Journal of Water Resource and Protection, 2012, 4, 877-884 Published Online October 2012 (
Seasonality in Abundance, Biomass and Production of the
Phytoplankton of Welala and Shesher Wetlands, Lake Tana
Sub-Basin (Ethiopia)
Tarekgne Wondmagegne1, Ayalew Wondie2, Minwyelet Mingist3, Jacobus Vijverberg4*
1Department of Animal Science, Debre Markos University, Debre Markos, Ethiopia
2Department of Biology, Bahir Dar University, Bahir Dar, Ethiopia
3Department of Fisheries, Wetlands and Wildlife Management, Bahir Dar University, Bahir Dar, Ethiopia
4Department of Aquatic Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Wageningen, The Netherlands
Email:, *
Received August 24, 2012; revised September 20, 2012; accepted October 19, 2012
The species composition and production of the phytoplankton community of the Shesher and Welala floodplain Wet-
lands, on the eastern side of Lake Tana, were studied during four seasons from July 2009 to May 2010. We investigated
the spatial and temporal dynamics of phytoplankton, densities, biomass, in relation to physico-chemical conditions.
Gross and net primary production was studied at one site in each Wetland. Temperature, dissolved oxygen, conductivity,
Secchi-disc depth, nitrate, phosphate and silicate concentrations showed significant temporal variation (p < 0.05),
whereas none of these parameters showed significant spatial variation (p > 0.05). Thirty six phytoplankton genera/spe-
cies, belonging to 7 higher taxa were identified. The Chlorophyta dominated the phytoplankton community and con-
tributed 42% - 53 % of the total observed phytoplankton numbers. Average phytoplankton biomass (chlorophyll a con-
tent) over four seasons ranged from 9 - 121 μg·l–1 in Shesher and from 27 - 206 μg·l–1 in Welala, whereas the average
gross primary production over three seasons was 10.5 in Shesher and 7.7 mg O2 l–1·d–1 in Welala. The peak concentra-
tion of chlorophyll a was observed in the pre-rainy season, which coincided with a bloom of Microcystis spp. Both
Shesher and Welala Wetlands are very productive and have a good water quality but they are threatened by low water
inputs since in the summer of 2009 the construction of a dam by local people and facilitated by local officials prevented
overflow from Ribb River into the Wetlands. We conclude that the good water quality, the relative high water tempera-
ture and high primary productivity make the two Wetlands suitable for culture based fisheries and/or aquaculture, but a
wise water resource management is crucially important.
Keywords: Biodiversity; Phytoplankton Biomass; Spatial and Temporal Dynamics; Water Resource Management;
Wise Use; Fogera Floodplain
1. Introduction
Temporal variability in the structure and function of phy-
toplankton communities is of fundamental importance to
the metabolism of an aquatic system [1]. Aquatic envi-
ronments are subject to high temporal variability, with
frequent reorganization of the relative abundance and
species composition of phytoplankton as a result of the
interactions between physical, chemical and biological
variables [2]. In floodplain Wetlands, there are additional
factors owing to the hydrodynamic differences arising
from the location, morphometry and the main function of
a given system. The hydrological cycle, which determin-
es precipitation, governs the flooding extent and the wa-
ter retention times in the system, generates pulses of ma-
terials and nutrients. The study of changes in phytoplan-
kton densities, biomass, species composition and primary
production is, therefore, fundamental to the understand-
ing of water quality [3]. Furthermore, fish yields depend
on the primary production of phytoplankton, particularly
in water bodies in which fish fauna is primarily com-
posed of planktivorous species such as tilapia [4].
A large number of studies have been made on the com-
munity structure and primary production of phytoplank-
ton in various East African Lakes and reservoirs [5],
however, seasonal floodplain reservoirs were rarely stud-
ied. This is also true for Ethiopia.
The Shesher and Welala Wetlands are highly exposed
to degradation because of unsustainable farming activi-
ties by local farmers such as drainage for recession agri-
culture [6]. The largest potential threat, however, is a
*Corresponding author.
opyright © 2012 SciRes. JWARP
huge irrigation project on Ribb River, which is under
construction [7,8].
The aim of this study was to assess the water quality
and productivity of these two Wetlands in order to evalu-
ate their potential for culture based fisheries. But, this
study may also serve as a baseline to investigate to what
extent the Wetlands are affected by changes due to the
large irrigation project on Ribb River which is now under
2. Material and Methods
2.1. Study-Area
Lake Tana located at an altitude of 1830 m is situated on
the basaltic Plateau of the north-western highlands of
Ethiopia covering an area of ca. 3050 km2. It is the
source of the Blue Nile River (Great Abbay), with a catch-
ment area of ca. 16,500 km2. Seven large permanent riv-
ers feed the lake as well as ca. 40 small seasonal rivers.
The main tributaries to the lake are Gilgel Abbay (Little
Blue Nile), Megech River, Gumara River and the Ribb
River. The Blue Nile is the only outflowing river. The
shallow lake is Ethiopia’s largest lake, containing more
than half the country’s freshwater resources, and the third
largest in the Nile Basin.
Shesher and Welala floodplain Wetlands are located
on the eastern side of Lake Tana within Amhara Region-
al State, South Gondar Zone of Fogera Woreda (Figure
1). The local community gets benefits from these Wet-
lands in the form of fishing, grazing for cattle and small-
scale irrigation. Most of the eastern portions of the
Shesher and Welala floodplain Wetlands are cultivated
for regression agriculture when the water shifts (“Bahir
Shesh”) and canals are made for irrigating field crops.
Rice farming is important in the area. On the moment ca.
30% of the land is used for rice cultivation [9].
Shesher and Welala Wetlands are also spawning and
nursery habitats for the African Catfish, Clarias garie-
pinus [10] and they harbor a large diversity of bird spe-
cies including internationally endangered and threatened
ones [6].
The Wetlands are very shallow, maximum depth of
Shesher is 1.75 m, whereas the maximum depth of We-
lala is 2.5 m. Shesher dries up usually during February-
March and Welala during April or May. In years with
much precipitation and much overflow from Ribb River,
Welala does not dry up at all [10].
Sufficient water inputs are vital for maintaining an
ecological connection with Lake Tana. During the rainy
season (July-October), the two Wetlands are charged with
overflow from Ribb River, overflow from Lake Tana,
overflow from Gumara River, catchment runoff and di-
rect precipitation. Before 2009, overflow from Ribb
River was the most important water source [6]. However,
during the rainy season of 2009 we observed that the
overflow from Ribb River was reduced because of the
construction of a dike along the bank of the River con-
structed by local people and facilitated by local officials
to minimize over flooding of the nearby villages (Figure
This research was conducted from July 2009 to May
2010 in four seasons. However, because of the very low
water levels during the pre-rainy season it was not possi-
ble to measure primary production during this season.
There were four sampling sites, two in each Wetland, one
in the littoral and one in the open water region of each
Wetland. GPS was used to define the sites. Sampling and
primary production measurements were carried out be-
tween 10:00 am and 3:00 pm when light conditions were
generally optimal.
2.2. Climate
The mean annual rainfall of the area is ca. 1200 mm and
ranges from 1103 to 1336 mm and its air temperature
ranges from 22˚C to 29˚C within years [11]. The climate
around L. Tana is characterized by four seasons: 1) A
main-rainy season with heavy rains during July-Sep-
tember; 2) A dry season during December-April; 3) A
pre-rainy season during May-June and 4) A post-rainy
season during October-November. After the main-rainy
season, there are the two cropping seasons.
Figure 1. Map of Lake Tana, Ribb River and Gumara Riv-
er, and associated Shesher and Welala floodplain Wetlands
(after [6]).
Copyright © 2012 SciRes. JWARP
2.3. Physico-Chemical Parameters
Temperature and dissolved oxygen (DO) were measured
with a portable oxymeter probe (OXY-315 WTW 8262),
whereas pH, total dissolved solids (TDS) and conduc-
tivity were measured simultaneously with the portable
probe Syberscan PC 300. Transparency of the pond was
measured with a standard Secchi-disc of 20 cm diameter.
Major nutrients (nitrate, phosphate, silicate) were meas-
ured using Hach portable spectrophotometer (Hach kit,
DR/2010). Nutrient analyses were made from water sam-
ples filtered through Whatman GF/C filters. Concentra-
tions were measured in the field immediately after col-
lecting the samples.
2.4. Phytoplankton Sampling
At each sampling site, the phytoplankton community was
sampled with a Van Dorn water sampler at different dep-
ths. Subsamples from different depths, representing equal
volumes, were pooled. These depth-integrated samples
were preserved with lugol solution and 100 ml was al-
lowed to settle in graduate cylinder overnight and the
supernatant was siphoned off till 10 ml remained. Of this
concentrated sample, 1 ml was used in a Sedgwick-Raf-
ter Cell of which 100 microscopic field were counted for
major species according to [12], using an Olympus com-
pound microscope (100×). Identification was predomi-
nantly based on [13].
2.5. Chlorophyll a and Primary Production
Chlorophyll a concentrations were measured once per
season in littoral and open water. From each site, 250 -
500 ml of the pondwater was filtered through Whatman
GF/C filters. The filters were folded with alluminium foil,
labeled and transported to the laboratory in an ice box
which was stored not more than one day. Pigments were
grinded and extracted in 90% acetone. After grinding, the
algal material was centrifuged. Then, the extract was
decanted into 5 ml cuvette and the absorbance of chlo-
rophyll a was measured spectrophotometrically at wave-
lengths 665 and 750 nm before and after acidification.
The concentrations were calculated according to [12].
Primary production was measured once per season in
the open water zone of each Wetland using the light-dark
bottles technique (Winkler’s method) with 3 h incubation
during midday (10:30 am - 2:30 pm). Bottles were sus-
pended at a depth of 15 - 30 cm, measured from the top
of the bottle to the water surface. After incubation, the
contents were fixed immediately with Winkler’s reagents.
Thereafter, they were acidified and thoroughly mixed
before titration in the laboratory 3 - 4 h after collection.
Calculations of gross and net photosynthetic rates and
respiration rates were done based on the change in oxy-
gen concentration between initial (I), light (L) and dark
(D) bottles. Gross primary productivity (GPP) = (L-D)/3 h
= mg O2 l–1·h–1, net primary productivity (NPP) = (L-I)/3
h = mg O2 l–1·h–1 and respiration rate (R) = (I-D)/3 h =
mg O2 l
–1·h–1 [12]. The results were extrapolated into
daily production per sampling date using a day length of
10 hrs [14].
2.6. Data Analysis
Physico-chemical parameters and phytoplankton compo-
sition were measured monthly (eleven sampling dates),
chlorophyll a content once per season (four sampling
dates) and primary production during 3 seasons only
(three sampling dates). At each sampling date, physico-
chemical parameters, phytoplankton composition and
chlorophyll a content were measured at four permanent
sites, two in each Wetland, one in the littoral and one in
the open water region of each Wetland (four samples per
sampling date). Primary production was only measured
in the open water region of each Wetland (two samples
per sampling date). For data analyses, the SPSS version
16.0 was used [15]. To evaluate the significance of spa-
tial and temporal variations of physico-chemical pa-
rameters the Kruskal-Wallis test was performed. Spear-
man bivariate correlation analysis was done to relate
phytoplankton densities to physico-chemical parameters.
3. Results
3.1. Physico-Chemical Parameters
Within each Wetland physico-chemical parameters did
not show significant spatial variation between pelagic
and littoral sites (Kruskal Wallis test; p > 0.05), but tem-
perature, dissolved oxygen, transparency and conductive-
ity showed significant differences among seasons (Kruskal
Wallis test; p < 0.01) (Table 1). Especially seasonal dif-
ferences in conductivity were large, whereas dissolved
oxygen ranged between 5.1 and 11.4 and 4.4 - 7.8 mg·l–1
in Shesher and Welala, respectively. Transparency was
found to be very low with lowest values during the main
rainy season and highest values during the post-rainy se-
ason. Average water temperatures varied between 22.0˚C
and 27.2˚C, Welala being slightly warmer than Shesher.
Dissolved nitrate, phosphate and silicate concentra-
tions varied seasonally (p < 0.05). Generally, concentra-
tions were highest during the main rainy season and low
during the post-rainy season (Table 1). During the dry
season, phosphate and silicate concentrations were also
low, but the nitrate concentration was relatively high,
whereas phosphate concentrations reached higher levels
in the pre-rainy season.
3.2. Phytoplankton Composition, Abundance
and Seasonality
A total of 36 genera/species, belonging to 7 taxonomic
Copyright © 2012 SciRes. JWARP
Copyright © 2012 SciRes. JWARP
Table 1. Seasonal variation of physico-chemical parameter in Shesher and Welala Wetlands, average ± 1 SD. Abbreviations
used: MRS = main rainy season, PORS = post-rainy season, DS = dry season, PRS = pre-rainy season, T = temperature, DO =
dissolved oxygen, TDS = total dissolved solids and SD = standard deviation. Differences are tested between seasons and water
bodies (Kruskal Wallis test). Means with identical letters of super scripts indicate a non-significant difference; physical fac-
tors (p 0.01), nutrients (p 0.05).
Shesher Welala
T (˚C) 24.3 ± 0.32b 26.8 ± 0.10 a 22.5 ± 0.5b 23.9 ± 1.3b 26.7 ± 0.5a 25.5 ± 0.4a 23.3 ± 1.3b 25.5 ± 0.2a
DO (mg/l) 5.2 ± 0.16c 7.6 ± 0.38b 10.4 ± 1.0a 5.8 ± 0.14c 6.0 ± 0.3c 5.0 ± 0.1c 7.6 ± 0.21b 4.7 ± 0.3c
Conductivity (μs/cm) 57.5 ± 6.45d 173.3 ± 10.4c 247 ± 6.8a 249.5 ± 5.3a80.75 ± 3.0 d150 ± 4.1 222.5 ± 6.5b 197.3 ± 0.2c
pH 6.74 ± 0.12 a 7.3 ± 0.10 a 7.23 ± 0.2a 8.54 ± 1.05a6.43 ± 0.3a 7.02 ± 0.0 a 7.87 ± 0.1a 6.8 ± 0.3a
TDS (ppt) 0.02 ± 0.01a 0.08 ± 0.01a 0.16 ± 0.03a0.12 ± 0.01a0.04 ± 0.0a 0.21 ± 0.3a 0.14 ± 0.03a 0.4 ± 0.4a
Secchi-disc depth (cm) 3.0 ± 0.00c 13.0 ± 0.41a 9.50 ± 0.9b 8.75 ± 2.06 b3.0 ± 0.00c 14.0 ± 0.4a 10.25 ± 0.5b 9.50 ± 2.7 b
NO3-N (mg/l) 3.1 ± 0.3a 0.2 ± 0.0d 1.26 ± 0.2c 0.25 ± 0.1d 2.09 ± 0.6b 0.16 ± 0.0d 1.06 ± 0.06c 0.38 ± 0.03d
PO4-P (mg/l) 0.7 ± 0.04c 0.00 ± 0.0g 0.2 ± 0.04d 0.68 ± 0.59c3.32 ± 0.6a 0.16 ± 0.0e 0.04 ± 0.07f 1.18 ± 0.12b
SiO2 (mg/l) 8.5 ± 0.67b 0.2 ± 0.01e 0.00 ± 0.0f 0.55 ± 0.01d19.36 ± 0.7a0.00 ± 0.0f 0.00 ± 0.0f 0.76 ± 0.01c
groups were observed during the survey (Table 2). In
this study, four genera of Cyanophycea (blue green al-
gae), 18 genera of Chlorophyceae (green algae), 10 gen-
era of Bacillariophyceae (diatoms) and Chrysophyceae,
Euglenophycea, Desmidiaceae and Dinophyceae with
one or two genera were identified. The most diverse and
abundant group was Chlorophyceae which contributed
42% - 53% of the total assemblage of phytoplankton.
Pediastrum spp., Actinastrum spp. and Closterium spp.
were the most dominant Chlorophyceae species in
Shesher and Closterium spp. and Pediastrum spp. were
the most dominant in Welala (Table 2). The second most
prominent group was the Cyanophycea, which were
dominated by Microcystis spp. in both Wetlands with
abundances of ca. 70% - 95% in Shesher and Welala
(Table 2). In Shesher, the Bacillariophyceae were the
third most dominant group, but in Welala the Dinophy-
ceae were more abundant than the Bacillariophyceae.
The most abundant diatoms in Shesher were Aulacoseira
spp., which accounted for ca. 70% of the total diatoms
(Table 2), but in Welala Cyclotella was the most domi-
nant diatom. Euglenophycea were represented by two
genera, Euglena and Phacus, the three other higher taxa
by one genus only: Chrysophyceae with Mallomonas,
Desmidiaceae with Cosmarium and Dinophyceae with
Peridinium (Table 2).
Generally, species diversity and abundance was low in
the main rainy season, whereas the highest densities were
observed during the pre-rainy season. Both Wetlands
showed seasonal variation in species composition. In the
pre-rainy season both Wetland were dominated by the
same genera, the green algae Closterium and Pediastrum
and the blue green alga Microcystis. But, in the main
rainy season, Shesher was dominated by the green alga
Actinastrum, while Welala was dominated by two green
algae, Crucigenia and Eudo rin a , and the diatom Cyclo-
tella. Also in post-rainy season, different species domina-
ted the phytoplankton community, Pediastrum in Shesher
and Microcystis and Peridinium (Dinophyceae) in We-
lala. In the dry season, Shesher was dominated by the
green alga Actinastrum and the diatom Aulacoseira
whereas Welala was dominated by two other green algae,
Closterium and Pediastrum.
There was a strong negative relationship between den-
sities of blue green algae and nitrate concentration (r2 =
–0.842; p < 0.001) and between diatom densities and
silicate concentrations (r2 = –0.624; p = 0.01; Table 3).
3.3. Chlorophyll a and Primary Production
The chlorophyll a concentration in the two Wetlands was
within the range between 5.4 and 217.8 μg·l–1 and show-
ed marked seasonal variation (Figure 2; p < 0.01). The
highest chlorophyll a concentrations were recorded in the
pre-rainy season. The mean value of Shesher and Welala
were 67.51 and 90.64 μg·l–1, respectively. On average
chlorophyll a concentrations were higher in Welala (av-
erage 91 μg·l–1) than in Shesher (average 68 μg·l–1) but
differences were not significant (p = 0.36). There was no
significant relationship between chlorophyll a content
and transparency (Secchi-disk depth) in the two Wet-
lands (r2 = 0.12, p = 0. 67).
Gross Primary Productivity (GPP) of Shesher and
Welala varied between 1.2 and 19.8 mg O2 l
–1·d–1 and
showed distinct significant seasonal variation (p < 0.05).
The average GPP values in Shesher and Welala were in
the same range (5.9 - 7.6 mg O2 l–1·d–1). Maximum values
were observed in the dry season (January 2010) in both
Wetlands. The average Net Primary Productivity (NPP)
was 7.7 mg O2 l–1·d–1 for Sheher and 3.65 mg O2 l–1·d–1 s
Table 2. Taxa of phytoplankton identified in Shesher and Welala Wetlands and their average abundance (%) during the
study period. Presence/absence in Lake Tana according to [17].
Taxa Genera/species Abundance Shesher (%)Abundance Welala (%) Presence/Absence Lake Tana
Cyanobacteria Anabaena sp. 2.13 1.35 –
Microcystis sp. 16.01 25.35 +
Oscillatoria sp. 3.55 0.11 –
Phormidium sp. 1.51 0.00 –
Chlorophyceae Actinastrum sp. 13.77 3.20 +
Ankistrodesmus sp. 1.13 0.19 +
Chlamydomonas sp. 0.05 0.00 +
Closterium sp. 5.82 20.44 +
Coelastrum sp. 0.12 0.30 +
Crucigenia sp. 0.18 3.49 –
Eudorina sp. 2.35 3.32 –
Kirchneriella sp. 1.92 0.02 –
Monoraphidium sp. 0.12 0.10 +
Oocystis sp. 0.48 0.11 +
Pandorina sp. 0.52 0.17 –
Pediastrum sp. 21.98 8.1 +
Scenedesmus sp. 1.91 0.74 +
Schroederia sp. 0.05 0.38 +
Sphaerocystis sp. 0.00 0.69 +
Staurastrum sp. 1.81 0.77 +
Volvox sp. 0.70 0.42 +
Bacillariophyceae Aulacoseira sp. 13.86 1.92 +
Cyclotella sp. 0.83 3.73 -
Cymatopleura sp. 0.07 0.08 +
Cymbella sp. 0.00 0.35 +
Navicula sp 0.82 0.22 –
Nitzschia sp. 2.93 1.09 +
Pinnularia sp. 0.02 0 +
Surirella sp. 0.00 0.02 +
Synedra sp. 0.46 0.43 +
Tabellaria sp. 0.06 0.02 +
Chrysophyceae Mallomonas sp. 0.30 0.46 +
Desmidiaceae Cosmarium sp. 0.00 0.29 –
Dinophyceae Peridinium sp. 0.78 19.90 +
Euglenophyceae Euglena sp. 3.05 1.88 +
Phacus sp. 0.69 0.33 +
Copyright © 2012 SciRes. JWARP
Table 3. Correlation between the concentrations of major
nutrients and the densities of major groups of phytoplank-
ton in Shesher and Welala Wetlands. N = 16.
Nutrients Phytoplankton taxa r2 p-value
Cyanophyceae –0.842 0.000
Chlorophyceae 0.047 0.862
Bacillariophyceae –0.287 0.281
Cyanophyceae –0.186 0.490
Chlorophyceae 0.032 0.905 PO4-P
Bacillariophyceae –0.471 0.065
Cyanophyceae –0.428 0.099
Chlorophyceae –0.126 0.643 SiO2
Bacillariophyceae –0.624 0.010
Figure 2. Temporal variation of average chlorophyll a con-
tent (µg·l–1) in Shesher and Welala Wetlands. Error bars
represent + 1 SD. Abbreviations used: MRS = main rainy
season, PORS = post-rainy season, DS = dry season, PRS =
pre-rainy season.
for Welala and showed the same seasonal pattern as that
of GPP (Table 4). There was a strong relationship be-
tween GPP and chlorophyll a content (r2 = 0.97, p <
4. Discussion
4.1. Hydrology
The hydrological conditions during the rainy season of
2009 (our study) were extraordinary because a new con-
structed dike along the bank of the Ribb River reduced
the overflow from Ribb River. As a result, the Wetlands
received less water than before. During our study period,
direct precipitation was the most important water source,
not the overflow from Ribb River as was the case in pre-
vious years [6]. Furthermore, one year after our study
period, during the rainy period in 2010, the dike had be-
come less efficient and Ribb River overflow was not
substantially reduced [10]. This was very fortunate be-
cause a reduced overflow from Ribb River would have
led to the drying up and disappearance of these two Wet-
lands. On the moment cultivation by draining the water
through pumping or channeling and flood prevention
practices by the local people are the main threats for
Table 4. Summary of gross primary production (GPP), net
primary production (NPP), respiration (R) and gross pri-
mary production per unit chlorophyll (GPP/Chl) in three
different seasons in Shesher and Welala Wetlands. Abbre-
viations used: MRS = main-rainy season, PORS = post-
rainy season, DS = dry season. Respiration and production
in mg O2 l–1·d–1, average chlorophyll a content in µg·l–1.
Shesher Welala
MRS1.20.6 0.60.130 1.8 0.84 0.960.065
PORS1.88 0.131.750.062 2.95 1.25 1.700.094
DS19.8 14.8 5.0 0.181 12.7 10.0 2.7 0.134
these Wetlands [10]. An even larger threat, however, is a
huge irrigation project on Ribb River, which is now un-
der construction [7,8]. This dam will retain most of the
water coming from Ribb watershed during the rainy sea-
son. Hence, without mitigating measures these Wetlands
will most probably dry up in the near future.
4.2. Physico-Chemical Parameters
Physico-chemical parameters did not differ between the
two Wetlands (Kruskal Wallis test (p > 0.05), probably
because both Wetlands receive water from the same
source, i.e. mainly direct precipitation and catchment
runoff. Temperature, dissolved oxygen, conductivity and
dissolved nutrients, however, showed significant differe-
nces among seasons. In both Wetlands dissolved oxygen
levels ranged between 4.7 and 10.4 mg·l–1, which are
generally considered favorable for most aquatic organis-
ms [16]. Chlorophyll a showed a weak relationship with
water transparency. This was also observed for Lake
Tana [14] and implies that water transparency was main-
ly controlled by the concentrations of suspended sedim-
ent particles rather than by phytoplankton. All major
nutrients showed relatively high concentration during the
rainy season and much lower concentrations during the
other seasons. The nutrient concentrations were affected
by phytoplankton, which are needed for their growth.
Therefore, it is not surprising that dissolved silicate conc-
entration was negatively correlated with diatoms densi-
ties and that dissolved nitrate concentrations were
negatively correlated with densities of blue green algae.
Most probably the dynamics of the nutrients were also
influenced by anthropogenic effect, i.e., channelization
and farming practices inside both Wetlands.
4.3. Phytoplankton
Seven higher taxa and 36 genera/species were identified
in the present study. The species compositions of the
phytoplankton of Shesher and Welala were similar to that
of Lake Tana, but the species richness was much lower
than Lake Tana (85 species; [17]). On average green al-
Copyright © 2012 SciRes. JWARP
gae contribute the greatest proportion in terms of num-
bers (around 50%) and blue green algae were subdomi-
nant (around 25%), but there were large seasonal differ-
ences in densities and species composition. The lowest
densities, dominated by several genera of green algae and
one diatom, were observed in the main rainy season. This
is probably due to the very low water transparency dur-
ing this period. The highest densities were observed in
the dry season and the pre-rainy season. The blue green
alga Microcystis became co-dominant in both Wetlands
during the post-rainy season, suggesting eutrophic condi-
4.4. Chlorophyll a Concentration
The average chlorophyll a concentrations of Shesher and
Welala were 67 and 91 μg·l–1, respectively. This is high-
er than in the Uba and Ruwe floodplain lakes in Tanzania
(range: 31 - 42 μg·l–1) [18] and much higher than in Lake
Tana (average: 4.7 μg·l–1) [14]. Over the different sea-
sons, chlorophyll a concentrations varied from 9 to 121
μg·l–1 in Shesher and from 27 to 206 μg·l–1 in Welala,
ranging from eutrophic to hypertrophic conditions. Chlo-
rophyll a concentrations showed a distinct seasonal pat-
tern, low in the main rainy season and a progressive in-
crease in dry and pre-rainy seasons. Such a low algal bio-
mass during periods of heavy rainfall is not unusual and
has also been reported for Lake Tana and Lake Victoria
[14,19]. In both Wetlands, the peak in chlorophyll a con-
centration was observed in the prerainy season when
primary production was stimulated by a relative high wa-
ter transparency in combination with increased phosphate
4.5. Primary Production
In this study, the average gross primary productivity
(GPP) of Shesher and Welala Wetlands was in the range
of 7 - 10.5 mg O2 l
–1·d–1. This is high compared with
most Ethiopian lakes, Lake Tana (average: 3.4 mg·O2
l–1·d–1) [14] and Lake Kuriftu (average: 4.0 mg O2 l–1·d–1)
[20]. This might be due to high availability of nutrients
which was facilitated by the shallowness of the ponds
and by evaporation as a result of which the nutrients be-
came more concentrated.
Primary production showed a distinct seasonal varia-
tion, low in the main rainy season and the post rainy
season, high in the dry season. The seasonal minimum
coincided with a period of heavy precipitation that re-
sulted in land runoff which brought particulate materials
into the floodplain Wetlands with consequently reduced
the light penetration even though inflow of nutrients
were high. GPP was strongly positively correlated with
Chlorophyll a, but primary production was also limited
by light and nutrients. The low gross photosynthetic rate
in the main rainy season was caused by the low water
transparency. During post-rainy season probably both
light and nutrients were limiting production. The maxi-
mum GPP in Shesher and Welala in the dry season were
not only the result of the high phytoplankton biomass,
but also the result of a relatively high GPP per unit of
phytoplankton biomass, suggesting less limitation by
light and/or nutrients than in the previous two seasons.
5. Conclusion
The Welala and Shesher Wetlands are valuable for the
local community. They get benefits in the form of fishing,
grazing for cattle and small scale irrigation. Furthermore,
the Wetlands are spawning and nursery habitats for the
African Catfish, C. gariepinus and they harbor a large
diversity of bird species. These Wetlands are threatened
by unwise farming practices and the construction of huge
dam, which will retain most of the water coming from
the Ribb River watershed during the rainy season. Miti-
gating management measures are urgently needed to
protect these Wetlands. The water of Shesher and Welala
Wetlands is productive, ranging seasonally from eutro-
phic to hypertrophic. The water quality is suitable for
fish and other aquatic organisms. The phytoplankton is
generally dominated by Chlorophyceae and diatoms,
which are indicators of good water quality and the dis-
solved oxygen concentrations are high enough to support
aquatic life. We conclude that the good water quality, the
relative high water temperatures and high water produc-
tivity make the Shesher and Welala Wetlands suitable for
culture based fisheries and/or aquaculture, but a wise
water resource management is crucially important.
6. Acknowledgements
The authors would like to thank the Amhara Regional
Bureau of Agriculture and Rural Development and South
Wollo Zone, Albko Woreda officers and officials, and
also IWMI and ARARI for their financial support. We
also thank the Staff members of the Bahir Dar Fishery
and Other Aquatic Resources Research Center of ARARI
as well as from the Bahir Dar University (BDU) Biology
and Chemistry Laboratories for their unreserved support
of our field and laboratory research. We acknowledge the
comments of Dr. Eshete Dejen (FAO, Subregional Office
for Eastern Africa) and Mr. Wassie Anteneh (BDU Bi-
ology) for useful comments on the manuscript.
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