Limnological studies in Lake Victoria (Kenyan portion) have been sporadic. Water quality and nutrient dynamics studies are being undertaken in fifteen sampling sites that have been divided into four ecological zones namely: Nyanza Gulf, Rusinga Channel, open waters inshore and open waters. The ongoing study will show how the physical and chemical paramenters affect fish distribution and abundance

Kenyanya, M.M.

English

Aquatic Commons

Water quality and nutrient dynamics Lake Victoria, Kenya
M.M. KENYANYA, Kenya Marine and Fisheries Research Institute, FO Box 1881,
Kisumu, Kenya
A.bstract Lirnnological studies in Lake Victoria (Kenyaii portion) have been
sporadic. Water quality and nutrient dynamics studies are being undertaken in fifteen
sampling sitcs that have been divided into foui ecological zones namely, Nyanza
Gulf, Rusinga Channel, open waters inshore and open waters. The ongoing study will
show how the physical and chemical parameters affect fish distribution and
abundance.
Introduction
Lake Victoria, the largest lake in Africa (68 000 kn2), is the most important
freshwater resouice foi the people living in its catchrnent Fshing and associated post-
harvest activities are amongst the most important economic activities of the region.
Also a rapidly gi owing population of more than i million in its catchment depends
ori the lake for supply of protein.
In the last 30 years, the waters of Lake Victoria have becii progressively enriched with
nitrogen and phosphorus (Ochumba & Kibaara 1989). This has led to the
accumulation of organic matter in the water and changes in its chemical properties
often resulting in the degradation of the water with respect to its suitability for
consumptive, industrial and agricultural uses
Ecological, studies in the Lake Victoria, especially those related to physical and
chemical characteristics of the water have received little attention. Studies have
focussed on alien fish introductions (e.g. Ogutu-Ohwayo 1990), macrophytes (e.g.
Gichuki & Odhiambo 1994) changes in fish compositions (e g Okemwa 1984) and
algal blooms (e.g. Ochuruba & Kibaara 1989). This is considered a nalToW focus, and
to understand the dynamics of the fisheries, an important food resource for the
peoples of the legion, there is a fundamental lequnement to determine ho physical
and chemical parameters affect the distribution and abundance of the fish stocks. This
paper presents preliminary findings ot limnnological studies in the Kenyan sector of
Lake Victoria, as a precursor to more in depth analysis of the way physical and
chemical parameters affect the fish stock dynamics.
Materials arid methods
The Kenyan part o Lake Victoria is at 1134 mn above sea level aud is located between
0°04'S - 00 32'S 34° arid l3'E - 34°52'E. Sampling Sites were selected to be
representative of the Kenyan waters of the lake and indicative of the experimental
trawling stations.
Fifteen sampling sites were selected in four ecological zones (Fig. 1). The Nyanza
Gulf zone was represented by eight sites: Kisumu Pier [10]; Sondu Miriu [2]; Awach
River Mouth. [3]; Homa Bay Pier [9]; Asembo Bay [26]; Gingra Rock [4]; Maboko;
118
and Madundu. The Rusinga Channel [34] was the second zone. The third zone was in
the open waters outside the Nyanza Gulf, and was sampled at Bridge Island [32]. The
fourth zone comprises the Yala River mouth [54], Sic) River mouth and Nzoia River
mouth[53], in the north of the lake, and the Kuja Rivei mouth and Muhuru Bay, in the
south.
Samples were collected, where possible, on a monthly basis between October 1997
and July 1999 to obtain informatíon on the main physical and chemical characteristics
and assess how they affect natural production.
In situ measurements for temperature and dissolved oxygen were made using a YS1
model 57 oxygen meter, pH was measured using a digital mini model 49, and
conductivity measured using a microprocessor conductivity meter LF 96. In addition,
redox potential was measured automatically using a Hydrolab Surveyor II connected
to an SVR 2 --DU display and 401 -CA circulator assembly. Turbidity was measured
using a Hatch wrhidimeter 2 lOOP. Light penetration was taken as extinction of a 20-
cm black and white secchi disc. Total alkalinity and total hardness were determined
by titrating 100 ml of each sample against a standard solution of a strong mineral acid
and ammoni urn hydroxide respectively.
At each station, and on each occasion, water samples were collected in polyethylene
bottles for nutrient analysis. They were preserved with I mL mercuric chloride and
stored at 4°C in the laboratory. The samples were filtered through a cellulose acetate
membrane with nominal pore size of 0.45 tm. The filtrate was saved for analysis of
dissolved chemical species as recommended by Gems (1992) and Mackereth et al.
(1978).
Results
Marked differences in the mean dissolved oxygen concentration [02] (mg U') and
temperature (°C) profiles were found between sampling stations and sampling zones
(Fig. 2). Two sampling stations had levels of [021 below 3 mg L', which is considered
the critical level for survival of the major commercial fish species (The optimum [07]
for fish survival is 5 mg U', critical [021 is 3 mg U' and levels below 3 mg L' are
considered lethal). These two sampling stations in the Nyanza Gulf zone were Awach
River mouth and Homa-Bay pier, both of which are characterised by extensive
coverage of water hyacinth, Eichhornia crassipes (Mart.) Soims, and shallow depths.
Kisumu pier and Mahoko stations in the Gulf zone had mean 107] of >5 mg U' at the
surface but critical (3 mg U') at the bottom. The remainder of the sampling stations in
the Gulf (Sondu Miriu, Gingra Rock and Madundu) baci mean dissolved oxygen
concentrations of 5 mg L' or more.
Stations in the inshore region of the open lake that exhibited a decrease in [021 to
critical levels as depth increased were Muhuru Bay, Yala River mouth and Nzoia river
mouW. Muhuru Bay is a deep sampling station (16 m deep) and exhibited thermal
stratification, hence the decrease in the level of oxygen in deeper water. Flowever,
Yala River mouth and Nzoia River mouth are no deeper than 5 m, and the low [Ox] is
probably linked to the influence of the river discharges which carry considerable
organic effluent and have high suspended solids loadings.
119
Bridge Island, another open water zone, is a deepwater station (41.2 m), which
exhibits the effect of thermal stratification, with a sharp drop in [02] from 5 mg U1 to
<3 mg U' around the thermocline at about 30 m. Although Rusinga Channel is a deep
water station (52 m), there was no evidence of thermal stratification or a degradation
of the [02] with increasing depth. This is probably because the water currents between
the inner gulf region and the open water of the lake continually mix the waters.
Mean chlorophyll-a (mg U') concentrations showed contrasting results between the
Nyanza Gulf sampling sites and those in the open waters (Table 1). Considerably
higher values were found within the gulf, especially around urban centres, possibly
reflecting nutrient input and cultural eutrophication.
Mean nitrate (mg U1) concentrations in the different zones (Table 1) showed similar
trends to those of chlorophyll-a, with the highest levels (0.33 mg U') in the Gulf
waters below the Sondu Miriu mouth, and the lowest concentrations (0.02 mg U') in
the open waters around the Nzoia River mouth.
Mean phosphates (pig U') concentrations (Table 1) showed no obvious trend, although
higher levels were again found around urban centres and around the discharge points
of the major rivers, the latter possibly indicative of fertilizer run-off.
Silicate concentrations (mg U') were much higher in the gulf than in the open waters
Table 1).
The eutrophic effects, were further supported by the mean secchi disc depths, which
were low in the gulf waters but generally higher in the open waters, the exceptions
being where major rivers discharge into the lake (Table 1).
Discussion
The limnological surveys of the Kenyan waters exhibit a number of features that are
indicative of the water movements, thermal stratification, pollution, eutrophication
and primary production processes.
Thermal stratification only occurs in the deeper waters and was most prevalent around
Bridge Island, and to a lesser extent at Yala river mouth and Muhuru Bay. Rusinga
Channel, which is also a deepwater site, did not exhibit stratification, presumably
because of the continuous mixing of waters between the open lake and the Nyanza
Gulf.
Depleted oxygen concentrations were prevalent throughout the lake but fell to critical
levels for survival of fish around Homa Bay and Awach River mouth. These sites
were characterised by extensive coverage of water hyacinth, which is known to
deplete oxygen below the vegetation.
The mean concentration of chlorophyll a in the water, an indicator of primary
productive, also varied between regions. Chlorophyll a levels were highest in the
Gulf zone and declined towards open waters. It is important to note that this trend is
not reflected in the nutrient (PO4 and NO3) levels, suggesting that the latter are not
limiting factors to primary production. This is probably more a reflection of nutrient
120
enrichment which is continually taking place around the inshore waters from urban
and agricultural sources. This is particularly noticeable around the mouths of the
major rivers which drain agricultural land, e.g. the Sondu Miriu, and large urban
centres like Homa Bay and Kisumu.
The high chlorophyll concentrations in the Gulf area are a result of higher densities of
diatoms and dinoflagellates, factors which suggest that eutrophication is prevalent in
the Gulf region. The Gulf also has higher silicate concentrations than open waters.
Silicates are important for diatom cell wall development.
These variations in dissolved oxygen concentrations, nutrient availability and
phytoplankton abundance all have implications for the fisheries production of the
lake. The loss of the many phytoplankivorous haplochromines following the
introduction of Nile perch, Lates niloticus (L.), has left a food niche in the lake which
has been occupied by Rastrineobola argentea (Pellegrin) and the resurgence of
Yssichroinísfusiformis (Greenwood and Gee) and Y. laparogramina (Greenwood and
Gee), two haplochminines which appear to be able to tolerate the predation pressure
of Nile perch. There is thus a need to continue to monitor the lìmnological
characteristics of the lake both in Kenyan waters and the other countries, to fully
understand the trophic dynamics of the system. Ultimately this information will be
fed into the ECOPATH model to elucidate factors affecting the status of the fish
stocks in the lake.
Acknowitedgements
The survey was ftnded by the European Union Lake Victoria Fisheries Research
Project (Ref: ACP-RPR 227). Other thanks go to the Director, Kenya Marine and
Fisheries Research Institute for both laboratory space and moral support. I also
extend my thanks to all researchers and technical staff of Kenya Marine and Fisheries
Research Institute without whom it would not have been possible.
121
References
Cml R.C.M., Silvester G.T., Posta D.J, Van Oijen M.J.P., Acere T.O. & Bongers G.
(1993) Monographs of the Great African Lakes. Limnology and Hydrology of
Lake Victoria. A bibliograpy of Lake Victoria (East Africa). Final draft. TAC
Fisheries Aquaculture unit 0CC No. 4, ICLARM contribution No. 577,
HEST/TAFIRI report No. 56. International Agriculture Centre, Wagenigen,
Netherlands, 174 pp.
Fish G.R. (1957) A seiche movement and its effects on the hydrology of L. Victoria.
Fisheries Publication, London 10, 1-68.
GEMS (1992) Water quality monitoring handbook. New York, UNEP, WHO, 1992.
Mackereth F.J.H., Heron J. & Tailing J.F. (1978) Water analysis. Some revised
methods for limnologist. Freshwater Biological Association, Scientific
Publication. No. 36. FBA, Windermere, England.
Mavuti K.M. & Litterick M.R. (1990) Composition and distribution and ecological
role of zooplankton community in Lake Victoria Kenya waters. Proceedings
of the International Association of Theoretical and Applied Limnology 24,
1117-1122.
Ochumba P.B.O. & Kibaara D.I. (1989) Observations on blue green algal blooms in
the open waters of Lake Victoria, Kenya. African. Journal of Ecology 27, 23-
24.
Okemwa E.N. (1984) Potential fishery of the Nile perch Lates niloticus Linne.
(Pisces: Centropomidae) in Nyanza Gulf of Lake Victoria East Africa.
Hydobiologia 108, 121-126.
122
Table 1. Mean nutrient concentrations and secchi depths for limnological sampling sites in Kenyan
waters of Lake Victoria.
123
Station Position Silicates Nitrates Phosphates Chlorophyll Secchi
(by GPS) (mg U1) (mg L') (tg L') (jig U') depth (m)
Nyanza Gulf
Kisumu Pier [10] 00°06'09"S 4.10 0.05 41.17 38.07 0.55
34°44'32"E
Sondu Miriu [2] 00°17'OO"S 7.14 0.33 44.75 12.35 0.50
34°44'42"E
Awach river mouth 00°20'17"S 6.30 0.13 77.25 10.68 0.70
[31 34°38'45"E
Homo Bay Pier [9] 00°30'53"S 4.87 0.03 77.25 20.04 0.75
34°27'4l'E
Asembo Ray 126] 0U°l2'35S 5.20 0.03 22.84 37.45 0.75
34°24'33"E
Gingra Rock [4] 00°20'37"S 3.23 0.11 52.63 16.44 1.03
34°26'30"E
Madundu 00°25'32"S 5.03 0.15 36.00 15.60 0.90
34°l9'59"E
Rusinga Channel
Rusinga Channel [34] 00°20'57"S 2.17 0.11 74.92 16.31 1.6
34° 13 '54"E
Open Lake
Bridge Island [32] 00°20'34"S 1.03 0.11 89.71 6.30 2.60
34°06'30"E
Open lake - inshore
Yala river mouth [54] 00°03'36"S 0.67 0.02 32.21 9.77 1.34
34°02' I9"E
Sio river mouth 00°13'47"N 1.89 0.03 22.42 10.80 1.03
34°00'lS"E
Kuja river mouth 00°55'20"S 1.73 0.08 74.71 14.13 0.98
34°07'4 I "E
Muhuru Bay 00°57'49"S 3.59 0.18 75.13 8.13 1.90
34°06'08"E
Nzoiarivermouth 00°03'46"N 1.53 0.02 41.35 7.86 0.75
153] 33°56'40"E
I
32
ft Sto F.M.
Kuja RM.
Muhuru KENYA
"
*
-.
TANZANIA
.-*.
34°O0E 34°30'E
124
D
L I aii0:.ytk*
o 10 20 30 40 50
t , I t I
35°OOE
L IL, 1,1
O°30S
kllornetxes 1OOS
Fig. 1. The Kenyan waters of Lake Victoria, showing the sampling sites () listed in
the text and in Table I *
op
o
I
(D
3
3
3.2
o
01
3
3.4
o
(D
3.2
Temp (°C)
25.5 26.0 26.5 27.0
O
3
O
i
2.0 4.0 6.0
02 (mg L1)
Temp (°C)
0.0 1.0 2.0
02 (mg L')
4.5 5.0 5.5 6.0
02 (mg L)
O
O
(D
3
3
4
o
i
o
(D
.3
4
Temp (°C)
26.1 26.2 26.2 26.3
4.8
1.9 2.0 2.1 2.2 2.3
02 (mg L')
0.0 5.0 10.0 0.0 5.0 10.0 15.0
02 (mg L') 02 (mgr1)
Temp (°C)
26.2 26.3 26.3 26.3 25.5 25.6 25.6 25.6
3.0 5.0 5.2 5.4 5.6
02 (mg L1)
5.8
Temp. C)
25.7 25.8 25.8
O
2
4
o
(D
38
10
12
t,
5
5.5
Fig. 2. Depth profiles for mean oxygen (dashed line, mg L and temperature (solid line C) from
October 1997 - July 1999.
125
5.0 5.5 6.0 6.5
02 (mg L1)
Temp eC)
24.0 26.0 28.0 30.0
Temp (°C)
25.4 25.6 25.8 26.0 26.2 24.0 26.0 28.0
Temp (°C)
25.7
o-
10
20
(D -
-t,
30
2 -
5°
5.0
O
5
lo
D 15(D0
3 25
30
35
41.2
Temp. C)
25.8 25.9 26.0 26.0 26.5 27.0
5.5 6.0 6.5
02 (mg L1)
Temp C)
24.0 24.5 25.0 25.5 26.0
O
2-
3-
3.6
26.0
0-
3i
4
4.5
O
1
4
4.9
Temp. C)
0.0 5.0 10.0
02 (mg L1)
Temp. eC)
26.1 26.1 26.2
Temp. C)
25.6 25.8 26.0 26.2
o
2
4
CD
-t,
3 10
12
14 -
16 -
0.0
O
I
2
5-
5.8 -
25.5
4.0 5.0 6.0 7.0
02 (mg L1)
Fig. 2 cont Depth profiles for mean oxygen (dashed line, mg L-1) and temperature (solid line, OC)
from October 1997-July 1999 126
00 5.0 10.0 5.5 6.0 6.5 0.0 5.0 10.0
02 (mg L1) 02 (mg L1) 02 (mgr1)
Temp. C)
25.4 25.6 25.8 26.0
10.05.0
02 (mgr1)
Temp. eC)
26.0 26.5


Water quality and nutrient dynamics - Lake Victoria, Kenya

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Water quality and nutrient dynamics - Lake Victoria, Kenya

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