Running Head: MARINE FOOD WEB AND NITRIFICATION
Marine Food Web and Nitrification
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Marine Food Web and Nitrification
According to Pomeroy (2001), the food web is one of the earliest, as well as essential
concepts in ecology. The article attempts to find the food webs, especially from the microorganisms from the ocean. Nonetheless, there have been attempts in the past to study the
significance of micro-organisms in the sea by Lohmann (1911) whereby during this period
microbiology had no technology to detail bacteria or to estimate their production in the ocean.
There has been no clear way to deal with the real complexity issue in food webs of real systems.
In the 20th century, micro-organisms were believed to be vital in regenerating nitrogen, as well as
phosphorous; however, not to be significant components of the flux of carbon based in the
marine food web. Therefore, the paper will critically examine the ocean food, specifically on
nitrification by the organisms living in the ocean (Pomeroy, 2001).
Biomass is a very important element in food webs among the microorganisms where it is
the main fraction of the total biomass, and very small organisms have a very huge ration of
production and respiration to biomass. Bacterial biomass in the ocean differs much less than the
biomass of phytoplankton and other microorganisms, and bacteria tend to be leading biomass in
enormously oligotrophic ecosystems, like the central ocean gyres (Pomeroy, 2001). Furthermore,
it is clear from the article that in majority of impoverished central parts of the ocean, a microbialloop dominates influx energy and biomass. All communities are present, comprising large fishes
and cetaceans, although the latter are rare.
Smith et al (2014) nitrification, which is the microbial oxidation of ammonium to nitrate,
is a very important part of the nitrogen cycle. The article further points out that in the ocean’s
surface layer, the processes changes the distribution of inorganic nitrogen species accessible to
phytoplankton , as well as produces nitrogen oxide. Based on the broader held concept of
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oceanographers is that the process of nitrification is inhibited by light in the ocean. These
sentiments were echoed by (Pomeroy (2001). Nonetheless, this thinking has been challenged by
the latest evidence that the primary organisms taking part in nitrification, the ammonia-oxidizing
archaea (AOA), are present, as well as active all through the surface ocean. Thus, the amount of
nitrogen that is supplied to the sunlit layers in the ocean controls the levels of primary
production, as well as phytoplankton population. This is because nitrate (NO3-) that enters the
photic region from deeper layers functions as an extra source of nitrogen required for “new”
primary production (Smith et al ,2014).
According to Yool et al (2007), the influx of material that sinks to the bottom of the
ocean is a leading control on the inventory of carbon present in the ocean. This implies the ocean
state of equilibrium demands that what goes down into the ocean should come up. As a result of
the difficulty in measuring the export of flux directly, it is normally projected indirectly through
quantifying the quantity of phytoplankton growth, or primary production, which is fuelled by the
upward flux of nitrate in the food web. Nonetheless, the production of nitrate through
nitrification process in ocean surface has recently received attention from researchers. The
authors say that the ocean is the Earth’s biggest active reservoir of carbon and at the same time a
leading sink for anthropogenic carbon dioxide. This is primarily due to solubility of CO2, as well
as the volume of ocean; however, also due to “biological pump” (Yool et al, 2007).
In addition, the nitrogen cycle potentially provided a method that will be used to separate
production driven by recycled nutrient from that dominant to export. Yool et al (2007) argue that
the inorganic nitrogen pool, which supports production takes place primarily as two nutrients,
nitrogen and ammonium, which are formed through distinctive pathways. Thus, when nitrogen is
metabolized via the food web, it returns to the dissolved pool in form of ammonium. This is
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because nitrogen in ocean waters will be generated via the oxidation of ammonium, which is a
biologically mediated process of nitrification. Like in Pomeroy (2001), the authors believe that
nitrification in oceans mainly takes place at depth, perhaps due to inhibition of light. Thus, the
presumed depth separation of the pathways utilized to define “new”, as well as “regenerated”
primary production: whereby the former is propelled by nitrate, which reaches surface waters of
the ocean principally from depth, through vertical transport; and the later is driven by ammonium
from recycling in situ in the euphotic region.
Furthermore, recent studies has supported the article by Pomeroy whereby in regions
whereby most primary production is driven by nitrate, like the mesotrophic high altitude, have
greater f-ratio as compared to regions, like oligotrophic subtropical gyres, whereby most of the
production is basically the product of recycled nutrient. Nonetheless, despite the advancement of
techniques and technologies used to measure nitrification in the oceans, nitrification remains
complex to measure, and directs observations for the open ocean are still rare.
Furthermore, studies by Beman et al (2011) indicate that through ocean acidification
through the dissolution of anthropogenic CO2 emissions in ocean water has noteworthy outcomes
for marine biogeochemistry, as well as ecology. It has been estimated that the oceans nearly
absorbs one-third of carbon dioxide emissions in the last two centuries. This changes the ocean
chemistry, lowering sea water PH, and impacts marine organisms and phytoplankton in many
ways. Thus, microbially mediated ocean biogeochemistry processes will be essential in
ascertaining how the earth system reacts to the environmental change; nevertheless, they way
they might be changed by ocean acidification are greatly uncertain. Therefore, microbial
nitrification rates in oceans declined in every instance when PH was practically reduced (Beman
et al ,2011).
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In the Pomeroy article, it was clear that during that period, there was no definite approach
used to quantify nitrification in oceans. At the present, there are several ways of measuring N2O
in the ocean. One of the methods that have been used of late is bottom-up approaches that
quantify the isotopic signatures, as well as size of individual N2O sources in the ocean waters.
The method of Freing et al (2009) pushed this half of the field through utilizing transiet chemical
tracers to partner N2O concentration measurements to water mass age distributions, permitting
them to calculate subsurface N2O production rates, as well as oxygen consumption rates
separately. Nevertheless, this approach can only be used below the depths, which are direct
exchange with the atmosphere and it does not offer mechanistic data concerning N2O formation.
This means that there is need to develop other methods that will measure N2O fluxes due to
nitrification in the top of thermocline or within the euphotic region (Freing et al, 2009).
Furthermore, top-down approaches to determining, as well as measuring N2O sources in
ocean waters are too gaining attention among many researchers around the world. Therefore, the
measuring techniques that utilize atmospheric models and constant measurements of atmospheric
N2O concentrations, especially in oceans were initially meant to estimate the regional
anthropogenic N2O emissions; nevertheless, these are at the present being used to recognize
seasonal, as well as inter yearly patterns in marine N2O fluxes. As these techniques are refined
over time, they will be able to isolate small seasonal changes in biological fluxes from larger,
physically fuelled fluxes, like air-sea gas exchange (Nevison et al, 2007). Isotopic measurements
at the moment being added to these studies might offer extra data on source identities and sizes.
In the geological precedent, atmospheric N2O concentrations changed speedily in glacialinterglacial transitions. Thus, human perturbation of the nitrogen cycle has moved the earth into
another period of quick N2O increase. Therefore, comprehending the fundamental
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biogeochemistry of this rise will permit significant projects of the effects of climate change on
future atmospheric N2O concentrations.
Finally, it is evident from the various studies that the ideas of Pomeroy in his article have
been expanded greatly. Nitrification, particularly in oceans is the process, which changes the
ammonium into nitrate and therefore, links the regeneration of organic nitrogen to fixed nitrogen
loss through denitrification process. Additionally, nitrification is important step in nitrogen cycle
in ocean towards sustaining the food web. Whilst nitrification does not lead to direct change in
fixed nitrogen inventory, it is a vital link between organic nitrogen and the ultimate loss the
system as nitrogen through nitrification. However, the article recommends the need for
researchers to further research the topic in order to eliminate the gap that exists in literature.
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References
Beman, J. Michael, Chow, Cheryl-Emiliane, King, Andrew L., Feng, Yuanyuan, Fuhrman, Jed
A., Andersson, Andreas, Bates, Nicholas R., Popp, Brian and Hutchins, David A.
(2011).Global declines in oceanic nitrification rates as a consequence of ocean
acidification. Proceedings of the National Academy of Sciences, 108, (1), 208-213.
Freing, A., Wallace, D. W. R., Tanhua, T., Walter, S., and Bange, H. W. (2009). North Atlantic
production of nitrous oxide in the context of changing atmospheric levels. Global
Biogeochemical Cycles, 23(GB4015).
Nevison, C. D., Mahowald, N. M., Weiss, R. F., and Prinn, R. G. (2007). Interannual and
seasonal variability in atmospheric N2O. Global Biogeochemical Cycles, 21(GB3017).
Pomeroy, L.R. (2001). Caught in the food web: Complexity mad simple? SCI. MAR., 65
(Suppl.2): 31-40.
Smith JM, Chavez FP, and Francis C.A. (2014).Ammonium Uptake by Phytoplankton Regulates
Nitrification in the Sunlit Ocean. PLoS ONE 9(9): e108173.
Ward.B.(2011). Measurement and distribution of nitrification rates in the oceans. Methods
Enzymol. 2011;486:307-23.
Yool, A., Martin, A., Fernandez, C., and Clark, D. (2007). The significance of nitrification for
oceanic new production. Nature, 447:999-1002.