Evidence for a maximum of sinking velocities of suspended particulate matter in a coastal transition zone Joeran Maerz 1 , Richard Hofmeister 1 , Eefke M. van der Lee 1 , Ulf Gräwe 2,3 , Rolf Riethmüller 1 , and Kai W. Wirtz 1 1 Institute of Coastal Research, Helmholtz-Zentrum Geesthacht (HZG), Geesthacht, Germany 2 Leibniz Institute for Baltic Sea Research, Warnemünde, Germany 3 Institute of Meteorology and Climatology, Leibniz University Hannover, Hannover, Germany Correspondence to: Joeran Maerz (joeran.maerz@hzg.de) 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 w s . Since w s of cohesive SPM aggregates varies significantly with size and composition of mineral and organic origin, w s probably exhibits large spatial variability along gradients of turbulence, SPM concentration 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. Analyzed with 5 respect to modeled prevailing energy dissipation rates ǫ, mean w s were significantly enhanced around log 10 (ǫ (m 2 s −3 )) ≈ −5.5. This ǫ region is typically found at water depths of approximately 15m to 20m on 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 landward- 10 oriented 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 15 retention is thus suggested as an important mechanism that underlies the often observed nutrient gradients towards the coast. 1 Introduction Biogeochemical cycling and functioning of marine coastal and shelf sea systems crucially relies on particle transport. Vertical fluxes of suspended particulate matter (SPM) are determined by sinking velocity w s and indirectly affect the horizontal trans- port. In coastal systems, SPM is composed of living and non-living particulate organic matter (POM) and fine cohesive and 20 non-cohesive resuspended minerals. Cohesive sediments and POM can undergo aggregation and fragmentation processes that change transport properties and thus their sinking velocity. As a consequence of flocculation, SPM aggregates ubiquitously possess a broad spectrum of size and composition (Fettweis, 2008). This heterogeneity in-between flocs increases the method- 1 Biogeosciences Discuss., doi:10.5194/bg-2015-667, 2016 Manuscript under review for journal Biogeosciences Published: 5 February 2016 c  Author(s) 2016. CC-BY 3.0 License.