Nutrient conversion in the marine environment
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Although there is great uncertainty about the estimates of nutrient conversion in the marine environment, it is widely believed that the underlying biogeochemical processes largely take place in the sediment of the continental shelf. This article provides a brief introduction to the processes involved in the conversion of the main nutrients: nitrogen, phosphorus and silicon.
Nutrients in coastal environments come from various sources - from rivers, atmospheric deposition, groundwater and from in situ biological fixation, see also What causes eutrophication? Rivers are the main pathway of nutrients to the coastal oceans. Riverine inputs of N (nitrogen, in the form of nitrate NO3- and ammonia NH3) and P (phosphorus, in the form of orthophosphate PO43-) have doubled in the period 1960-1990. This is especially due the strongly increased use of synthetically produced fertilizers in agriculture. Other inputs include point-source discharges of wastewater from urban sewer networks and industrial wastes. Around 2010, the discharge of N to the coastal waters was estimated to be about 50 Tg N/yr  and the discharge of reactive (available for uptake) P probably close to 5 Tg P/yr (1 Tg = 1 billion kg). Groundwater discharge of nutrients is much smaller, not more than a few percent of the riverine discharge. Atmospheric deposition contributes about 8 Tg N/yr to the coastal waters and about 45 Tg/yr to the global ocean. However, atmospheric deposition differs between regions. For instance, atmospheric deposition amounts to 30% of the total land-based nitrogen input to the North Sea, mainly as oxidized N, and 50% to the Baltic Sea . The N:Si:P ratio for wet deposition in the North Sea is 503:2:1. Riverine Si fluxes, originating predominantly from weathering, have generally been little altered by human activity. However, human management of rivers has, in some cases, altered the Si fluxes extensively, often leading to a reduction in diatom blooms as a result of damming.
In addition to imports from land, certain cyanobacteria convert N2 into ammonia (NH4+) in the sea. This N2 fixation process (diazotrophy) is catalyzed by the enzyme nitrogenase. The amount of nutrient N that through this process becomes available in the global ocean is estimated to be 106-120 N Tg/yr. In low-oxygen regions of the deep sea, N2 is also fixed through another process, in which certain aphotic (not requiring light) bacteria and archaea play a role. Although the fixation rate is very low, due to the enormous ocean volume this process can still make a major contribution to the oceanic stock of nutrient N. Estimates range from 13 to 134 Tg N / yr .
Nutrient cycling refers to the uptake of nutrients by phytoplankton and the subsequent release of these nutrients during respiration and mineralization by bacteria, making these nutrients available again for uptake, see Algal bloom dynamics. Coastal shelf seas are zones of intense nutrient cycling which enhances primary productivity. It is estimated that shelf seas account for up to 80% of global benthic mineralization, despite covering only 7% of the seafloor . Most mineralization takes place when detrital plankton settles to the bottom, either under aerobic or under anaerobic conditions. In coarse grained sediments, organic matter is rapidly mineralized, because of the high availability of oxygen that penetrates deep into the sediment. The oxygen consumption is therefore high and the stock of organic carbon low. Mineralization is enhanced in the presence of bioturbating benthic macrofauna(Fig. 1). In fine-grained cohesive sediments, oxygen hardly penetrates the sediment bed; anoxic mineralization is relatively more important and the organic carbon content is larger. There is relatively less deposition of organic material on the seabed in deeper parts of the continental shelf and the ocean because decomposition of detrital organic material (phytoplankton, zooplankton, …) also takes place in the water column.
N species in aquatic environments include dissolved (nitrate, nitrite, ammonium, organic N) and particulate (organic N) constituents. The removal of N occurs by deposition and permanent burial in sediments and, most importantly, by release to the atmosphere as a result denitrification coupled with organic matter decomposition. Denitrification is the conversion of nitrate into quasi-inert gases N2 and NO2, by microbial oxidation of organic matter. It is a heterotrophic process that takes place at low oxygen concentrations of the order of 0.2 mg O2 /l or less. Bacteria capable of denitrification are ubiquitous; the determining factors are therefore the conditions of nitrate or nitrite availability, low oxygen concentrations, and sufficient organic matter. Before denitrification can take place, ammonia must first be converted into nitrite and then nitrate. The nitrite step can be skipped if comammox bacteria are present that convert ammonia directly into nitrate. Nitrification is a process of biological oxidation performed by autotrophic bacteria and archaea that require oxygen. Therefore, simultaneous nitrification and denitrification can only take place in special environments. Muddy shelf-sea sediments are a special environment that lends itself very well to the nitrification and denitrification steps. Nitrate formed by nitrification in the aerobic surface sediment layer diffuses into the deeper suboxic layer where denitrification occurs. Biological activity of macrofauna can considerably extent the vertical area extent of the oxic/suboxic interface and therefore increase the amount of denitrification. Globally, most denitrification is believed to take place in shelf-sea sediments. The amount of N denitrified in shelf-sea sediments is estimated to be about 200 N Tg / yr. Denitrification also takes place in other ocean regions, especially the hypoxic zones in the Eastern Tropical North Pacific, the Eastern Tropical South Pacific and the Arabian Sea (see Possible consequences of eutrophication). Here it is estimated that about 50 N Tg / yr nitrate is denitrified, which is supplied to these regions via ocean currents. Denitrification can also occur in estuaries. Significant denitrification has been observed where a strong estuarine turbidity maximum is present. It is surmised that aerobic degradation of suspended organic particles creates hypoxic microniches for the growth of denitrifying bacteria. However, most denitrification takes place in shelf sea sediments and hypoxic ocean regions.
P species discharged by rivers into the coastal ozone include dissolved constituents (inorganic DIP, organic DOP) and particulate constituents (inorganic PIP, organic POP). Most of the non-reactive PIP (not available for biologic up take, such as apatite) is deposited on the continental shelf and does not reach the ocean. Much of the phosphate discharge is adsorbed onto clay particles with iron-manganese oxide / oxyhydroxides on their surface. It is trapped in estuaries, but the phosphate is released into the sea under conditions of increased salinity. It has been estimated that the total load of P which desorbs from clay particles is 2-5 times more than the dissolved phosphate load which enters the ocean via rivers. Part of the non-reactive P is released into sediment pore waters during diagenesis and the subsequent flux into bottom waters may also contribute to the oceanic P cycle. The dissolved inorganic phosphorus (usually as orthophosphate PO43-) is transformed into DOP when assimilated by phytoplankton and returns to the P pool after mineralization of planktonic detritus or detritus of higher trophic organisms. In the oceans, most of the DOP is mineralized in the surface layer. The DOP that sinks to deep ocean layers persists several thousand years. The amount of (mostly reactive) P buried in the ocean marine sediments is estimated in the order of 3-10 Tg / yr.
Some removal of P can occur through bacterial reduction of phosphate to gaseous phosphine. However, little is known on the rate of phosphate-phosphine transformation and its contribution to overall P cycling .
Relevant Si species in the aquatic environments include dissolved Si (DSi), mainly as undissociated monomeric silicic acid, Si(OH)4, and particulate Si (biogenic silica, BSiO2), which includes the amorphous silica in both living biomass and biogenic detritus in surface waters, soils and sediments. The main transformation processes are the uptake of DSi and the biomineralisation as BSiO2 in plants and organisms, as well as the dissolution of BSiO2 back to DSi. Over sufficiently long time scales, BSiO2 may undergo significant chemical and mineralogical changes , even including a complete diagenetic transformation of the opaline silica into alumino-silicate minerals .
The major producers of BSiO2 in marine environments are diatoms. However, other organisms such as radiolarians, sponges and chrysophytes may be important local sources of BSiO2 . Large quantities of DSi are also fixed on land by higher plants, forming amorphous silica deposits, known as phytoliths . Their role in the Si cycle has only recently been studied  . In general, riverine Si fluxes have been much less altered by human activity than those of N and P. However, increased damming of major rivers has promoted siliceous phytoplankton blooms , and therefore, reduced Si fluxes to the coastal zone. For example, the damming of the Danube has reduced the DSi concentration by more than 50%.
- What causes eutrophication?
- Possible consequences of eutrophication
- Algal bloom dynamics
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