1. 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

2. Introduction
Envisat-MERIS: TSM
Annual average 2011

3. 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

4. Sampling procedure
Water inlet

5. 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

6. 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)

7. 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 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)

8. Higher relative velocity destroys particles
Dependence r on v
LISST measurements 2013:
Higher relative velocity destroys particles
r increases slightly with vADCP

10. Dependence b / spmc on vrel

11. 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

12. Dependence b / spmc on LoI

13. 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?