Download Measuring Light Attenuation in Shallow Coastal Systems

Brito et al., J Ecosys Ecograph 2013, 3:1
http://dx.doi.org/10.4172/2157-7625.1000122
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Measuring Light Attenuation in Shallow Coastal Systems
Ana C. Brito1*, Alice Newton2,3, Teresa F. Fernandes4, and Paul Tett5
CO-FCUL, Centro de Oceanografia, Faculdade de Ciências da Universidade de Lisboa, Campo Grande 1749-016 Lisboa, Portugal
IMAR- FCT Gambelas, University of Algarve, 8005-139 Faro, Portugal
3
NILU-CEE, Box 100, 2027 Kjeller, Norway
4
School of Life Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
5
Scottish Association for Marine Science (SAMS), Oban, Argyll PA37 1QA, UK
1
2
Abstract
Photosynthetic Active Radiation (PAR) was measured using single planar and two-bulb spherical light sensors.
The attenuation coefficient (Kd) was found to vary significantly during the year. The highest Kd values were obtained
in the station with higher influence of currents and run-off. Our data suggested a reflection of 50% of light that reaches
the bottom, which is associated with a decrease in the Kd value obtained with the spherical sensor of 0.15 m-1. This
means that flat sensors may underestimate PAR and that spherical sensor may underestimate Kd. This is a critical
issue given that knowledge on light attenuation is essential for modeling approaches and quality assessments.
Keywords: Light attenuation; Flat and spherical light sensors;
Shallow coastal lagoons; Ria Formosa; Portugal
Introduction
Amongst other factors, light attenuation is limiting for primary
production [1]. Generally, shallow systems have lighted bottoms and
a rich benthic algal community, which may have a significant role in
reducing the susceptibility to eutrophication [2]. Benthic algae take
nutrients up from the water column and retain a very important part of
the nutrient flux from the pore water to the water column [3,4]. This is
key in the assessment of ecosystem function and health.
The Photosynthetic Active Radiation (PAR), which covers
wavelengths from 400 to 700 nm, is the part of the solar irradiance used
by the photosynthetic apparatus [5]. However, not all the photons in the
water column will be enrolled in the photosynthesis process. Photons
will either be absorbed, leading to a change in the energy state of atoms
or molecules, or scattered in the environment, leading to a change of
direction in their propagation [5]. The decrease of photons, vertically
through the water column, i.e. light attenuation, can be expressed by
the diffuse attenuation coefficient for downward irradiance (Kd) [68]. However, in shallow sites with clear bottoms, special attention has
to be given to the upward and scalar irrandiance, as they are likely to
be important. The vertical attenuation coefficients for the appropriate
irradiance, Kd for downward irradiance, Ku for upward irrandiance and
K0 for scalar irradiance, follow the Beer-Lambert Law, decreasing in an
approximately exponential manner with depth [1].
Historically, the vertical attenuation coefficient for downward
irradiance has been estimated through the measurement of Secchi depths.
However, this method is not viable for shallow systems with clear waters,
such as Ria Formosa lagoon. From the 1970’s onwards submersible
sensors to measure photon flux density started to be used [9]. This allows
the direct calculation of Kd following the Beer-Lambert Law.
Nevertheless, the type of sensor also needs to be considered
carefully in shallow systems. Spherical sensors are able to receive
photons not only downwards from the top, as the planar sensors, but
also scattered photons from all sides (scalar irradiance) and reflected
photons going upwards, from the seabed bottom (upward irradiance)
[10]. Therefore, studies on light availability and attenuation carried out
in shallow waters may result in a confusion of estimates because what
is measured by the spherical sensor is not only the vertical attenuation
coefficient for downward irradiance (Kd).
J Ecosys Ecograph
ISSN:2157-7625 JEE, an open access journal
The objectives of this study were to investigate: 1) the effect of
bottom reflection on the estimates of the vertical attenuation coefficient
obtained in shallow clear waters; and 2) the temporal variation of Kd in
the Ria Formosa lagoon.
Methodology
Study site and sampling
Ria Formosa is a shallow mesotidal lagoon which is cut-off from
the normal coastal circulation (Figure 1). It is located in the south of
Portugal, extending along the eastern part [10]. The mean depth of the
lagoon is around 1.5 m and the mean residence time of waters is 2.4 days
[11,12]. Measurements were taken at Ponte and Ramalhete (P and R in
Figure 1) twice a month during 2007-08, using a single planar sensor
(flat collector) and on the 13th and 14th of June 2007, using the two-bulb
sensor (spherical collector), connected to a Seabird 19 conductivity,
temperature, depth sensor (CTD).
Single planar light sensor
The single planar light sensor used was a Li-Cor (Li-192)
Underwater Quantum sensor. The PAR diffuse attenuation coefficient
for downward irradiance was calculated using the Beer-Lambert Law
equation:
Ed (z ) = Ed (0).e −kd.z (1)
Ed(z) is the PAR measurement at z depth, Ed(0) is the PAR measurement
when the sensor is just under the water surface, kd is the PAR diffuse
attenuation coefficient and z is the depth. In Ria Formosa, on every
sampling date, PAR was measured just below the water surface and at
0.25 m depth at Ponte and Ramalhete.
*Corresponding author: Ana C. Brito, CO-FCUL, Oceanography Centre, Faculty
of Science, University of Lisbon, Campo Grande 1749-016 Lisbon, Portugal, Tel:
00351217500148; Fax: 00351217500009; E-mail: [email protected]
Received November 23, 2012; Accepted January 10, 2013; Published January
14, 2013
Citation: Brito AC, Newton A, Fernandes TF, Tett P (2013) Measuring
Light Attenuation in Shallow Coastal Systems. J Ecosys Ecograph 3: 122.
doi:10.4172/2157-7625.1000122
Copyright: © 2013 Brito AC, et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited.
Volume 3 • Issue 1 • 1000122
Citation: Brito AC, Newton A, Fernandes TF, Tett P (2013) Measuring Light Attenuation in Shallow Coastal Systems. J Ecosys Ecograph 3: 122.
doi:10.4172/2157-7625.1000122
Page 2 of 4
the different sensitivity of the sensors. Sensor 1 measures approximately
1.2 times more than sensor 2. Sp is the distance of separation between
sensors. Measurements were taken by lowering the equipment, i.e.
during the downcast, as well as during the upcast. A set of instant
coefficients were obtained for each cast.
In the Ria Formosa, due to its shallowness, the profile was generally
not divided into layers. If divided, the profile was separated in only
two layers. The second approach consists of the calculation of the Kd
for each depth interval as the slope of the regression line of the logtransformed PAR in that given depth interval.
At the end of the analysis of a given profile, two estimates of Kd
are obtained using the second approach (regression), one for the down
cast and another for the up cast. In addition, two estimates of Kd from
instant measurements are also obtained from the down and up cast.
The mean of the coefficients obtained from each approach was then
calculated.
Statistical analysis
Statistical tests were carried out using Minitab 14 software.
Data were tested for normality and homoscedasticity of variance
and parametric tests conducted, when possible. Otherwise, the
correspondent nonparametric tests were performed.
Results
A total of 36 point measurements (single planar light sensor; twice
a month from March 2007 to February 2008) and 17 profiles (two-bulb
light sensor; several profiles on two days) were obtained at the two
stations of Ria Formosa, Ponte and Ramalhete.
Figure 1: Map of Ria Formosa showing sampling stations at P=Ponte and
R=Ramalhete. Water exchanges in the western part of the lagoon are mainly
through the inlet B.
Two-bulb light sensor
The two-bulb light sensor was constituted by two Li-Cor Underwater
Spherical Quantum sensors and a coupled CTD positioned below
(Figure 2). The sensor was developed in order to obtain concomitant
measurements of the available PAR in the water column and to
obtain vertical profiles of PAR measurements. This approach aimed at
improving the previous method used to calculate Kd, by using profiles
and instant PAR measurements at both depths, rather than isolated and
non concomitant PAR values.
Single planar light sensor
During the period from March 2007 to February 2008, the values
of the Kd coefficient varied from 0.25 to 1.10 at Ramalhete and from
0.68 to 1.28 at Ponte (Table 1). Mean Kd values found were 0.69 at
Ramalhete and 0.93 at Ponte. Positive Pearson’s correlations were
found between values of Ponte and Ramalhete (p < 0.005). In addition,
a linear regression analysis was performed between the Kd coefficient
Numerical approach for the two-bulb sensor
Two numerical approaches were applied to the datasets from the
two-bulb light sensors. One calculates the Kd for each optical depth
using solely the instant PAR measurements recorded in each specific
depth interval. The other goes further and performs a regression
between the log (x) transformed measurements of PAR and depth
within each optical depth. The coefficient of this relationship is the Kd
value. This will be further explained below.
For the first approach, it is necessary to calculate, for a given profile,
the instant coefficients for the down and the up casts, at a specific time.
Thus
 PAR

bottom × P1P 2ratio × 1 
Kd = − log 
PARtop
sp  

(2)
PARbottom and PARtop are PAR measurements of the top (sensor 1)
and bottom (sensor 2) sensors. P1P2ratio is a coefficient to correct for
J Ecosys Ecograph
ISSN:2157-7625 JEE, an open access journal
Figure 2: A) Scheme of the two-bulb underwater light sensor. Distances
between sensors are adjustable. B) Two-bulb underwater light sensor with
protective caps on the bulbs.
Volume 3 • Issue 1 • 1000122
Citation: Brito AC, Newton A, Fernandes TF, Tett P (2013) Measuring Light Attenuation in Shallow Coastal Systems. J Ecosys Ecograph 3: 122.
doi:10.4172/2157-7625.1000122
Page 3 of 4
Ria Formosa Single planar light sensor–2007/2008
Kd (m-1)
mean
Ram 0.25 0.79 0.59 0.57 0.53 - 0.90 0.59 1.10 -
0.66 0.90 0.69
Ponte 0.68 0.96 0.93 0.77 1.28 - 1.27 1.10 1.3 0.96 1.17 0.75 0.93
M
A
M
J
J
A S
O
N
D
J
F
Table 1: Mean values of the diffuse attenuation coefficient (m ) measured at Ponte
and Ramalhete (Ria Formosa) from March 2007 to February 2008 with the single
planar light sensor.
-1
Ria Formosa
Figure 3: Estimates of Kd obtained from regression (colour lines; Red: top
sensor and blue: bottom sensor; both correspond to the coloured PAR slopes)
and from instant measurements (single values-black triangles and the black line
corresponds to the mean value).
Profile
Maximum water depth
Mean Kd
Method
Ponte 1
0.97
0.60
Instant
Ponte 2
1.09
0.70
‘’
Ponte 3
1.80
0.72
‘’
Ponte 4
2.24
0.55
‘’
Ponte 5
2.34
0.48
‘’
Ponte 6
2.86
0.34
‘’
Ponte 7
2.66
0.37
‘’
Ponte 8
2.11
0.40
‘’
Ponte 9
1.58
0.63
‘’
Ponte 10
2.43
0.63
‘’
Ponte 11
3.00
0.55
‘’
Ponte 12
3.02
0.57
‘’
the Kd mean values obtained at Ponte and Ramalhete using solely the
instant measurements. The mean Kd values obtained were 0.55 and 0.57
at Ponte and at Ramalhete, respectively. Profiles 1 to 6 at Ponte were
done during the flood and high water periods, as represented by the
maximum water depths. Profiles 7 to 9 at Ponte were recorded during
the ebb period. Profile 10 at Ponte was recorded during the flood period
again. Profiles 11 and 12 at Ponte were recorded during the high water
period on the following day. Profiles 1 to 5 at Ramalhete were collected
during low water and flood periods. No significant differences were
found between Kd values obtained at Ponte and at Ramalhete (p > 0.05;
Mann-Whitney).
Due to its morphology, the spherical sensor is able to measure the
irradiance that is reflected by the seabed. The spherical light sensor
responds to upwards as well as downwards light. Therefore, it considers
data that should be used for the estimation of K0, the coefficient of
scalar irradiance. If the spherical sensor is used for the assessment of
Kd, it may underestimate the coefficient, if the sensor is lowered towards
a reflecting seabed. The importance of the sea-bed reflection in shallow
systems should be evaluated. Artificial data for PAR measurements in
the bottom and top sensor were therefore produced, according to the
Beer-Lambert Law described previously (Equation 1). These data were
produced using different values of Kd from 0.5 to 1, which is a range
that consideres the majority of values found with the planar sensor
during 2007-2008 (Figure 4A and 4B). The sea-bed reflection term
was then introduced in the Beer-Lambert Law equation as being 0.5,
which represents a reflection of 50% of light that reaches the bottom.
0.55
Ram 1
0.21
0.75
‘’
Ram 2
0.14
0.96
‘’
Ram 3
0.19
0.42
‘’
Ram 4
0.17
0.36
‘’
Ram 5
0.13
0.38
‘’
0.57
Table 2: Maximum water depths and mean Kd of each profile at Ponte and
Ramalhete.
and chlorophyll concentrations (data not shown) and no significant
relationship was found.
Two-bulb light sensor
In Ria Formosa, it was not possible to determine the diffuse
attenuation coefficient for downward irradiance using the regression
method. Due to the shallowness of the lagoon, there were insufficient
measurements made through the water column to conduct an accurate
regression. In most cases, the maximum depth was approximately
1 meter and sensors were 0.75 m apart, which left an insufficient
depth of less than 0.25 m to derive a profile. Estimates of Kd obtained
from the regression method (coloured vertical lines) and from the
calculation of the instant Kd values (black triangles), obtained from
profile 1 at Ponte are represented in Figure 3. Negative values of Kd were
obtained using the regression method. This is presented to show that
the regression method is not viable at these sites. Table 2 comprises
J Ecosys Ecograph
ISSN:2157-7625 JEE, an open access journal
Figure 4: A) Log (x) transformed PAR values produced following the BeerLambert Law for each optical depth during the down (∇) and upcast (∆) through
the water column. B) Estimates of Kd obtained from regression (vertical coloured
lines: Red: top sensor and blue: bottom sensor) and from instant measurements
(single values - black points and the black vertical line corresponds to the mean
value). C) Estimates of Kd, considering a sea-bed reflection of 50%, obtained
from regression (vertical coloured lines) and from instant measurements (single
values-black points and the black vertical lines correspond to the mean value).
Sensors: Top and bottom. Casts: down (solid line) and up (dashed line).
Volume 3 • Issue 1 • 1000122
Citation: Brito AC, Newton A, Fernandes TF, Tett P (2013) Measuring Light Attenuation in Shallow Coastal Systems. J Ecosys Ecograph 3: 122.
doi:10.4172/2157-7625.1000122
Page 4 of 4
This value was considered as reasonable by the observation of the clear
sea bottom. The artificial dataset was produced once again (Figure 4C).
The result was an increase of 0.15 in the Kd estimate, which was 0.7 m-1
instead of the 0.55 m-1 found without considering the bottom reflection
(Figure 4B).
Discussion
In Ria Formosa, higher mean Kd values were observed in Ponte
rather than Ramalhete. This might express the higher influence of
currents, resuspension of sediments and run-off at this site, which is
a main channel, compared with Ramalhete, which is an inner channel
[13]. Profiles recorded at Ponte and Ramalhete clearly express differences
of light attenuation between low and high water. This shallow region is
expected to have higher concentrations of suspended particles due to
the run-off and re-suspension of sediments than the open ocean [14].
Therefore, a decrease in light attenuation is expected during the flood
and high water periods [6,8]
One of the most striking findings is the relevance of bottom
reflection in shallow systems such as Ria Formosa. This reveals the
importance of the type of sensor used in systems such as this coastal
lagoon (spherical vs planar sensor).
The increase of 0.15 in the vertical attenuation coefficient when
considering the bottom reflection was very interesting. After applying
the correction factor, the coefficient would be 0.7 m-1 in Ponte, which
was the same number as obtained with the flat sensor for the same
period. The shallowness of the system means that a significant part of
the incident light will reach the bottom and be reflected [15]. Bottom
reflection is therefore an important phenomenon that should be taken
into account when studying shallow waters. Reflected light will be
available again for phytoplankton and turns the water clearer. The twobulb light sensor was spherical and because of that it would also record
the reflected light by the sea-bed. In this way, the spherical sensor was
also measuring the scalar irradiance and should also be used for the
estimation of the scalar coefficient (K0) and not solely the Kd. When
the upwelling irradiance is very small, K0 may be approximated to Kd.
However, in this case, the bottom reflection is very important. The
influence of the reflected light on the flat sensor (single planar light
sensor) should be smaller because the sensor was only recording light
reaching from above.
The fact that these sensors are measuring different types of
irradiance should explain the observed differences between the ‘Kd’
values obtained with the flat and spherical sensors. In fact, our spherical
light sensor could provide more accurate measurements of the available
irradiance for photosynthesis in the shallow waters of Ria Formosa.
The flat sensor will always provide an underestimation of these values.
Furthermore, the use of two sensors collecting PAR measurements
at the same time is always more reliable than using only one sensor.
Nevertheless, if the main aim of an investigation is to study the light
attenuation due to particle concentration using spherical collectors, a
correction for bottom reflection should be applied, or the flat collector
should be considered.
from Edinburgh Napier University and a Portuguese Post-doc grant from FCT
(BPD/63017/2009). This study was also supported by the Fundação para a Ciência
e a Tecnologia (FCT) (PEst-OE/MAR/UI0199/2011).
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Acknowledgements
Special features:
Ana Brito was funded by a Portuguese PhD grant from FCT (POCI 2010
BD/21525/2005), an initial studentship (from October to December 2005)
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Citation: Brito AC, Newton A, Fernandes TF, Tett P (2013) Measuring
Light Attenuation in Shallow Coastal Systems. J Ecosys Ecograph 3: 122.
doi:10.4172/2157-7625.1000122
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ISSN:2157-7625 JEE, an open access journal
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