Evidence for a maximum of sinking velocities of suspended particulate matter in a coastal transition zone

Abstract

Marine coastal ecosystem functioning is crucially linked to the transport and fate of suspended particulate matter (SPM). Transport of SPM is, amongst others, controlled by sinking velocity ws. Since ws of cohesive SPM aggregates varies significantly with size and composition of mineral and organic origin, ws probably exhibits large spatial variability along gradients of turbulence, SPM concentration 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. Analyzed with respect to modeled prevailing energy dissipation rates , mean ws were significantly enhanced around log10((m2 s−3)) −5.5. This region is typically found at water depths of approximately 15mto 20mon a cross-shore transect. Across this zone, SPM concentration declines drastically towards the offshore 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 landwardoriented bottom currents are likely 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 re-mineralized. Residual surface currents transport dissolved nutrients towards the off-shore, 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.