Estuary-type circulation as a factor sustaining horizontal nutrient gradients in freshwater-influenced coastal systems
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Estuary-type circulation is a residual circulation in coastal systems with horizontal density gradients. It drives the accumulation of suspended particulate matter in coastal embayments where density gradients are sustained by some freshwater inflow from rivers. Ebenhöh et al. (Ecol Model 174(3):241–252, 2004) found that shallow water depth can explain nutrient gradients becoming established towards the coast even in the absence of river inflow. The present study follows their concept and investigates the characteristic transport of organic matter towards the coast based on idealised scenarios whereby an estuary-type circulation is maintained by surface freshwater fluxes and pronounced shoaling towards the coast. A coupled hydrodynamical and biogeochemical model is used to simulate the dynamics of nutrient gradients and to derive budgets of organic matter flux for a coastal transect. Horizontal nutrient gradients are considered only in terms of tidal asymmetries of suspended matter transport. The results show that the accumulation of organic matter near the coast is not only highly sensitive to variations in the sinking velocity of suspended matter but is also noticeably enhanced by an increase in precipitation. This scenario is comparable with North Sea conditions. By contrast, horizontal nutrient gradients would be reversed in the case of evaporation-dominated inverse estuaries (cf. reverse gradients of nutrient and organic matter concentrations). Credible coastal nutrient budget calculations are required for resolving trends in eutrophication. For tidal systems, the present results suggest that these calculations require an explicit consideration of freshwater flux and asymmetries in tidal mixing. In the present case, the nutrient budget for the vertically mixed zone also indicates carbon pumping from the shelf sea towards the coast from as far offshore as 25 km.
KeywordsSuspended Particulate Matter Dissolve Inorganic Carbon Particulate Organic Matter Dissolve Inorganic Nitrogen Horizontal Gradient
The work of Richard Hofmeister has been funded by the Lower Saxony Ministries for Science and Culture (MWK) and the Ministry of Environment, Energy and Environmental Protection (MU) through the project WIMO and by the German Federal Ministry of Education and Research (BMBF) through the project MOSSCO. We thank Karsten Bolding and Jorn Bruggeman for maintaining the open-source modelling software FABM, GOTM and GETM. Constructive assessments by three reviewers are acknowledged.
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Conflict of interest
The authors declare that there is no conflict of interest with third parties.
- Dentener F, Drevet J, Lamarque JF, Bey I, Eickhout B, Fiore AM, Hauglustaine D, Horowitz LW, Krol M, Kulshrestha UC, Lawrence M, Galy-Lacaux C, Rast S, Shindell D, Stevenson D, Van Noije T, Atherton C, Bell N, Bergman D, Butler T, Cofala J, Collins B, Doherty R, Ellingsen K, Galloway J, Gauss M, Montanaro V, Müller JF, Pitari G, Rodriguez J, Sanderson M, Solmon F, Strahan S, Schultz M, Sudo K, Szopa S, Wild O (2006) Nitrogen and sulfur deposition on regional and global scales: a multimodel evaluation. Glob Biogeochem Cycles 20:GB4003. doi: 10.1029/2005GB002672 CrossRefGoogle Scholar
- Eckart C (1952) The propagation of water waves from deep to shallow water. Natl Bur Stand Circ 20:165–173Google Scholar
- Grunwald M, Dellwig O, Kohlmeier C, Kowalski N, Beck M, Badewien TH, Kotzur S, Liebezeit G, Brumsack HJ (2010) Nutrient dynamics in a back barrier tidal basin of the Southern North Sea: time-series, model simulations, and budget estimates. J Sea Res 64(3):199–212. doi: 10.1016/j.seares.2010.02.008 CrossRefGoogle Scholar
- Kumar N, Voulgaris G, Warner JC, Olabarrieta M (2012) Implementation of the vortex force formalism in the coupled ocean-atmosphere-wave-sediment transport (COAWST) modeling system for inner shelf and surf zone applications. Ocean Model 47:65–95. doi: 10.1016/j.ocemod.2012.01.003 CrossRefGoogle Scholar
- Lenhart H-J, Mills DK, Baretta-Bekker H, van Leeuwen SM, van der Molen J, Baretta JW, Blaas M, Desmit X, Kühn W, Lacroix G, Los HJ, Ménesguen A, Neves R, Proctor R, Ruardij P, Skogen MD, Vanhoutte-Brunier A, Villars MT, Wakelin SL (2010) Predicting the consequences of nutrient reduction on the eutrophication status of the North Sea. J Mar Syst 81:148–170. doi: 10.1016/j.jmarsys.2009.12.014 CrossRefGoogle Scholar
- Painting S, Foden J, Forster R, van der Molen J, Aldridge J, Best M, Jonas P, Hydes D, Walsham P, Webster L, Gubbins M, Heath M, McGovern E, Vincent C, Gowen R, O’Boyle S (2013) Impacts of climate change on nutrient enrichment. MCCIP Sci Rev 2013:219–235. doi: 10.14465/2013.arc23.219-235 Google Scholar
- Soulsby RL (1997) Dynamics of marine sands. Thomas Telford, LondonGoogle Scholar
- Winter C, Herrling G, Bartholomä A, Capperucci R, Callies U, Heipke C, Schmidt A, Hillebrand H, Reimers C, Bremer P, Weiler R (2014) Scientific concepts for monitoring the ecological state of German coastal seas (in German). Wasser und Abfall 07–08(2014):21–26. doi: 10.1365/s35152-014-0685-7 CrossRefGoogle Scholar