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Geo-Marine Letters

, Volume 37, Issue 2, pp 179–192 | Cite as

Estuary-type circulation as a factor sustaining horizontal nutrient gradients in freshwater-influenced coastal systems

  • Richard Hofmeister
  • Götz Flöser
  • Markus Schartau
Original

Abstract

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.

Keywords

Suspended Particulate Matter Dissolve Inorganic Carbon Particulate Organic Matter Dissolve Inorganic Nitrogen Horizontal Gradient 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgments

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.

Compliance with ethical standards

Conflict of interest

The authors declare that there is no conflict of interest with third parties.

Supplementary material

367_2016_469_MOESM1_ESM.pdf (430 kb)
ESM 1 (PDF 429 kb)

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Copyright information

© Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  • Richard Hofmeister
    • 1
    • 2
  • Götz Flöser
    • 1
  • Markus Schartau
    • 3
  1. 1.Helmholtz-Zentrum GeesthachtInstitute of Coastal ResearchGeesthachtGermany
  2. 2.Institute of Hydrobiology and Fisheries ScienceUniversity of HamburgHamburgGermany
  3. 3.GEOMAR Helmholtz Centre for Ocean ResearchKielGermany

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