Environmental Engineering Reference
In-Depth Information
Experimental studies on phytoplankton biomass showed that phytoplankton has
a strong absorption capacity at 400-700 nm and that its absorption decreases dur-
ing 33 days of dark incubation (Fig. 4 ) (Zhang et al. 2009 ). An increase in UV
absorption by PM may result from additional algal biomass during the phyto-
plankton bloom (Whitehead and Vernet 2000 ). The algal community composition
in term of dominant cell size and, therefore, of pigment packaging is the key fac-
tor driving the phytoplankton specific absorption in the water column (Vantrepotte
et al. 2007 ). The scattering coefficient of particulate materials increases approxi-
mately linearly with decreasing wavelength where suspended sediments domi-
nate the optical signal in natural waters (Belzile et al. 2002 ; Morel 1988 ; Ahn
et al. 1992 ; Roesler and Zaneveld 2258; Haltrin 1999 ; Pegau et al. 1999 ; Roesler
1998 ; Morel and Loisel 1998 ). The experimental and theoretical efficiency
factor for scattering by the picocyanobacteria Synechococcus sp., Synechocystis
sp. and Anacystis marina increases with decreasing wavelength (Ahn et al. 1992 ).
However, an increase of the scattering efficiency with decreasing wavelength is
not systematically detected in larger algal species (Ahn et al. 1992 ). The spectral
slope coefficients (300-700 nm) of CDOM samples increase by as much as 20 %
after mixing with 10 g L 1 sediment and by 5 % after mixing with 1 g L 1 sedi-
ment (Shank et al. 2005 ). This suggests that sorption to particles has the potential
to significantly alter the optical properties of CDOM in the water column of turbid
shallow environments or in areas of high benthic exchange (Shank et al. 2005 ).
Phytoplankton is the key driver of the spatial-temporal variations of the light
attenuation coefficient, which accounts on average for 44 % of the total light
attenuation (Obrador and Pretus 2008 ). The backscattering coefficient is highly
correlated with turbidity and suspended matter (R 2 = 0.98), but it is poorly corre-
lated to chlorophyll (R 2 = 0.42) (Dupouy et al. 2010 ), suggesting the importance
of the inherent optical properties of PM in waters. The concentration-specific
absorption coefficient of mineral particles is generally found to decrease expo-
nentially with wavelength towards a constant non-zero value in the red (Bowers
and Binding 2006 ). Specific scattering coefficients of mineral particles show a
tendency to decrease from the open ocean into energetic shelf seas and estuaries,
but then to increase again within shelf seas as turbulent energy increases (Bowers
and Binding 2006 ). Light attenuation and scattering by particles can account for
11-52 % of the total attenuation/scattering in a variety of waters (Smith et al.
2004 ; Belzile et al. 2002 ; Lund-Hansen 2004 ; Smith et al. 1999 ). PM can con-
tribute an estimated 25-90 % of the attenuation coefficients for the first-year sea
ice at wavelengths <500 nm (Fritsen et al. 2011 ). The total particulate absorption
coefficients at 300 nm are 0.1-0.3 m 1 in Southern Ocean waters (Holm-Hansen
et al. 1993 ). Specific absorption coefficients for Antarctic phytoplankton is 0.1 m 2
(mg chl a) 1 within the UV range (Mitchell et al. 1989 ; Arrigo 1994 ). Bacterial
attenuation at 390 nm ranges from 0.002 m 1 for Micrococcus sp. to 2.80 m 1
for Moraxella sp. at concentrations of 1012 cells m 3 , and increases markedly at
shorter wavelength (Kopelevich et al. 1987 ). These studies show that light atten-
uation by suspended particles is very variable depending on the water (clear or
turbid) and the particle loading.
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