes in the stratosphere to form N2, NO and nitrogen dioxide (NO2) (Codispoti, 2010). The nitric oxide reacts with the ozone (O3) to produce oxygen and NO2 as a by product. The NO2 can then go on to further react with the ozone. During the initial reaction with NO, the NO supplies the free oxygen radicals that act as the catalysts which cause the destruction of the highest levels of the ozone layer, the stratosphere, by replacing it with oxygen. The ozone is produced as a result of a reaction between oxygen and light, and its main role is to reduce ultraviolet (UV) light coming from the sun (Prather, 2007). Expanding OMZs will thereby enhance its destruction which will mean that more UV light can extend to the surface of the earth and ultimately have damaging effects on living organisms and climate due to the extra radiation (Codispoti, 2010, Pratt, 1977).
Effects on the carbon cycle
It is the global ratio of N2 fixation and denitrification in the nitrogen cycle that is related to the sequestration of CO2 (Falkowski, 1997). The important role the oceans play in this effect when concerning carbon dioxide takes place as they act to absorb carbon during the photosynthetic pathway used by the phytoplankton in the water (Paulmier at al., 2008). Whilst the majority of the gas diffuses back into the atmosphere, some of it is transported down into the depths of the ocean via the sinking of dead organic matter. However as the organic matter decays CO2 is released into the water, after which ocean currents eventually return it to the surface (Ward et al., 2008). This ties in with the nitrogen cycle as the overall nutrient concentration in the oceans has a net effect on the carbon cycle. However, since it is denitrification that is the predominant process that occurs in OMZs in relation to N2 fixation, they sequester less CO2 and are therefore intense sources of CO2. This on a global scale has important implications on the warming of the planet as CO2 is also an extremely potent greenhouse gas (Falkowski, 1997, Paulmier et al., 2008).
It is even believed that the nitrogen cycle has evolved for this sequestration of CO2 (Falkowski, 1997). Phytoplankton in the euphotic layer of the ocean are mainly associated with the N2 fixation process occurring in the upper boundary of the OMZs. When the nitrate formed as a result of this process, and other nutrients such as phosphate, become depleted, the organisms can no longer survive and sink as dead organic matter to the depths of the ocean (Capone and Knapp, 2007). Since the zooplankton and bacterioplankton occur at depths where no primary production can take place, they survive by consuming the more dense organic material which sinks from the surface waters above (Rosenburg et al., 1983, Deutsch et al., 2007). As the organic matter sinks, it decomposes and is converted into the biologically important elements of inorganic carbon, nitrogen, and phosphorus, according to the Redfield Ratio (106:16:1) (Arrigo, 2005).
Whilst this process of remineralisation transports carbon down to the sediments, nitrogen and phosphorous are utilised as nutrients. This is how carbon absorbed from the atmosphere by the phytoplankton in the upper layer of the water, and is deposited down to the sediments in association with the nitrogen cycle.
Expansion of the OMZs will lead to more regions of nutrient deprived waters which will therefore, not be able to sustain life. This means that there will be a lot less phytoplankton capable of absorbing CO2 from atmosphere in the oceans, thereby increasing global warming as CO2 levels continue to rise and the resultant change in temperature that occurs in response to the increased CO2 levels, will further shift food webs and nutrient limitation will cause primary production to cease (O'Connor et al., 2009).This is therefore an example which serves the purpose of showing just how inter-linked global biogeochemical cycles are. It thereby implies that an expansion of the oceans OMZs could have colossal effects on the balance of all biogeochemical cycles as the nitrogen cycle is thrown out of sync and has a knock on effect on the oceans role as a carbon sink.
Effect on the sulphur cycle
In addition, it is not only the carbon and nitrogen cycles that affect the climate in association with expanding OMZs as the sulphur cycle also plays a role. The oceans also represent a major sulphur reservoir and the less stable forms of sulphur often occur in the low oxygen conditions of OMZs. These are mainly released from the surface of the ocean into the atmosphere as dimethyl sulphide (DMS) by phytoplankton in the surface waters of the ocean, and as H2S by sulphide oxidising bacteria in the OMZs. When the DMS enters the atmosphere it is oxidised to acidic aerosol particles which can ultimately affect the properties of clouds and their role in the warming of the planet via solar radiation (Sievert et al., 2007, Shaw et al., 1998). Similarly, when the H2S enter the atmosphere, it undergoes a reaction with the ozone in which it is oxidised to sulphuric acid and the sulphuric acid is what causes acid rain (Janssen et al., 1999). Expansion of the oxygen deprived waters will, therefore, amplify the effect of the volatile forms of the sulphur in the atmosphere which can ultimately lead to more severe climate change and maybe even the alteration of biological habitats, as factors such as increased acid rain and OMZs can be associated with oceanic acidification (Paulmier and Ruiz-Pino, 2009).
The expansion of OMZs will have serious implications for the functioning of biological life and the ecosystems they inhabit and the biogeochemical cycles that the oceans play a key role in. In biological terms, the consequences of an expansion may be faster evolution rates and expanded distributions of smaller organisms which are adapted to low oxygen conditions. These adaptations can include a change in body size and shape, and a development of respiratory pigments and enzymes to enable them to obtain oxygen more efficiently in the low oxygen conditions. Another consequence is massive species extinction and diminished biodiversity of larger organisms which cannot adapt and survive in the new environment which ultimately leads to the collapse of predator-prey relationships within the entire ecosystem as microbial processes dominate the energy flows. Alterations of planktonic migration patterns both into and out of the OMZs are yet another consequence of the expansion. The plankton migrate in response to greater survival factors, such as a greater food supply and release from predation, within the OMZs, or in accordance with an avoidance of the low oxygen regions due to an inability to survive in them.
When concerning the biogeochemical consequences of OMZ expansion, the major penalty will be on the balance of the global nitrogen cycle as the smallest change in the size of oxygen-deprived waters will cause an enormous anomaly on the stability of the nitrogen being removed from the oceans and how these go on to affect the climate as they are released into the atmosphere.
Other biogeochemical cycles that would be affected by the expanding OMZs include the carbon cycle, as the sequestration of CO2 into the oceanic sediments is affected by the changes the nitrogen cycle incur as a result of the expanding zones, and the sulphur cycle as some of the marine life that the OMZs support release volatile gases such as H2S and DMS into the atmosphere. Many of the by products of the biological processes that occur within the OMZs regulate the biogeochemical cycles and eventually go on to affect climate change. This results in the continual warming of our planet, which has serious implications in itself.