Assessment of Nutrients and Physical Parameter Distribution as Indicators of the
Kandaga Shallow Closed Lake Fluctuations
Student: Fatuma Kyando
Mentor: Hudson Nkotagu
Introduction
Lake Kandaga is located at S 29o 52’ and E 4o 58.35’ at elevation of 951m. It is a shallow closed lake with
a maximum depth of 3.7 m at its west side. The lake has a total area of approximate 0.4 km2 (Figure 1).
The wind stress on the lake causes shear fluctuation. Relaxation of the wind is followed by density
redistribution. The lake has direct entrainment effected by turbulent mixing and convective penetration at
the surface. The wind stress causes both surface and internal waves that ultimately result in almost total
mixing of the lake.
Objective and Hypothesis
The main objective of this study is to assess the distribution of nutrients and other physical parameters in
the Kandaga closed lake. The study hypothesis is that the Kandaga shallow closed lake is a well-mixed
water body with little or no stratification.
Material & Methods
Field methods
A two-day sampling campaign was conducted. During the first day, samples were taken from the surface of
the lake to the bottom, at regular intervals, at each of eight sites. The second sampling campaign was
conducted at two sites, with samples taken from the surface to the lake bottom at each site at three hours
intervals, from morning to evening. The following parameters were determined: temperature, dissolved
oxygen and conductivity (measured using a Multi Probe); pH (measured using a pH meter); turbidity
(measured by HACH Turbidmeter); and transparency (measured with a 20 cm diameter secchi disk). Water
samples were kept in a cooler box and taken to the laboratory for nutrient and other physical parameter
determinations.
Laboratory Methods
The colorimeter based HACH Analytical procedures for DR/2010 were used. Total phosphorous from
unfiltered water samples was determined after acid digestion, followed by ascorbic acid addition. Alkalinity
was determined by using titration with sulfuric acid, after addition of phenolphthalein and bromcresol
methyl red indicators. Water samples were filtered before analyzing PO4-3, NH4+, NO3-, NO2- SO4-2, Cl- and
Fe+2. Soluble Phosphorus was determined following PhosVer3 method. Nitrite was analyzed using
Diazotization method, while Nitrate was analyzed using the Cadmium reduction method. Ammonia was
measured with the Nessler method for high range ammonium. Silica was determined following the
Silicomolybdate method for high range.
Results
Water temperature decreased with depth, being high during the afternoon, but ranged between 22.3 to 25.0o
C. Electrical Conductivity (EC) values ranged between 185 and 199µs/cm, being high during the
afternoons, but decreasing with depth. The average surface EC value is 191.4µs/cm. pH values were
recorded between 7.9 and 8.7 averaging at the surface at 8.3. The highest readings were determined during
the afternoons and decreased with depth.
Dissolved Oxygen (DO) concentrations decrease with depth. The highest dissolved oxygen observed was
122% during afternoon, with the lowest values being recorded in the morning at 80% (see Figures 2 and 3).
Turbidity ranged between 5.6 to 5.85 NTU. It fluctuated with time, with the highest turbidity being
observed during the evening at the lake bottom and the lowest value observed during the afternoon (see
Figure 4). Transparency fluctuated between 0.7 m and 1 m, with the highest transparency depth recorded at
Sites 8 and 5 at 1 m depth. In general, alkalinity decreases with depth. The highest values of alkalinity as
bicarbonate were recorded during the morning, at 90.0 mg/l with the lowest value recorded at 70.0mg/
(Figure 5). Chlorophyll a recorded its highest value of 24.0 mg/l at the lake bottom and lowest value of
13.0 mg/l at the surface in the afternoon (Figs.3,4,& 6).
Nutrients
Total phosphate concentrations varied over time. The highest concentration was observed at the surface
during the afternoon. Total phosphates ranged between 0.1 mg/L and 4.66 mg/L (Fig. 5). Soluble
phosphorus concentration ranged between 0.41mg/L and 0.55mg/L with the highest concentration obtained
at the lake bottom during the afternoon (Fig.6.0) Ammonia ranged between 0.02 to 0.18mg/L. the peak
concentration was at 1m during the mornings. Nitrate ranged from 0 to 0.04mg/L with peak concentration
observed during the afternoon. Nitrite concentration varies from 0.002 to 0.004mg/L. The highest
concentrations of nitrite were observed during the morning. (Fig.6.0). However, the concentration values
for sulfate are almost constant at 1mg/L over both time and space. Generally silica concentration decreases
with depth, fluctuating between 25.6 and 37.1mg/L with the highest concentration recorded during the
mornings. (Fig.5). At site 4.0 where measurements were conducted at different times of the day the values
ranged from 21.0 to 24.5 mg/l (Fig.6.0).
Discussion
The range of temperature from the surface to the bottom is small but the diurnal fluctuation is high
indicating that the lake is a very shallow well mixed system. The secchi disk results show that the incident
sunlight penetrates down to the lake bottom albeit at different intensities.
Dissolved oxygen and pH values fluctuate on diurnal bases with the lowest pH and dissolved oxygen
observed in the mornings and a subsequent slow rise during the day. This is probably due to turbulent
mixing and photosynthetic activity. During the daytime photosynthesis process results in increased oxygen
as per the following equation CO2 + 2H2O Light CH2O + 2O2.
High concentration of Chlorophyll-a at the lake bottom was recorded and may be attributed to nutrients
accumulation at the bottom that resulted in a lot of algae biomass attraction.
However, the lowest values of dissolved oxygen at the lake bottom are observed probably due to the
oxidation of the organic matter and the respiration of the organisms that results in the consumption of
oxygen. Alternatively low values of dissolved oxygen may reflect decreased photosynthetic activity due to
decreased mixing or light levels.
Alkalinity as bicarbonate decreases with depth indicating that a lot of CO2 is consumed at the lake bottom
during both photosynthetic and non photosynthetic processes and probably also as a result of Fe reduction
processes.
The concentration of reactive phosphorus is relatively high (Fig 6), according to Hutchinson (1957) the
highest concentration of the reactive phosphorus is due to the fall of the oxygen concentration with
subsequent liberation of soluble phosphorus that occurs at low pH.
Different parts of the lake show variable nutrient concentrations and other physical parameters this may be
attributed to variable exposure of different parts of the lake to the wind intensities due to the lake’s variable
physiography (Figs.1, 2 and 5). Wind intensities produce kinetic energy to water thus generating both
surface and internal waves that result in variable distribution of nutrients concentrations and other physical
parameters.
Nutrients show strong variation with time. This might be due to their redistribution through mixing caused
by both surface and internal waves generated by variable wind action on the lake (Fg. 6). In addition
photosynthesis process may result in production of oxygen that affects the solubility of inorganic nutrients
like NO2-, Fe+2 etc.
Generally, the highest concentrations of the nutrient are observed at the lake bottom during the mornings
and this may probably be attributed to regeneration from the lake sediment and particulate matter found at
the bottom.
Conclusions
The nutrients and other physical parameters data show that the lake is a well mixed water body. In addition,
the results also reflect a very strong diurnal lake fluctuation patterns. Furthermore, the results indicate that
the lake mixing is highly variable in both time and space and is predominantly caused by wind action.
Finally, different parts of the lake have variable nutrient concentrations as well as other parameters.
Recommendations
1.
Detailed sampling covering many sites and seasons in the lake should be conducted in order to
find out seasonal influences on the nutrients concentration as well as other physical parameters.
2.
Meteorological data at the lake including wind intensities should be collected on adaily basis in
order to evaluate their influence on the hydrodynamics of the nutrients and other physical
parameters at the lake.
3.
Nutrients and other physical parameters should be determined from sediment interstitial waters in
order to evaluate their regeneration influences.
Acknowledgements
I wish to thank the Nyanza project for financial support of this project. In addition I would like to
acknowledge the following: Dr H.H. Nkotagu for his critical suggestions and mentoring this work. W.
Mbemba for his support during analytical and fieldwork. K. Chororoka and Christine Gans for field
assistance and computer work, Dr. A. Cohen, Dr C. O’Reilly, Dr. E. Michel and Dr. K. Lezzar for their
logistical support. All the Nyanza students and staff for lively discussions and exchange of ideas that
rekindled my scientific curiosity.
References
Cohen, A.S. 2003. Paleolimnology: The history and evolution of Lake Systems. Oxford University Press.
New York.
Ramadhani, S. 2000. Nutrient redistribution in relation to hydrodynamic changes and primary production: a
short-term study at Kigoma station. The Nyanza Project 2000 Annual Report.
HACH Company. 1992. Water Analysis Handbook (Analytical procedures for DR/2010) 2nd Ed Loveland
Colorado USA 831 pp.
Hutchinson, G.E., 1957. A Treatise on limnology ,V1 Geography, Physics and chemistry. J. Wiley and
Sons, New York.
Figure 1: Location of the Lake Kandaga showing the sampling sites.
Chlorophyll a and DO% Vs Time
124
Variation of DO% with Depth
25
122
120
114
112
10
110
108
5
106
Surface
0.6-1m
120
1.8-2.4m
2,8-3.5m
110
DO%
15
Chlorophyll a mg/L
116
DO%
130
20
118
100
90
104
102
0
Morning
DO %
Aftrernoon
Time
Evening
80
70
Chlorophyll a
0
2
4
6
Sites
Figure 2
Figure 3
8
10
Turbidity and Chlorophyll a Vs Time
25
5.9
5.85
5.8
5.75
15
5.7
10
Turbidity
Chlorophyll a
20
5.65
5.6
5
5.55
0
5.5
Morning
Aftrernoon
Time
Chlorophyll a
Evening
Turbidity NTU
Figure 4
Nutrients Vs sites in bottom and surface
3.5
2.5
1.5
0.5
-0.5
1 2 3 4 5 6 7 8
surface
1 4 6 3 7 5 8 2
Bottom
sites
T.Phos mg/L
Nitrate (mg/L)
Figure 5
R.Phos(mg/L)
Silicamg/L
NH4 (mg/L)
alkalinity
0.03
0.50
0.02
0.40
0.02
0.30
0.01
0.20
0.01
0.10
0.00
0.00
Morning
Afternoon
Evening
Time
R. PO4-3(mg/L)
Nitrite(mg/l)
Figure 6
NH3-N(mg/L)
NO3-N(mg/L)
NO3-N and NH3-N (mg/L)
Nutrients (mg/L)
4.5
0.60
R.PO4-3 (mg/L)
100
90
80
70
60
50
40
30
20
10
0
silica and alkalinity
5.5
Nutrients concetration Vs Time