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 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 (m2 s−3)) ≈ −5.5. This ϵ 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.
Highlights
Biogeochemical cycling and functioning of marine coastal and shelf sea systems crucially relies on particle transport
As suggested by Dyer (1989) and Pejrup and Mikkelsen (2010), in addition to SPM concentration (SPMC), turbulent shear can be regarded as the major determinant for ws
Our study suggests a change in particle composition, with as proxied by the fluorescence-to-SPMC ratio
Summary
Biogeochemical cycling and functioning of marine coastal and shelf sea systems crucially relies on particle transport. SPM is composed of living and nonliving particulate organic matter (POM) and fine cohesive and non-cohesive resuspended minerals. Fine-grained minerals of sizes typically up to 8 μm (Chang et al, 2006) and POM can undergo aggregation and fragmentation processes that change sinking velocity and transport properties. As a consequence of flocculation, SPM aggregates ubiquitously possess a broad spectrum of size and composition (Fettweis, 2008). This heterogeneity between flocs increases the methodological effort required to analyze ws in situ (Fettweis, 2008). Tidal and wind-induced currents are the major driver for resuspension and subsequent horizontal transport, while biological processes, such as algae growth and bio-induced sediment stabilization (Black et al, 2002; Stal, 2003), interfere and shape the complex
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