A Preliminary Investigation on Turbidity and Chlorophyll-a patchiness
in Lake Tanganyika, Kigoma Bay
Student: Sandra Baldwin
Mentor: Pierre-Denis Plisnier
Introduction
A possible linkage may exist between biological patchiness and hydrodynamic mixing events. This was
investigated by measuring the spatial and temporal variations in chlorophyll-a, temperature, and wind
events. The hypothesis is that during periods of cool temperatures and high wind velocities the water
column is unstable and deep nutrient rich water is more likely to mix with the epilimnion bringing useful
nutrients to phytoplankton in the euphotic zone. The mixing due to internal waves may be a controlling
factor for higher chlorophyll-a concentrations, higher turbidity levels and lower Secchi disc transparency
throughout the first 60 meters.
Study Area, Materials, and Methods
For the study beginning on the 7/19/00, and continuing through 8/4/00, three nearshore to offshore transects
were chosen based on location and coastal substrate. Transect A is located in Kigoma Bay, partially
protected from open water. Transect B abuts the mountains that extend down to the water line, and transect
C extends from a cliff point out into the lake, and is exposed to open water. The shore of transect A has a
sandy substrate while B and C have large pebbles. Along each transect, three sites were chosen for study,
one at the shore, one intermediate site 200m offshore and an offshore site about 500m from shore. The
exact latitude and longitude positions are detailed in table 1. In an effort to remain constant through the
experiment, it was important to sample at the same time each morning. The sampling period began around
8:00 am at line A and continued with line B and C before returning to site A for a second sampling at about
12:00.
The following materials were used: 20’ boat, GPS, spectrophotometer, Whatman filters, Secchi disc,
turbidity meter, temperature gauge, thermistors and a Nansen sampling bottle.
Samples were taken at depths of 0, 20, 40 and 60 meters at each of the three pelagic sites in order to
investigate vertical change. The pelagic sample of site A was measured in the early morning and again in
the early afternoon for a daily vertical comparison. Horizontal change was monitored using surface
samples filtered from the coastal, intermediate and pelagic sites. A recording thermistor was placed at each
of the coastal sites near the shore, and a recording thermistor chain with thermistors positioned at 10, 40,
70, 100, and 110 m was placed at a position of 4°51.13 S, 29°35.79 E. A standard 25cm Secchi disc was
used to measure water transparency. Turbidity was measured using a turbidity meter (Hach) and results
were expressed as nephelometric turbid units (NTU). Chlorophyll-a was measured by filtering four liters of
sample water through a Whatman filter, and extracting the chlorophyll-a pigment using methanol over a
period of 24 hours, and then measuring the absorbence of 665nm wavelength light using a
spectrophotometer.
Results
Vertical Analysis
A temporal chlorophyll-a comparison revealed that the morning values ranged between 0.79 and 2.5
micrograms per liter, decreased throughout the morning in the surface to range between 0.55 and 1.08
micrograms per liter by the early afternoon (figure 1). This follows closely with the data recorded by R.E
Hecky and E.J. Fee in their study of chlorophyll-a values at Kigoma Bay (Hecky and Fee, 1981). Spatially
there seems to be little change in chlorophyll-a with depth, and the water was basically homogeneous
except a chlorophyll-a peak that seemed to develop around 20-40 meters in the afternoon data.
TABLE 1: Location of sites sampled
Site location
Coastal
Intermediate
Pelagic
A
4 52.82 S
29 37.35 E
4 52.55 S
29 36.53 E
4 52.66 S
29 35.17 E
B
4 51.17 S
29 36.61 E
4 51.12 S
29 35.20 E
4 51.18 S
29 34.75 E
C
4 55.38 S
29 36.70 E
4 55.58 S
29 36.63 E
4 56.05 S
29 36.60 E
TABLE 2: Chlorophyll A and turbidity average, standard deviation and number of observations.
Chlorophyll a
average
STD
Count
Trubidity
average
STD
Count
Coastal
2.30
1.02
19
Coastal
0.31
0.16
20
Intmediate
1.43
0.75
19
Intermediate
0.18
0.05
20
Pelagic
1.25
0.37
20
Pelagic
0.18
0.10
20
A. Vertical Comparison of Chlorophyll (am)
Chlorophyll-a (ug/L)
Depth (m)
0
0.5
1
1.5
2
2.5
0
10
20
30
40
50
60
7/19/00
7/21/00
2/8/00
4/8/00
7/25/00
7/27/00
7/31/00
B. Vertical Chlorophyll Comparison (pm)
0
0.5
Chlorophyll-a (ug/L)
1
1.5
2
2.5
0
Depth (m)
10
20
30
40
50
60
7/19/00
7/21/00
7/25/00
Figure 1:Vertical Chlorophyll Comparison
7/31/00
2/8/00
4/8/00
0,60
3,00
0,50
2,50
0,40
2,00
0,30
1,50
0,20
1,00
0,50
0,10
0,00
0,00
Coastal
Intermediate
Turbidity (NTU)
Chl a (mg/l)
3,50
Pelagic
Chlorophyll a
Turbidity
Figure 3 : Average concentrations (+ standard deviations) of chlorophyll a and turbidity
at three coastal, intermediate and pelagic sites near Kigoma.
Chlorophyll a - turbidity relationship
R2 = 0.504
Turbidity (NTU)
y = 0.0948x + 0.0634
0,40
0,30
0,20
0,10
0,00
0,5
1,5
2,5
3,5
Chl a (mg/l)
Figure 4: Linear regression of average chlorophyll concentrations v.s. turbidity measurements
at the sampled sites.
Temperature (c)
Wind Speed (m/s)
0
0.5
1
1.5
2
2.5
3
7/19/00 12:00 AM
7/21/00 12:00 AM
Figure 5: Coastal water temperature (A) and wind speed (B)
24.50
7/17/00 12:00 AM
25.00
25.50
26.00
26.50
27.00
27.50
TsiteA
7/23/00 12:00 AM
TsiteC
7/25/00 12:00 AM
TsiteB
B.Wind Speed
96 per. Mov. Avg. (TsiteB)
7/27/00 12:00 AM
7/31/00 12:00 AM
96 per. Mov. Avg. (TsiteA)
7/29/00 12:00 AM
96 per. Mov. Avg. (TsiteC)
A.Coastal water temperature
8/2/00 12:00 AM
8/4/00 12:00 AM
8/6/00 12:00 AM
Horizontal Analysis
A temporal chlorophyll-a comparison revealed two distinct periods. The first between 7/25/00 through
7/30/00 where the chlorophyll-a values were high and then a second from 7/30/00 through 8/2/00, where
the chlorophyll-a values decreased (figure 2). The first few days of data are ignored due to sampling error.
Spatially there was not much change between transects, but a large difference was observed (1
microgram/L) between near-shore and offshore sites within each transect (figure 4), with higher
concentrations at the near shore sites.
Discussion
Vertical Analysis
Temporally the noticeable peak of chlorophyll-a in the afternoon around 20-40 meters was most likely
caused by photoinhibition, as some of the plankton may deepen in the water column because of the increase
in sunlight throughout the day. They may adapt their depth to find the optimal conditions of radiation.
(figure 1). Deeper in the water column the decrease in chlorophyll-a concentrations was most likely a
result from a decrease in sunlight, and nutrients.
Horizontal Analysis
Horizontally at the surface, the most probable controls over the fluctuating chlorophyll-a values were
meteorological events. Figure 5 compares wind speed data and coastal water temperatures over the study
period. There is a noticeable decrease in water temperatures and a noticeable increase in wind speed during
the period between 7/25/00, and 8/2/00. This is the same period of time during which an increase in
chlorophyll-a concentrations can be seen (figure 2). There may be a link between high wind speed, colder
temperatures, and an increase in the mixing of nutrient rich water from the deep as can be seen in figure 6.
Two processes effect the nutrient distribution. Winds induce internal waves, and colder water temperatures
induce mixing of the epilimnion, and when these two processes are combined nutrient rich water is
supplied to the surface, providing the phytoplankton with nutrients. This is supported by the Secchi disc
data, and turbidity measurements. As the chlorophyll-a values increase, the Secchi disc depths decrease,
and turbidity measurements increase (figure 2). The period between 7/30/00, and 8/2/00 saw an increase in
coastal temperatures, a decrease in wind speeds, and a decrease in chlorophyll-a values. Again this
supports the link between meteorological events, and chlorophyll-a fluctuations.
Spatially there was not a large difference between transect lines perhaps due to their relatively close
proximity, however there was a noticeable increase from offshore to inshore. The shore site had about a 1
microgram/liter higher concentration than the offshore sites (table 2). This could suggest a close proximity
to a nutrient source, coastal upwelling, or the resuspension of material due to wave energy (figure 3+4).
Conclusion
Chlorophyll-a may have a possible link to meteorological processes such as wind speed and water
temperature. By comparing the chlorophyll-a values at sites chosen intentionally with different spatial
locations, different distances from shore and different vertical depths in the pelagic, a possible link is
suggested from observations of extreme wind events, water temperature anomalies and the changes seen in
the chlorophyll-a levels. High winds and cold temperatures cause less stratified water, which allows the
epilimnion to mix, bringing nutrients to the surface and possibly raising chlorophyll -a values.
Recommendations
Some points to improve the sampling method are to make sure the positions are all the same distance from
shore. This regularity is important to maintain so the sites are comparable. Also sampling should occur at
the same time every day due to daily fluctuations, again so they can be compared easily. For accurate wind
data ideally measurements should be taken at each site, instead of relying on data collected from a weather
station in a protected bay such as Kigoma. The most important change however, would be to sample over a
Temperature (C)
26.5
26
25.5
25
24.5
24
23.5
23
7/21/2000 12:45
7/21/2000 23:15
7/22/2000 9:45
7/22/2000 20:14
7/23/2000 6:44
7/23/2000 17:14
7/24/2000 3:44
7/25/2000 0:44
7/25/2000 11:14
7/25/2000 21:44
7/26/2000 8:14
Temperature Depth Comparison
Figure 6: Temperature of water as recorded by the thermistors
7/24/2000 14:14
7/26/2000 18:44
7/27/2000 5:14
7/27/2000 15:44
7/28/2000 2:14
7/28/2000 12:44
7/28/2000 23:14
7/29/2000 9:44
7/29/2000 20:14
7/30/2000 6:44
7/30/2000 17:14
7/31/2000 3:44
7/31/2000 14:14
8/1/2000 0:44
8/1/2000 11:14
8/1/2000 21:44
8/2/2000 8:14
8/2/2000 18:44
8/3/2000 5:14
8/3/2000 15:44
8/4/2000 2:14
T0m
T30m
T60m
T70m
T90m
T100m
longer period of time in order to document the changes in chlorophyll-a and changes in weather, and make
a better comparison.
Acknowledgements
I would like to thank Pierre-Denis Plisnier, Wilfried Mbemba, Robert, Kadula and professor Halfman for
their help and support. I would like to extend a special thanks to the Nyanza project for an incredible
experience!
References
Hecky, R.E and Fee, E.J. 1981. Primary production and rates of algal growth in Lake Tanganyika. Limnology & Oceanography,
26(3): 532-547.