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 in the period 1960-1990  . 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. See also What causes eutrophication?.
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  , see the article Submarine groundwater discharge and its influence on the coastal environment. 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. 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 (marine-fresh groundwater interface zone) 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 (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%.
- Howarth, R., H. Jensen, R. Marino, and H. Postma, in Phosphorus in the Global Environment:Transfers, Cycles and Management, H. Tiessen, Ed., Scientific Committee on Problems of the Environment 54. (Wiley, New York, 1995), pp. 323–356.
- Duce, R., P.S. Liss, J.T. Merrill, E.L. Atlas, P. Buat-Menard, B.B. Hicks, J.M. Miller, J.M. Prospero, R. Arimoto, T.M. Church,. W. Ellis, J.N. Galloway, L. Hansen, T.D. Jickells, A.H. Knap, K.H. Reinhardt, B. Schneider, A. Soudine, J.J. Tokos, S. Tsunogai, R. Wollast, and M. Zhou (1991), The atmospheric input of trace species to the world ocean, Global Biogeochemical Cycles 5, 193-296.
- Jickells T.D. (1998), Nutrient Biogeochemistry of the Coastal Zone, Science, 281 217 – 222
- Billen, G., J. Garnier, J. Nemery, M. Sebilo, A. Sferratore, S. Barles, P. Benoit, and M. Benoit (2007), A long-term view of nutrient transfers through the Seine river continuum, Science of the Total Environment 375, 80-97.
- European Environment Agency (1999), Nutrients in European Ecosystems. Environmental Assessment Report No. 4, Office for Official Publications of the European Communities, Luxembourg, pp. 156.
- Billen, G., C. Lancelot, and M. Meybeck (1991), N, P and Si retention along the aquatic continuum from land to ocean. Ocean Margin Processes in Global Change, R.F.C Mantoura, J.-M. Martin, and R. Wollast, Eds. (John Wiley & Sons Ltd.), pp. 19-44.
- Humborg, C, S. Blomqvist, E. Avsan, Y. Bergensund, E. Smedberg, J. Brink, and C.-M. Morth (2002), Hydrological alterations with river damming in northern Sweden: implications for weathering and river biogeochemistry, Global Biogeochemical Cycles, 16 (3), 1039
- Johannes, R.E. (1980), The ecological significance of the submarine discharge of groundwater, Marine Ecology-Progress Series 3, 365-373.
- Capone, D.G., and M.F. Bautista (1985), A groundwater source of nitrate in nearshore marine sediments, Nature 313, 214 216.
- Church, T.H. (1996), An underground route for the water cycle, Nature 380, 579-580.
- Valiela, I., J. Costa, K. Foreman, J. Teal, B. Howes, and D. Aubrey (1990), Groundwater-borne inputs from watersheds to coastal waters, Biogeochemistry 10, 177-198.
- Dollar, S.J., and M.J. Atkinson (1992), Effects of nutrient subsidies from groundwater to nearshore marine ecosystems off the island of Hawaii, Estuarine, Coastal and Shelf Science 35, 409-424.
- Moore, W.S. (1996), Large groundwater inputs to coastal waters revealed by 226Ra enrichments, Nature 380, 612-614.
- Uchiyama, Y., K. Nadaoka, P. Rolke, K. Adachi, and H. Yagi (2000), Submarine groundwater discharge into the sea and associated nutrient transport in a sandy beach, Water Resources Research 36, 1467-1479.
- Garrison, G.H., C.R. Glenn, and G.M. McMurty (2003), Measurement of submarine groundwater discharge in Kahana Bay, O’ahu, Hawaii, Limnology and Oceanography 48, 920-928.
- Hanshaw, B.B., and W. Back (1980), Chemical mass-wasting of the northern Yucatan Peninsula by groundwater dissolution, Geology 8, 222-224.
- Conley D.J., C.L. Schelske, and E.F. Stoermer (1993), Modification of the biogeochemical cycle of silica with eutrophication, Marine Ecology-Progress Series 101, 179–192.
- Conley D.J., P. Stalnacke, H. Pitkanen, and A. Wilander (2000), The transport and retention of dissolved silicate by rivers in Sweden and Finland, Limnology and Oceanography 45, 1850–1853.
- Galloway J., W. Chlesinger, H. Levy, A. Michaels, and J. Schnoor (1995), Nitrogen fixaton: Anthropogenic enhancement and environmental response, Global Biogeochemical Cycles 9, 235-252.
- North Sea Task Force (1993), North Sea Quality Status Report, Oslo and Paris Commissions, London. Olsen & Olsen, Fredensborg, Denmark.
- Rendell, A. R., Ottley, C. J., Jickells, T. D. & Harrison, R. M. Tellus 45, 53−63 (1993).
- Tappin, A.D. (2002), An Examination of the Fluxes of Nitrogen and Phosphorus in Temperate and Tropical Estuaries: Current Estimates and Uncertainties, Estuarine, Coastal and Shelf Science 55, 885-901.
- Barnes, J., and N.J.P. Owens (1998), Denitrification and nitrous oxide concentrations in the Humber Estuary, UK, and adjacent coastal zones, Marine Pollution Bulletin 37, 247–26.
- Dong, L.F., D.C.O. Thornton, D.B. Nedwell, and G.J.C. Underwood (2000), Denitrification in sediments of the River Colne estuary, England, Marine Ecology Progress Series 203, 109–122.
- Seitzinger, S.P. 1988. Denitrification in freshwater and coastal marine ecosystems: ecological and geochemical importance. Limnology and Oceanography 33:702-724.
- Malcolm, S.J. and Sivyer, D.B., 1997. Nutrient recycling in intertidal sediments. in Jickells, T. and Rae, J.E. (Eds) Biogeochemistry of Intertidal Sediments. Cambridge University Press, pp. 59–83.
- Nixon, S.W. (1995), Coastal marine eutrophication: A definition, social causes, and future concerns, Ophelia 41, 199–219.
- Horrigan, S.G., and Capone, D.G (1985), Rates of nitrification and nitrate reduction in nearshore marine sediments under varying environmental conditons, Marine Chemistry 16, 317-327
- Nowicki, B.L., E. Requintina, D. van Keuren, and J. Portnoy (1999), The role of sediment denitrification in reducing groundwater-derived nitrate inputs to Nauset Marsh Estuary, Cape Cod, Massachusetts, Estuaries 22, 245-259.
- Starr, R.C., and R.W. Gillham (1993), Denitrification and organic-carbon availability in two aquifers, Ground Water 31, 934–947.
- Slater, J.M., and D.G. Capone (1987), Denitrification in aquifer soil and nearshore marine sediments influenced by groundwater nitrate, Applied and Environmental Microbiology 53, 1292-1297.
- DeSimone, L.A., and B.L. Howes (1996), Denitrification and nitrogen transport in a coastal aquifer receiving wastewater discharge, Environmental Science and Technology 30, 1152-1162.
- Capone, D.G., and J.M. Slater (1990), Interannual patterns of water-table height and groundwater derived nitrate in nearshore sediments, Biogeochemistry 10, 277-288.
- Giblin, A.E., and A.G. Gaines (1990), Nitrogen inputs to a marine embayment: The importance of groundwater, Biogeochemistry 10, 309-328.
- Jickells, T.D., T.H. Blackburn, J.O. Blanton, D. Eisma, S.W. Fowler, R.F.C. Manroura, C.S. Martens, A. Moll, R. Scharek, K.I. Suzu, and D. Vaulot (1991), What determines the fate of material within ocean margins? Ocean Margin Processes in Global Change, R.F.C Mantoura, J.-M. Martin, and R. Wollast, Eds. (John Wiley & Sons Ltd.), pp. 211–234.
- Milliman, J.D. (1991), in Ocean Margin Processes in Global Change, R.F.C Mantoura, J.-M. Martin, and R. Wollast, Eds. (John Wiley & Sons Ltd.), pp. 69–90.
- Krom, M.D., and R.A. Berner (1980), Adsorption of phosphate in anoxic marine sediments, Limnology and Oceanography 25, 797-806.
- Frossard, E., M. Brossard, M.J. Hedley, and A. Metherell (1995), Reactions controlling the cycling of P in soils. Phosphorus in the global environment, H. Tiessen, Ed. (John Wiley & Sons Ltd.), pp. 107-138.
- Spiteri, C., C.P. Slomp, K. Tuncay, and C. Meile (2007), Modeling biogeochemical processes in subterranean estuaries: The effect of flow dynamics and redox conditions on submarine groundwater discharge, Water Resources Research, doi:10.1029/2007WR006071.
- Charette, M.A., and E.R. Sholkovitz (2002), Oxidative precipitation of groundwater-derived ferrous iron in the subterranean estuary of a coastal bay, Geophysical Resources Letters 29, art. no.-1444.
- Spiteri, C., P. Regnier, C.P. Slomp, and M.A. Charette (2006), pH-Dependent iron oxide precipitation in a subterranean estuary, Journal of Geochemical Exploration 88, 399-403.
- Froelich, P.N. (1988), Kinetic control f dissolved phosphate in natural rivers and estuaries: A primer o the phosphate buffer mechanism, Limnology and Oceanography 33, 649-668.
- Lebo, M.E. (1991), Particle-bound phosphorus along an urbanized coastal plain estuary, Marine Chemistry 34, 225-246.
- Van der Zee, C., N. Roevros, and L. Chou (2007), Phosphorus speciation, transformation and retention in the Scheldt estuary (Belgium/The Netherlands) from the freshwater tidal limits to the North Sea, Marine Chemistry doi:10.1016/j.marchem.2007.01.003.
- Gassman, G. (1994) Phosphine in the fluvial and marine hydrosphere, Marine Chemistry 45, 197–205.
- Carpenter, K. (1997) A critical appraisal of the methodology used in studies of material flux between saltmarshes and coastal waters. Biogeochemistry of Intertidal Sediments, T.D. Jickells, and J.E. Rae, Eds. (Cambridge University Press), pp. 59–83.
- Ruddy, G., C. M. Turley, and T.E.R. Jones (1998a), Ecological interaction and sediment transport on an intertidal mudflat I. Evidence for a biologically mediated sediment-water interface. Sedimentary Processes in the Intertidal Zone, K.S. Black, D.M. Paterson, and A. Cramp, Eds. Geological Society of London Special Publications 139, pp. 135–148.
- Ruddy, G., C.M. Turley, and T.E.R. Jones (1998b), Ecological interaction and sediment transport on an intertidal mudflat II. An experimental dynamic model of the sediment-water interface. Sedimentary Processes in the Intertidal Zone. K.S. Black, D.M. Paterson, and A. Cramp, Eds. Geological Society of London Special Publications 139, pp. 149–166.
- Billen, G., C. Lancelot, and M. Meybeck (1991), N, P and Si retention along the aquatic continuum from land to ocean. Ocean Margin Processes in Global Change, R.F.C Mantoura, J.-M. Martin, and R. Wollast, Eds. (John Wiley & Sons Ltd.), pp. 19-44. Cite error: Invalid
<ref>tag; name "”Billen1991”" defined multiple times with different content
- Howarth, R., H. Jensen, R. Marino, and H. Postma, Phosphorus in the Global Environment:Transfers, Cycles and Management, H. Tiessen, Ed., Scientific Committee on Problems of the Environment 54. (Wiley, New York, 1995), pp. 323–356.
- Van Cappellen, P., S. Dixit, and J. van Beusekom (2002), Biogenic silica dissolution in the oceans: Reconciling experimental and field-based dissolution rates, Global Biogeochemical Cycles 16, 1075, doi:10.1029/2001GB001431.
- Michalopoulos, P., R.C. Aller, and R.J. Reeder (2000), Conversion of diatoms to clays during early diagenesis in tropical, continental shelf muds, Geology 28, 1095-1098.
- Simpspon, T.L. and B.E. Volcani (1981), Silicon and Siliceous Structures in Biological Systems, Springer-Verlag NY, 587 pp
- Piperno, D.L. (1998), Phytolith analysis. An archaeological and geological perspective. London: Academic Press.
- Bartoli, F. (1983), The biogeochemical cycle of silicon in two temperate forest ecosystems, Ecological Bulletins (Stockholm) 35, 469–476.
- Meunier, J.D., F. Colin, and C. Alarcon (1999), Biogenic silica storage in soils, Geology 27, 835-838.
- Humborg, C., V. Ittekot, A. Cociosu, and B. v. Bdungen (1997), Effect of Danube River dam on Black Sea biogeochemistry and ecosystem structure, Nature 386, 385 – 388.
Please note that others may also have edited the contents of this article.
Please note that others may also have edited the contents of this article.