Wetlands Ecol Manage (2008) 16:89–96
DOI 10.1007/s11273-007-9072-4
ORIGINAL PAPER
Papyrus wetlands, nutrients balance, fisheries collapse,
food security, and Lake Victoria level decline in 2000–2006
Yustina Andrew Kiwango Æ Eric Wolanski
Received: 19 November 2007 / Accepted: 19 November 2007 / Published online: 25 January 2008
Ó Springer Science+Business Media B.V. 2007
Abstract The future of Lake Victoria and its people
is highly related to the future of its papyrus wetlands.
This appears to be threatened by the overdrawing of
water at two dams at the outlet of Lake Victoria in
Uganda, which can lead to wetland loss, tilapia
fisheries collapse, enhanced eutrophication of the lake,
loss of food security for the empoverished population,
and a measurable contribution to global warming.
Keywords Eutrophication Lake level Papyrus wetlands Dams Tilapia Lake Victoria
Introduction
Lake Victoria, East Africa (Fig. 1), has a surface area
of 96,000 km2. Its shores are shared by Uganda,
Kenya, and Tanzania. It drains 25 large rivers with a
total catchment area of about 184,000 km2, supporting 30 million people.
The mean lake rainfall is 1,315 mm year-1, which is
82% of the total inflow; the remaining 18% is contributed
Y. A. Kiwango (&)
Tanzania National Parks (TANAPA), P.O. Box 3134,
Arusha, Tanzania
e-mail: [email protected]
E. Wolanski
ACTFR, James Cook University, Townsville, QLD,
Australia
from river inflow. Of the outflow, 76% is due to
evaporation from the lake and the remaining 24%
(*23.5 km3 year-1) is the outflow forming the White
Nile River at Jinja (Scheren et al. 2000; COWI 2002).
The lake is increasingly eutrophicated since 1990
(World Bank 1996; Kansiime and Nalubega 1999;
COWI 2002; Odada et al. 2004; Kansiime et al.
2005; Rutagemwa et al. 2006). As much as 75% of
the riverine nitrogen now entering the lake basin
comes from agriculture, and probably 80% of the
riverine phosphorus comes from municipal and
industrial sewage, discharge from urban and agricultural drainage channels, and the dumping of untreated
sewage from villages and small settlements (Scheren
et al. 2000; Odada et al. 2004). There are inadequate
treatment facilities for effluents from point sources
(industries and factories) discharging to the lake
system (Scheren et al. 2000) and no remediation at all
exists for non-point sources effluent. Eutrophication
is apparent near most river mouths and more serious
inshore than offshore (Kansiime and Nalubega 1999).
The watershed includes vast wetlands, both within
the catchments and along the shores of the lake, most of
which are composed of monotypic stands of papyrus
plant (Kansiime and Nalubega 1999). Papyrus is a
large herbaceous reed species with a culm growing up
to a height of 5 m (Jones and Muthuri 1985). Fringing
papyrus wetlands are an important buffer zone protecting the lake from eutrophication. Growing papyrus
absorb organic nutrients from both water and sediment
and thus trap nutrients especially nitrogen and
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Wetlands Ecol Manage (2008) 16:89–96
Fig. 1 Maps of Lake
Victoria basin system
(modified from Kayombo
and Jorgensen (2005)) and
Mlaga Bay at Rubondo
Island
phosphorus (Gaudet 1979; Kassenga 1997; Kansiime
and Nalubega 1999; Azza et al. 2000; Kyambadde
et al. 2004; Gichuki et al. 2005). During senescence,
the papyrus plants accumulate nutrients in their root
zones and the decaying papyrus release nutrients back
in the water (Asaeda et al. 2002). About one third of
the dead biomass is deposited back in the wetland; the
rest is lost to elution, rain and decomposition (Gaudet
1977; Muthuri and Jones 1997). Denitrification is the
only process in the nitrogen cycle that permanently
removes nutrients (Kansiime and Nalubega 1999).
Papyrus wetlands are also important to the ecology
of the tilapia fish, by providing a refuge for the
juveniles (Mnaya and Wolanski 2002), and this fish
provides food security to the empoverished section of
the human population living along the lake’s shore.
The capacity of the wetlands to filter nutrients and
to sustain the fisheries was compromised by the
2.5 m decrease of the lake level between 2000 and
2006, which exposed all the papyrus wetlands around
the lake (Awange and Ong’ang’a 2006). There are
diverging hypotheses on the reasons for this decrease.
To generate hydroelectricity the Nalubaale dam was
constructed in 1957 (Fig. 2). The discharge forms the
White Nile River and was set by an international
agreement between Uganda and Egypt to be equal to
the ‘‘Agreed Curve,’’ which is the natural discharge
of the White Nile controlled by the lake level. Thus
the lake level fluctuated in time but remained that
determined by natural conditions until 2000 when the
new Kiira dam (Fig. 2) was completed. Because the
dams operate in parallel, it became impossible for
Uganda to adhere to the ‘‘Agreed Curve’’; the White
Nile River discharge was increased, possibly as much
as by 50% (Zaake pers. com., EAC 2006). EAC
123
(2006), Kull (pers. com.) and Mubiru (pers. com.)
attributed the 2.5 m decrease of the lake level
between 2000 and 2006 to both lack of rain and
excessive water extraction at Kiira dam, although
their relative contribution was not quantified.
In this study, we estimate the areal cover of
papyrus wetlands in the lake catchment area, we
Fig. 2 Aerial photographs of (a) the Nalubaale and Kiira dams
on the White Nile River at the outlet of Lake Victoria, and (b)
papyrus wetlands in Mlaga Bay, Rubondo Island
Wetlands Ecol Manage (2008) 16:89–96
91
quantify the role of papyrus wetlands in minimizing
eutrophication of Lake Victoria, and we link wetland
function with lake level. We quantify the quantity of
nutrients extracted through denitrification and
through above-ground biomass harvest as a proposed
solution to reduce the lake eutrophication. We
quantify the role of the dams in Uganda in relation
to the lake level decrease, and the associated decrease
in the fisheries and food security for the people.
Methods
Water level and mean catchment rainfall data for the
period 1899–2001 were available from Mnaya and
Wolanski (2002). Lake rainfall and level data were
provided by the Mwanza Meteorology Department
and the Lake Victoria Water Basin Office at Mwanza
respectively.
The monthly rainfall data at Mwanza during 1986–
2006 were used to calculate the mean catchments
rainfall from 2001 to 2006 from a correlation
between the mean catchment rainfall from 1986 to
2000 with the rainfall at Mwanza.
The model of Nicholson and Yin (2001) was used
to calculate water inflow and outflow from Lake
Victoria, and to predict what the lake level should
have been in natural conditions, i.e., in the absence of
the Kiira dam parallel turbines. Accordingly, the
water balance equation was:
DH(i) = Pw + I (E + D)
ð1Þ
where i is the year number, DH(i) (in mm) is the
change in the lake level H(i) (in m) during year i from
the preceding year i-1, Pw is the annual precipitation
over the lake (in mm), I is the tributary inflow (in mm
of lake level), E is the evaporation (in mm) over the
lake and D is the White Nile River discharge (in mm
of lake level). The variables on the right-hand side of
the equations are determined from the following
empirical relationships
I = 0.33395 Hi 0.24311 H(i 1)
0:266 Pl(i) + 0.2356 Pl(i 1) 726
ð2Þ
D = 0.15913 H(i 1) + 0.07054 Hi 223
ð3Þ
Pw = 1.3533 Pl(i) 87
ð4Þ
where Pl(i) is the catchment rainfall (in mm) during
year i.
Fig. 3 Time-series plot of the annual rainfall for Lake Victoria
from 1986 to 2006. (a) Mean catchment rainfall; (b) measured
rainfall at Mwanza, and (c) calculated mean catchment rainfall
During December 2006, fish larvae were sampled
each night at Mlaga Bay, Rubondo Island, using the
same field procedure, the same 5 l light trap, and at
the same location as those of Mnaya and Wolanski
(2002) during December 2000.
Results
Water budgets
The time series plot (Fig. 3) of the mean catchment
rainfall during 1985–2006, based on the rainfall at
Mwanza, reveals the occurrence of droughts in 1992–
1993 and in 2000 and near normal rainfall during the
rest of the time. Droughts thus do not explain the
continuous decrease in the lake level during 2000–
2006 (Fig. 4). The model suggests that the lake level
Fig. 4 Time-series plot of the (a) observed and (b) predicted
(what the lake level should have been without the Kiira dam)
level of Lake Victoria from 1985 to 2006
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Wetlands Ecol Manage (2008) 16:89–96
Discussion
The overdrawing of water in Uganda exposed the
papyrus of Lake Victoria, lead to the collapse of
tilapia recruitment, and probably promoted
invasive plant species
Fig. 5 Comparison of fish larvae recruitment at Mlaga Bay,
Rubondo Island, in December 2000 and December 2006
would have remained about constant during 2000–
2006 if the ‘‘Agreed Curve’’ was followed.
Fish larvae recruitment
The substrate of the papyrus wetlands at Mlaga Bay
was underwater in 2000. It was exposed in 2006, and
thus it offered no refuge to the fish. The fish larvae
recruitment at that site decreased by 80% from 2000
(10.6 fish per sample ± 13.4) to 2006 (2.4 ± 1.5).
This decrease is statistically significant (t(23) = 2.967,
P \ 0.05). The comparison is shown in Fig. 5.
Nutrient budgets
The area of the papyrus wetlands in Lake Victoria
basin is estimated to be about 10,235 km2, i.e., about
5.5% of the total basin area (Table 1). An average of
0.35 g N m-2 day-1 is removed from these wetlands
by the process of denitrification (Sloey et al. 1978).
Papyrus therefore removes 1.3 * 106 tonnes N year-1
in the entire basin.
The total input of N into the lake is 2.43 * 106 tonnes N year-1 (COWI 2002). Therefore the papyrus
wetlands remove 53% of the total annual input of N to
the lake, helping to slow down the lake eutrophication.
Table 1 Distribution and
size of the Lake Victoria
basin wetlands
The model suggests that the decrease of the level of
Lake Victoria is exclusively due to the overdrawing
of water at Kiira dam, supporting the finding of
Awange et al. (2007). The model shows that the
overdrawing of water from the lake started in 2000,
closely agreeing with the findings of EAC (2006) and
Awange et al. (2007).
This water level exposed all the papyrus wetlands
along the lake’s shores. Since papyrus wetlands
provide a refuge to tilapia juveniles, and this refuge
was unavailable when the wetlands were exposed, the
recruitment of tilapia juveniles in papyrus-fringed
Mlaga Bay collapsed by 80%. This finding agrees
well with other studies that report that a decrease in
water level modifies near-shore habitats and unfavorably changes the breeding area available to many
fish species (Lung’aiya et al. 2001).
It is important to note that mainly tilapia was
caught in the light trap in papyrus-fringed Mlaga Bay
wetlands (Mnaya and Wolanski 2002). The Nile
Perch does not depend on papyrus, while the tilapia
does. The Nile Perch supports the commercial
fisheries, while the tilapia supports the artisanal
fisheries. When the papyrus was exposed, the tilapia
lost its larval refuge, while the Nile Perch was not
affected. This suggests that the future of the artisanal
fisheries is at stake, while the commercial fisheries
continue to flourish. The majorities of the empoverished people living along the lake’s shore depend on
tilapia for their livelihood and food security; these are
the people who do not have access to the Nile Perch
(Odada et al. 2004) and whose food security is
compromised by the overdrawing of water for
hydroelectricity in Uganda.
Country
Area (km2)
References
Tanzania
4,220
Arcadis Euroconsult (2001)
Kenya
2168.60
SMEC (2005)
Uganda
3846.57
Awange and Ong’ang’a (2006) and Kasoma (2006)
Total wetland area of Lake Victoria basin
123
10,235.17 km2
Wetlands Ecol Manage (2008) 16:89–96
Fig. 6 Photograph showing the proliferation of the water
hyacinth in Mlaga Bay, Rubondo Island, on 23 March 2007
Until 2004, the infestation of water hyacinth exotic
weed was minimal in Mlaga Bay and all around
Rubondo Island. But from mid-2006 onward, extensive coverage of the weed was seen around the lake
again (Fig. 6). This may be due to the seed bank in
the substrate being exposed again (Awange and
Ong’ang’a 2006).
Unseasonal rainfall saved the papyrus
The decrease of the lake level lowered the water level
below that of the intake at Kiira dam and stopped the
water extraction. Unseasonal large rainfalls occurred
in November–December 2006 and in January–
February 2007. The lake level rose by 1 m in the
first six months of 2007 and inundated the papyrus
wetlands again. We observed that the papyrus at
Mlaga Bay had recovered fully by September 2007.
Why did the papyrus survive 18 months of exposure? By digging 1 m deep holes in the soil in Mlaga
Bay papyrus, we observed that the peat that the
rhizome and roots grew into retained enough moisture at 0.5–1 m depth for the plant to remain alive.
What could have happened to the papyrus if
normal rainfall had occurred?
What would have happened if this unseasonal large
rainfall did not occur and the lake level did not rise?
Presumably over time the peat in the papyrus
wetlands would have dried out and ultimately died.
93
The above-ground papyrus biomass would either be
burned for land clearance for cultivation, access to
the lake, settlements, and removal of pest animals
(Awange and Ong’ang’a 2006), or would have
decayed and ultimately ended up in the lake. The
total below-ground biomass of papyrus is
6.67 * 103 tonnes km-2 and the above-ground biomass is 9.58 * 103 tonnes km-2 (Mnaya et al. 2007).
The ratio of below-ground to above-ground biomass
is thus 0.7. In the entire lake wetland basin the
amount of N in above-ground biomass is thus
equivalent 4.5 * 1010 tonnes N. If this entire nutrient
ended up in the lake, this would be equivalent to
18,500 years of nutrient inputs to the lake, greatly
accelerating eutrophication. Alternatively, if the
papyrus was burned, a fraction would end up in the
lake and the remaining in the atmosphere. From
experiment with papyrus in Lake Naivasha (Boar
et al. 1999), the resultant N input into the lake from
the burned biomass would have been 4.1 * 105 tonnes, i.e., 17% of the annual input of N into the lake,
thus also increasing the lake eutrophication.
The burning of the papyrus would also have
contributed to the Greenhouse effect. Papyrus biomass material is about 40% C by weight (Levine
1999). Hot, dry, fires with a good supply of Oxygen
produce mostly CO2 with little CO, CH4, and
NMHCs (non-methane hydrocarbons), while the
smoldering phase approximates incomplete combustion, resulting in the production of CO, CH4,
NMHCs, and Carbon ash/particulate Carbon (Levine
1999). The total mass M(C) of the Carbon species
(CO2 + CO + CH4 + NMHCs + particulate Carbon) is related to the mass of the burned biomass
(M) by (Bowen 1979)
M(C) = F * M
ð5Þ
where F = mass fraction of Carbon in the biomass
material (40% by weight).
The sum of below-ground and above-ground
papyrus biomass is 16.26 kg m-2 (Mnaya et al
2007). For the Lake Victoria’s papyrus wetlands thus
the total papyrus biomass is 1.66 * 108 tonnes, or
6.66 * 107 tonnes of C. Assuming that the average
particulate carbon/carbon ash is 13% of the carbon
species produced in the combustion (Lobert et al.
2001), the total contribution of carbon due to burning
all the papyrus wetlands would be 8.658 * 106 tonnes
particulate carbon and carbon ash. The rest (87%) is
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Wetlands Ecol Manage (2008) 16:89–96
carbon material (CO2, CO, CH4, and NMHCs), of
which 90% is CO2 (Levine 1999) which is released in
the atmosphere. Therefore, burning the papyrus
would release 5.2 * 107 tonnes of CO2 to the
atmosphere, i.e., about 5% of the CO2, released by
the catastrophic peat and forest fires in Indonesia
during 1997 (Pege et al. 2002).
matter for use in the watershed may be a practical
alternative worth considering for removing nutrients
from the lake, thus reducing Lake Victoria’s eutrophication. It would extend to the scale of the lake the
practice of harvesting emergent plants from ponds in
urban settings to remove nutrients from the water
(Zalewski 2002).
Harvesting of papyrus as a solution for
eutrophication?
Is eutrophication self-accelerating?
Papyrus harvesting may help reduce the eutrophication of Lake Victoria. Harvesting of papyrus plants
for the purpose of nutrient removal requires careful
timing. This is because depending on the age of the
plants, only 5–20% of the total nutrients may be
stored in harvestable parts of the plants (Wetzel
1975). In the Nakivubo channel, papyrus harvesting
is done after every 6–8 months (Kansiime and
Nalubega 1999), but Muthuri et al. (1989) suggest
an interval of 12 months or more for sustainable
harvesting.
Since the entire stock of N in the lake is
2.43 * 106 tonnes, if there was no more input of N
into the lake, the time required for papyrus to filter
the entire lake by denitrification is 1.3 year.
Field studies suggest that about 5.69 * 104 kg N
year-1 and 9.38 * 103 kg P year-1 can be removed
by harvesting the above ground biomass in a
0.92 km2 papyrus wetland area (Kansiime and
Nalubega 1999). The above-ground biomass of
papyrus is 4.77 * 103 tonnes km-2, while its nutrients content is 1.30% for N and 0.21% for P. Thus if
all the above-ground papyrus of Lake Victoria was
harvested in one year, this would remove
6.3 * 105 tonnes N year-1, or about 30% of the
annual load of N to the lake.
For P, the average nutrient content in aboveground biomass is 1.02 * 105 tonnes P year-1. About
5.34 * 105 tonnes P year-1 enter the lake (COWI
2002). The amount of P that could be removed by
harvesting all the above-ground biomass of papyrus
wetlands is thus equal to 19% of the annual input of
P. If there was no more input of P, the time scale
required for removing the P from the lake if all
papyrus wetlands were harvested is 2 years.
Therefore, regular harvesting of the above-ground
biomass of papyrus plants and removing this organic
123
The nutrient input of N to the lake may possibly be
greater than what is assumed so far from river and
atmospheric input because of biological processes at
the lake bottom. There is often zero dissolved oxygen
in waters at the lake’s bottom (Rutagemwa et al.
2006) and this may accelerate the leaching of N due
to anoxic bacteria denitrification from the mud on the
bottom which has thousands of years of N input
locked in. Phosphorus is also released in anoxic
conditions. As the anoxic layer grows with increasing
eutrophication, it inundates new substrates, accelerates the nutrient release from the substrate, and this
accelerates eutrophication. There are no data on that
process and studies are clearly warranted.
A lake without papyrus?
A decrease in the lake level exposes the upper
papyrus and floods the lower papyrus. If the lake
remains stationary, the lower papyrus grows and its
detritus accumulates and raises the substrate, and
ultimately the wetland translates lakeward. If the lake
level decreases rapidly, the papyrus is exposed and
new papyrus does not have time to establish on the
new shore. A rise in the lake level drowns the lower
papyrus and floods new lands, which can be transformed in a new wetland; ultimately the papyrus
translates landward. During transition periods thus,
the total area available for wetland-water interaction
is decreased, hence minimizing the functionality of
papyrus wetlands (Kansiime and Nalubega 1999).
The human factor exacerbates this process both by
accelerating the lake level decrease and by land
reclamation. Indeed, in 2005–2006 the newly
exposed shorelines of Lake Victoria were quickly
encroached for settlements, grazing, and cultivation
and the exposed papyrus wetlands were replaced by
Wetlands Ecol Manage (2008) 16:89–96
agricultural crops (Awange and Ong’ang’a 2006).
These crops, such as cocoyams, are far less efficient
than papyrus for nutrient removal from polluted
waters (Kansiime and Nalubega 1999; Kansiime
et al. 2005).
If Uganda resumes overdrawing water from the
lake and permanently dries out the papyrus of Lake
Victoria, the resultant eutrophication of Lake Victoria
may be large-scale and could also result in the
collapse of artisanal fisheries and threaten food
security for the empoverished fraction of the population living on the lake’s shores, while also possibly
exacerbating the infestation of the water hyacinth
exotic weed. Global warming may also be
accelerated.
Except for Rubondo Island which is fully protected as a national park in Tanzania, the rest of the
papyrus wetlands all along the shores of Lake
Victoria are not protected. Since these wetlands
protect Lake Victoria from eutrophication and support the artisanal fisheries and thus provide food
security for the population, in addition to other
ecosystem services that they provide, it is important
that more papyrus wetlands obtain full protection
from human intervention along the lake shores in
Uganda, Kenya, and Tanzania. For the remaining
papyrus, each country bordering Lake Victoria
should be encouraged to put in place policies for
their sustainable use, including sustainable harvesting
to combat eutrophication.
This study suggests that the future of Lake
Victoria and its people is highly related to the future
of its papyrus wetlands.
Acknowledgements This study was made possible by
TANAPA and the EU Erasmus Mundus Programme. It is a
pleasure to thank G. Bigurube, A. Newton, E. Gereta, I. Lejora,
and S. Ndaga as well as W. Losioyo, B. Mnaya, O. Myanza,
and C. Kibwe.
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