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