Götz Flöser, Rolf Riethmüller, HZG 24 February 2016 The use of optical backscatter as a proxy for suspended matter concentration in the German Bight Götz Flöser, Rolf Riethmüller, HZG 24 February 2016
Introduction Envisat-MERIS: TSM Annual average 2011
Seapoint turbidity meter Introduction Seapoint turbidity meter 27 campaigns of ~ 5 days each in Wadden Sea / German Bight 4 pole stations with 9 months / year Seapoint’s sensing volume
Sampling procedure Water inlet
Correlation turbidity – SPMC Ship campaigns with “Ludwig Prandtl”, “Heincke”, pole measuring stations North Frisia East Frisia East Frisia East Frisia German Bight North Frisia - GB
Why is the slope different? Why is the scatter so large? Correlation turbidity – SPMC German Bight: slope = 0.88 Hörnum: slope = 0.69 Why is the slope different? Why is the scatter so large? Büsum-Helg: slope = 0.59 East Frisia: slope = 0.46 (0.48, 0.43, 0.46)
b = 2 K N π r2 (Baker & Lavelle, 1984) Theory b = 2 K N π r2 (Baker & Lavelle, 1984) (seapoint tur represents constant fraction of total scatter) TSM = N 4/3 π r3 ρ (Spherical particles) b / TSM = 3/2 K / (r ρ) b optical backscatter intensity @ 880 nm (FTU) K Mie scattering coefficient. For particle size » λ, K=2 N Number concentration (particles m-3) r particle radius (m) TSM suspended matter concentration (kg m-3) ρ ‚homogeneous‘ particle density (kg m-3)
Higher relative velocity destroys particles Dependence r on v LISST measurements 2013: Higher relative velocity destroys particles r increases slightly with vADCP
Dependence b / spmc on vrel
Slack water, high organic content, fuzzy particles Dependence b / spmc on LoI Slack water, high organic content, fuzzy particles Full current, low organic content, solid particles
Dependence b / spmc on LoI
Proxy for density is organic content or LoI Conclusions Conclusions: Mass specific optical backscatter depends mostly on density, not particle size Proxy for density is organic content or LoI The tidal cycle is characterized by resuspension and deposition of inorganic particles with current Questions: Why is the slope (b / spmc vs. LoI) at a particular location so different? Why is North and East Frisia different in dependence of b / spmc on LoI? Why is the scatter – particularly in North Frisia – that large?