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Underwater light and eelgrass – what is wrong with the Secchi disc?

Underwater light and eelgrass – what is wrong with the Secchi disc?. Søren Laurentius Nielsen Roskilde University. Apparently eelgrass needs more and more light?. Light attenuation is the sum of absorbance and scattering: Attenuation = Absorption + Scattering

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Underwater light and eelgrass – what is wrong with the Secchi disc?

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  1. Underwater light and eelgrass – what is wrong with the Secchi disc? Søren Laurentius Nielsen Roskilde University

  2. Apparently eelgrass needs more and more light?

  3. Light attenuation is the sum of absorbance and scattering: • Attenuation = Absorption + Scattering • Absorption depends on water, particles and colored dissolved matter: • Abstot = Absw + Abspig + Absdet + AbsCDOM • Scattering depends on water and particles: • Scatttot = Scattw + Scattpart

  4. An absorbed photon is gone • A scattered photon is still available for photosynthesis, but… • Scattering might be away from eelgrass on the bottom (upwards, horizontal) • Scattering enhances the probability of absorption (longer pathway)

  5. PAR sensors measure the true available light – you get Kd • Secchi discs are however a different matter

  6. Secchi depth (Zsd) is a measure of image attenuation rather than light attenuation • Zsd is much more sensitive to scatter than Kd • Kd is often estimated from Zsd, assuming that Kd*Zsd is constant (often ≈ 1.44)

  7. Modified from Davies-Colley & Vant, Water Resources Bulletin, 1988

  8. Roskilde Fjord 2008 - 09

  9. (based on eelgrass depth limits corresponding to 10 % of incident light)

  10. Conclusions: • Systematic changes in the relative contribution of particles and colored dissolved matter during eutrophication/oligotrophication → • Systematic changes in scatter/absorption ratio → • Systematic changes in Zsd * Kd → • Use of Secchi discs lead to overestimation of improvements in light availability and thus potential eelgrass re-colonization during oligotrophication • In Roskilde Fjord 0.5 – 1.5 m

  11. The fluffy bottom layer

  12. : Extrapolated, sandy : Actual, sandy : Extrapolated, muddy : Actual, sandy

  13. Summary: • Estimations of available light from Secchi disc measurements in the water column may overestimate actually available light due to • Changes in relative contributions from particles and colored dissolved matter (and thus b/a) during oligotrophication • The presence of a fluffy layer close to the bottom

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