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

Søren Laurentius NielsenRoskilde University

Apparently eelgrass needs more and more light?

1 2 3 4 5 6 7 8 9 10 110

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4

6

8

10

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f(x) = 1.02177777777778 xR² = 0.983284684433282

f(x) = 0.657706046701676 xR² = 0.963649502719956

f(x) = NaN xR² = 0

Nielsen et al. 1994 & 2002

Linear (Nielsen et al. 1994 & 2002)

DMU 1992

Linear (DMU 1992)

Ostenfeldt 1908

Linear (Ostenfeldt 1908)

Secchi depth (m)

Eel

gras

s de

pth

limits

(m)

• 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

( )a

• 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)

• PAR sensors measure the true available light – you get Kd

• Secchi discs are however a different matter

• 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)

Modified fromDavies-Colley & Vant, Water Resources Bulletin, 1988

Roskilde Fjord 2008 - 09

Station Period

Average

Zd

Average

Kd

Chl a

att (%)

POM*

att (%)

Residual

att (%) KdZd

St 1 1985 1.82 1.10 45.6 16.5 37.9 1.83

St 2 1985 2.39 0.80 18.0 16.9 65.1 2.10

St 3 1985 na 0.62 17.4 22.1 60.5 na

St 4 1985 3.46 0.49 21.4 21.3 57.2 1.54

St 1 2008-9 3.78 0.67 12.0 4.0 84.1 2.48

St 2 2008-9 4.22 0.54 12.9 3.9 83.2 2.14

St 3 2008-9 4.00 0.53 17.3 5.5 77.2 na

St 4 2008-9 4.58 0.53 17.2 6.2 76.7 2.03

1985 2008-09

Station

Seagrass depth limit

at Secchi depth (Zd)

(m)

Seagrass depth

limit based on Kd

(m)

Seagrass depth limit

at Secchi depth (Zd)

(m)

Seagrass depth

limit based on Kd

(m)

Increase in seagrass depth limit

from 1985 to 2008-09 based on

Secchi depth (m)

Increase in seagrass depth

limit from 1985 to 2008-09

based on Kd (m)

1 1.82 2.09 3.78 3.44 1.96 1.35

2 2.39 2.88 4.22 4.26 1.83 1.38

3 NA NA 4.00 4.34 NA NA

4 3.46 4.70 4.58 4.34 1.12 - 0.36

x 2.56 3.22 4.15 4.10 1.64 0.79

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

• 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

The fluffy bottom layer

: Extrapolated, sandy: Actual, sandy: Extrapolated, muddy: Actual, sandy

• 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