Continental Nutrient Sources and Nutrient Transformation
Nutrients in coastal environments come from various sources - through rivers, groundwater or atmospheric deposition. The main species - Nitrogen, Phosphorus and Silicon - undergo different transformation processes and states.
Continental nutrient sources
On a global scale, riverine inputs of N and P to coastal seas have possibly increased by factors of 2 to 3  , ,. Agriculture, in the form of fertilizers, leachates and animal wastes, is the largest contributor of N and P in aquatic systems  . Other major inputs include point-source discharges of wastewater from urban sewer networks , and industrial wastes. The direct discharge of P exchanged with soils and sediments also contributes significantly to the budget of this element.
Riverine Si fluxes, originating predominantly from weathering, have generally been altered little 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.
The direct discharge of groundwater into the ocean, termed submarine groundwater discharge (SGD), has been recently recognized as an additional pathway of nutrients from the land to coastal waters,. On a global scale, SGD rates vary between 0.01-10 % of river runoff. However, the concentrations of nutrients in groundwater are typically higher than those in coastal and river waters,,,,. Therefore, in terms of fluxes, such high concentrations can compensate for the relatively low SGD rates. At the local scale, SGD of nutrients is a prominent transport pathway, particularly in enclosed bays, karstic and fractured systems (e.g., Hawaii), or at locations where rivers are small or non-existent (e.g., Yucatan peninsula).
Atmospheric deposition is a significant source of N compounds to the coastal zone, particularly in summer and autumn, but is only a minor source of Si and P,,. Nitrogen delivered by the atmospheric pathway can be either in the oxidized or reduced form . 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 .
Nutrients are significantly altered by biogeochemical processes during their transport along the land-ocean transition zone, especially in estuarine systems. Figure 1 summarizes the major N sources and transformation processes in an estuary. Estuaries are usually turbid, and hence primary production is often limited by light availability. Light conditions generally improve towards the coastal zone and primary production becomes a dominant process in controlling the biogeochemical cycles of nutrients.
Sediments cover most of the seabed and hence most of the earth. Recycling of carbon and nutrients within this habitat (both subtidally and intertidally) is critical both at small and large scales. The availability of essential nutrients, such as nitrogen and phosphorus, and metals is essential for life. Processes that aid nutrient cycling are crucial to ecosystem functioning, as this increases the availability of nutrients and thus maintains productivity of the system. For example, in the marine benthic environment, bioturbation by marine worms, mainly through burrowing in the sediment, moves nutrients from deep sediment layers to the surface and vice versa (Fig. 1). Nutrient cycling is also maintained through processes such as ingestion and excretion of materials by organisms e.g. fish mineralise nitrogen and phosphorus through excretion.
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, loss to the atmosphere by bacterial denitrification. This process is coupled with organic matter decomposition and reduces nitrate to gaseous N2/N2O under anoxic conditions. Part of the nitrate pool originates from coupled nitrification/denitrification, in which the ammonium produced from organic matter degradation is first oxidized to nitrate, and subsequently denitrified . In temperate and tropical estuaries the estimated loss of nitrate N via denitrification varies widely, and also varies in time and space within estuaries , . Because denitrification requires low oxygen concentrations, this process is particularly important in muddy sediments , . It is also quantitatively more important in ecosystems characterized by relatively long residence times . In groundwater systems, the nitrate supplied either by infiltrating water or produced through nitrification , is also commonly removed through denitrification. As in surface estuaries, a set of conditions, namely the presence of labile organic matter, a low redox potential and sufficient time for reaction, are prerequisite for effective denitrification to occur. However, field studies often report only limited nitrate removal prior to discharge to coastal waters primarily due to a lack of labile dissolved organic matter , ,, as is the case in many shallow groundwater aquifers or sandy nearshore sediments, or due to high groundwater velocities, .
P species in aquatic systems include dissolved (inorganic, organic P) and particulate (inorganic, organic P) constituents . The retention of P in the land-ocean transition zone is often attributed to adsorption on solid particles, which are constantly trapped in estuarine sediments , or forms part of the solid matrix in coastal aquifers. However, in the case of very large rivers that discharge directly in the continental shelf, P retention in the mixing zones between freshwater and seawater will be limited . Adsorption onto solids such as iron and aluminum oxides is particularly effective , , and thus may be also coupled to the redox conditions . For instance, removal of P is very efficient in subterranean estuaries characterized by zones of iron oxide accumulation, (“Iron Curtains”  , ). The behavior of P in estuarine systems is also influenced by the strong physico-chemical gradients, which result from the variations in pH, ionic strength and solution composition between the freshwater and seawater end-members (e.g.  , , ). The 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 ,  .
Tidal and marginal sediments are considered important sinks of N and P, although a quantitative estimation of their role remains uncertain ,.  ,.  . On the global scale, it is generally accepted that intertidal sediments are more efficient for P burial than for N  .
Relevant Si species in the aquatic environments include dissolved Si (DSi), mainly as undissociated monomeric silicic acid, Si(OH)4, and particulate Si (biogeneic 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 BSiO<sub<2 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%.
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