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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 13, issue 17 | Copyright

Special issue: COSYNA: integrating observations and modeling to understand...

Biogeosciences, 13, 4863-4876, 2016
https://doi.org/10.5194/bg-13-4863-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 05 Sep 2016

Research article | 05 Sep 2016

Maximum sinking velocities of suspended particulate matter in a coastal transition zone

Joeran Maerz1,a, Richard Hofmeister1, Eefke M. van der Lee1, Ulf Gräwe2,3, Rolf Riethmüller1, and Kai W. Wirtz1 Joeran Maerz et al.
  • 1Institute of Coastal Research, Helmholtz-Zentrum Geesthacht (HZG), Geesthacht, Germany
  • 2Leibniz Institute for Baltic Sea Research, Warnemünde, Germany
  • 3Institute of Meteorology and Climatology, Leibniz University Hanover, Hanover, Germany
  • apresent address: Max Planck Institute for Meteorology (MPI-M), Hamburg, Germany

Abstract. Marine coastal ecosystem functioning is crucially linked to the transport and fate of suspended particulate matter (SPM). Transport of SPM is controlled by, amongst other factors, sinking velocity ws. Since the ws of cohesive SPM aggregates varies significantly with size and composition of the mineral and organic origin, ws exhibits large spatial variability along gradients of turbulence, SPM concentration (SPMC) and SPM composition. In this study, we retrieved ws for the German Bight, North Sea, by combining measured vertical turbidity profiles with simulation results for turbulent eddy diffusivity. We analyzed ws with respect to modeled prevailing dissipation rates ϵ and found that mean ws were significantly enhanced around log10(ϵ (m2s−3)) ≈ −5.5. This ϵ region is typically found at water depths of approximately 15 to 20m along cross-shore transects. Across this zone, SPMC declines towards the offshore waters and a change in particle composition occurs. This characterizes a transition zone with potentially enhanced vertical fluxes. Our findings contribute to the conceptual understanding of nutrient cycling in the coastal region which is as follows. Previous studies identified an estuarine circulation. Its residual landward-oriented bottom currents are loaded with SPM, particularly within the transition zone. This retains and traps fine sediments and particulate-bound nutrients in coastal waters where organic components of SPM become remineralized. Residual surface currents transport dissolved nutrients offshore, where they are again consumed by phytoplankton. Algae excrete extracellular polymeric substances which are known to mediate mineral aggregation and thus sedimentation. This probably takes place particularly in the transition zone and completes the coastal nutrient cycle. The efficiency of the transition zone for retention is thus suggested as an important mechanism that underlies the often observed nutrient gradients towards the coast.

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We investigated sinking velocity (ws) of suspended particulate matter (SPM) in the German Bight. By inferring ws indirectly from an extensive turbidity data set and hydrodynamic model results, we found enhanced ws in a coastal transition zone. Combined with known residual circulation patterns, this led to a new conceptual understanding of the retention of fine minerals and nutrients in shallow coastal areas. The retention is likely modulated by algal excretions enhancing flocculation of SPM.
We investigated sinking velocity (ws) of suspended particulate matter (SPM) in the German Bight....
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