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

Abstract

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 w s . Since the w s of cohesive SPM aggregates varies significantly with size and composition of the mineral and organic origin, w s exhibits large spatial variability along gradients of turbulence, SPM concentration (SPMC) and SPM composition. In this study, we retrieved w s for the German Bight, North Sea, by combining measured vertical turbidity profiles with simulation results for turbulent eddy diffusivity. We analyzed w s with respect to modeled prevailing dissipation rates and found that mean w s were significantly enhanced around log 10 ( (m 2 s −3 )) ≈ −5.5. This region is typically found at water depths of approximately 15 to 20 m 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.

The acclimative biogeochemical model of the southern North Sea

“…These are in some cases in the form of higher concentrations below the pycnocline but in some others appear as thin layers at around the pycnocline. While the deep chlorophyll maxima are prevalently found in stratified offshore regions, the well-mixed shallower regions mostly show homogeneously distributed high chlorophyll concentrations throughout the water column due to higher dissipation rates (Maerz et al, 2016). The MAECS simulation agrees qualitatively well with these patterns and captures the spatial variability of the observed vertical chlorophyll distribution (Fig.…”

“…A better representation of the SPM-caused turbidity might be achieved by an explicit description of the SPM dynamics (e.g., as in van der Molen et al, 2016). Coupling the biogeochemical model with such an SPM model would then also allow the description of the two-way interactions, i.e., not only light limitation (Tian et al, 2009), but also the acceleration of sinking of SPM by the production of transparent exopolymer particles Maerz et al, 2016). At the stations within the ROFIs of major rivers, such as the Norderelbe and S. Amrum (Fig.…”

“…Continuous Scanfish and FerryBox measurements were performed within the operation of the Coastal Observing System for Northern and Arctic Seas (COSYNA, Baschek et al, 2016). Data collection, processing and quality control of the Scanfish data are described by Maerz et al (2016) and of the FerryBox data by Petersen (2014). The satellite dataset used here is the Ocean Colour Climate Change Initiative (CCI), version 3.1, European Space Agency (ESA), available online at http://www.esa-oceancolour-cci.org/.…”

“…The latter is largely determined by suspended particulate matter (SPM) concentrations (Tian et al, 2009;Su et al, 2015). These gradients are driven by a complex interplay of riverine and atmospheric fluxes, complex topography, residual tidal currents, density gradients, biological processing of organic matter (OM), benthic-pelagic coupling and sedimentation-resuspension dynamics (Postma, 1961;Puls et al, 1997;van Beusekom and de Jonge, 2002;Burchard et al, 2008;Hofmeister et al, 2016;Maerz et al, 2016). A number of modeling studies previously addressed the biogeochemistry of the North Sea, including the German Bight.…”

Spatial and Seasonal Variation of Biomineral Suspended Particulate Matter Properties in High-Turbid Nearshore and Low-Turbid Offshore Zones

“…Proliferation of a specific algae group could enhance flocculation and store sediments in the near-bed layer, and hence could cause large but suspended biological aggregates and a low SPM concentration in the water column. Maerz and co-workers [51] have been looking at the whole gradient from the nearshore TMZ to the OSZ; they have found a maximum settling velocity in the transition zone between the TMZ and the OSZ where the aggregates are larger as compared to near-coast TMZ and denser as compared to the low turbid OSZ. This maximum in settling velocity is caused by similar gradients in aggregate size, POC content, density, and chlorophyll concentration than found in our data.…”