CDOM can be assumed to be zero, in this case, the average over 700-800 nm.
Total Particulate absorption a
TP
was measured using the Transmittance-Reflectance
quantitative technique Tassan, 2002, where reflectance and transmission measurements were
taken using
the dual-bean
UV-2600 Shimadzu
spectrophotometer coupled with an integrating sphere. TP optical density OD
TP
was measured between 300 nm and 800 nm at 1 nm intervals, with a blank filter saturated with Fresh
Milli-Q water as a reference. Pigmented matter was then bleached for 5 to 20 min with 5 mL of a 10 sodium
hypochlorite solution before being rinsed with distilled water prior to re-scanning. It has already been demonstrated that
bleaching using sodium hypo-chlorite oxidizes phytoplankton pigments faster than other particulate organic matter enabling
the separation of their absorption signals Ferrari Tassan, 1999. Therefore, Non-Algal Particles NAP absorption
fraction OD
NAP
will include unbleached organic detritus Organic NAP and mineral sediments Inorganic NAP.
The Total Particulate absorption a
TP
and NAP absorption a
NAP
were converted from OD to absorption coefficients m
-1
following equation 2: � =
2.303 �
� � 2
Where R is the ratio of the filtered volume to the filter clearance area in meters and x is the component TP or NAP. The
difference between a
TP
and a
NAP
leads to the absorption due to phytoplankton pigments a
ph
a
ph
= a
TP
- a
NAP
. Total Laboratory Absorption was calculated as the sum of Total
Particulate and CDOM absorptions a
TLab
= a
TP
+ a
CDOM
.
2.5 Inherent optical property measurements
At each reservoirs sampling station, profiles of attenuation c and absorption a coefficients were acquired with a 25 cm
pathlength Spectral Absorption and Attenuation meter AC-S WetLabs Inc., whereas in lakes of Amazon river floodplain,
due to attenuation larger than 20 m
-1
, a 10 cm pathlength AC-S meter was used. To prevent air bubbles, measurements started
from the limit of euphotic zone towards the subsurface. Two profiles with spectral resolution of 4 nm between 400 and 750
nm were acquired at each station. The AC-S meters were set to a sample rate of 4 Hz.
Both AC-S measurements, attenuation and absorption coefficients, were corrected for temperature dependency along
the wavelengths Pegau et al., 1997. Absorption coefficient overestimation Zaneveld et al., 1994; Tzortziou, 2004 was
corrected using the proportional method Zaneveld et al., 1994 as follows:
� = � −
� ∗ �
� 3
Where, a
c
λ is the corrected absorption coefficient, a
mts
λ the absorption coefficient corrected for temperature, a
mts
λ
r
the absorption coefficient corrected for temperature at a reference
wavelength, b
m
the scattering coefficient and b
m
λ
r
the scattering coefficient at the reference wavelength.
The spectral scattering profiles were obtained, by subtracting the absorption coefficient from the attenuation coefficients [b
= c – a ].
Profiles of volume scattering function VSF measured at six wavelengths 420, 442, 470, 510, 590, 700nm and at 140º
angle β140
using a HydroScat-6 instrument HOBI Labs, were converted to backscattering coefficient b
b
profiles, as follows:
= 2 ���140º − � 140º +
4 Where,
β
w
140 is the volume scattering function of pure
water, b
bw
the backscattering coefficient of pure water and χ is a
non-dimensional variable relating β140
to b
b
. The value χ
= 1.08, recommended in the user’s manual HOBI Labs, 2010,
was then adopted, even though χ can vary according to acquisition angle Boss Pegau, 2001 and environmental
conditions. Underestimation of backscattering values, in Case II waters, due to high attenuation along the optical path of the
instrument, were corrected as described in Carvalho et al. 2014.
2.6 Apparent optical property measurements
