Johanna Theroux-7613677
BIOL 4374: Aquatic Botany
Lab 1: Light
Introduction:
In this laboratory, we determined the light availability in four lakes of Fort Whyte Alive. Light availability
is important for aquatic plant life because it is a direct input in photosynthesis. Light quantity and quality
are determining factors in assessing the suitability of the aquatic environment for plant life. Light quality
reaching the water surface is affected by atmospheric particles; altitude, cloud, and canopy cover
because these forms of interception may absorb the wavelength of light that is absorbed by most
chlorophyll during photosynthesis. This wavelength is known as PAR (photosynthetically active
radiation) and is between 400-700 nm. Light quantity can also be affected by the characteristics of the
water column such as turbidity and water depth which influence the reflection and absorption by
sediments and organic particles.
Materials and Methods:
In this experiment, the PAR values were measured at one meter intervals from one to six meters in the
four study lakes: Devonian Lake, Muir Lake, Lake Two, and Cargill Lake. The measurements were made
from a canoe by inserting a spherical light sensor attached to a light meter into the water column and
recording the corresponding PAR values. The Secchi disk depth was also calculated by inserting a black
and white weighted metal disk into the water column on the shady side of the boat and recording the
depth at each interval where the disk disappears and reappears. From the data, the log10 of the PAR
values were plotted to form an absorption curve and a linear regression line was calculated. The 1%
surface light value was calculated by taking the antilog of the interception value “A” in the regression
line formula Y=A+BX and multiplying by 0.01. The euphotic zone depth was calculated by inputting the
log10 of the 1% light value into the regression line formula. The slope in the regression line equation
corresponds to the PAR extinction coefficient. The euphotic zone depth was then divided by the Secchi
disk depth to determine if the ratio corresponds to the theoretical value of three.
Results:
The results show that Muir Lake, on average, had the highest PAR values recorded followed by Cargill,
Lake Two, and Devonian Lake (Table 1). The Secchi disk depths did not follow the same pattern. Lake
Two had the largest mean depth, followed by Muir, Cargill, and Devonian. (Table 2). The euphotic zone
depth was largest for Lake Two, followed by Muir Lake, Cargill Lake, and Devonian Lake. Based on the
Table 1: PAR (photosynthetically active radiation) values for the four study lakes at 6 depth intervals
PAR
(μmol/m²/s)
Muir Lake
0m
log10
1m
log10
2m
log10
3m
log10
4m
log10
5m
log10
6m
log10
1404.00
3.15
300.00
2.48
54.00
1.73
15.00
1.18
4.00
0.60
1.00
0.00
0.00
n/a
average
PAR
254.00
Cargill Lake
1513.00
3.18
102.00
2.01
9.00
0.95
3.00
0.48
0.30
-0.52
0.06
-1.22
0.00
n/a
232.48
Lake Two
944.00
2.97
165.00
2.22
29.00
1.46
5.00
0.70
2.00
0.30
1.00
0.00
0.00
n/a
163.71
Devonian Lake
754.00
2.88
49.00
1.69
8.00
0.90
0.60
-0.22
0.10
-1.00
0.00
n/a
0.00
n/a
115.96
size of the Secchi disk depth and the euphotic zone depth, Lake Two had the largest light penetration
(Table 2 &3). The euphotic zone depth to Secchi disk depth ratio did not equal 3 in any of the study
lakes. Lake Two showed the smallest ratio where the euphotic zone was calculated to be 4.02 times the
size of the Secchi disk depth. Cargill Lake had the highest ratio where the euphotic zone was 4.88 times
the Secchi disk depth, which is still well over the theoretical value of 3. Overall the average ratio of the
four lakes showed that the ratio of euphotic zone to Secchi disk depth was 4.46.
Table 2: Average Secchi Disk Depths for the 4 study lakes
Secchi Disk Depths (cm)
Rep 1
Rep 2
Rep 3
Mean
(m)
Muir Lake
63
72
64
66
0.66
Cargill Lake
51
47
45
48
0.48
Lake Two
77
81
88
82
0.82
Devonian Lake
54
45
52
50
0.50
Table 3: Ratio calculations for Secchi disk depth to euphotic zone depth
euphotic zone
A value
1% surface light depth (m)
log10 1% surface light
depth
Muir
3.09
12.25
Cargill
2.96
9.14
1.09
0.96
3.19
2.33
4.81
4.88
Lake two
Devonian
2.80
2.78
6.36
6.07
0.80
0.78
3.27
2.07
4.02
4.11
Average
2.91
8.46
0.91
2.72
4.46
Table 4: Study Lake dimensional statistics
Lake
Area (km2)
Devonian
0.15
Two
0.05
Muir
0.06
Cargill
0.06
Volume (m3)
658487
206693
257165
175439
Euphotic zone depth
Max depth (m)
8.1
11.2
9.6
7.7
ratio
Mean Depth (m)
4.5
3.8
4.0
3.0
Discussion:
The data show that the ratio of euphotic zone depth to Secchi disk depth was not equal to 3 in any of
the study lakes. The source of error is likely due to the subjectivity in the use of the Secchi disk. When
doing experimentation with the Secchi disk, measurement protocols include measurement of the depth
at which the disk disappears, and then re-appears and then using the average of both values to
eliminate errors imposed by possible residual memory effects, and by different operators (Hou et al.,
2007). This protocol was not followed in the experiment which could have led to errors in the Secchi disk
data collection. Other factors that can affect the Secchi disk reading are personal characteristics such as
the eyesight and age of the viewer, their personal experience, the distance between the observer and
the water surface, cloudiness, and height of the sun (Bledzki, 2010). Devonian Lake had the smallest
eutrophic zone depth, this could be due to the large area of the lake and the volume that it holds. This
lake is over twice the area and holds approximately more than twice the volume of the other lakes
(Table 4). This large surface area could result in greater re-suspension of organic and inorganic
sediments from winddriven currents and wave action, causing erosion, and resulting in increased
turbidity (Howick & Wilhm, 1985). Cargill Lake had the largest ratio of Sechi disk depth to euphotic zone
depth (Table 3). This could be from the type of particles in the lake. The lake may have large quantities
of particles which affect the sight of the Secchi disk, but do not absorb in the 400-700nm range.
Another source of error could have occurred statistically because of the generally small sample sizes for
Secchi disk measurements.
Conclusion:
In this experiment, we observed that the ratio of Secchi disk depth to euphotic zone depth is highly
variable. Because of the imprecision of the methods, there is a chance that a variety of errors could
occur. In general, the values that were obtained were not extraordinary and the euphotic zone averaged
roughly 4 times the Secchi disk depth. Although the results were variable between study lakes, the
suitability of the light environment in each lake was relatively similar ranging from 2-4 meters of
euphotic zone depth.
References:
Bledzki, Leszek. 2010. "Secchi disk". Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.:
Environmental Information Coalition, National Council for Science and the Environment). Retrieved October 16,
2011 from: http://www.eoearth.org/article/Secchi_disk
Hou, W., Lee Z., Weidemann A.D. 2007. Why does the Secchi disk disappear? An imaging Perspective. 15: 6. Optics
Express. Pp. 2791-2802. Retrieved from : http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-15-6-2791.
Howick, G.L., Willhm J. 1985. Turbidiy in Lake Carl Blackwell: Effects of Water Depth and Wind. Proc. Okla. Acad.
Sci. 65:51-57. Retrieved from: http://digital.library.okstate.edu/OAS/oas_pdf/v65/p51_57.pdf.
Appendix:
Light values at various depths on Muir Lake
3.500
3.000
LOG10(PAR) light values (μmol/m²/s)
2.500
y = -0.6262x + 3.0881
2.000
1.500
LOG10 PAR light values
1.000
Linear (LOG10 PAR light values)
0.500
0.000
0
1
2
3
-0.500
4
5
6
Depth (m)
Figure 1: PAR value data for various depths on Muir Lake
Light at various depths in Cargill Lake
LOG10(PAR) light values (μmol/m²/s)
3.500
3.000
2.500
y = -0.8594x + 2.9611
2.000
1.500
1.000
LOG 10 of PAR light values
0.500
0.000
-0.500
0
1
2
3
-1.000
-1.500
-2.000
Depth (m)
Figure 2: PAR value data for various depths on Cargill Lake
4
5
6
Light values for various depths on Devonian Lake
3.500
LOG10(PAR) light values (μmol/m²/s)
3.000
2.500
2.000
1.500
Log10 PAR light values
1.000
Linear (Log10 PAR light values)
0.500
0.000
0
1
2
3
4
5
-0.500
-1.000
y = -0.9667x + 2.7831
-1.500
Depth (m)
Figure 3: PAR value data for various depths on Devonian Lake
Light values at varous depths on Lake Two
3.500
3.000
LOG10(PAR) light values (μmol/m²/s)
2.500
y = -0.6111x + 2.8035
2.000
LOG10 of PAR light values
1.500
Linear (LOG10 of PAR light values)
1.000
0.500
0.000
0
-0.500
1
2
3
Depth (m)
Figure 4: PAR light values for various depths on Lake Two
4
5
6