Ecosystem structure, trophic link and functioning of a shallow rift valley lake: the case of Lake Ziway (Ethiopia
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Date
2011-06
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Addis Ababa University
Abstract
A trophic model was constructed for the Lake Ziway ecosystem using Ecopath with
Ecosim application software. This model was used to evaluate and analyze the food web
structure and other properties of this ecosystem. The mean Chlorophyll a biomass of
70.46 μg L−1was found which is higher than those of earlier reports. Wet weight value of
phytoplankton was estimated to be 15.99 tons km-2. A total of 4 cladocerans and 2
copepods were identified. The biomass of zooplankton is dominated by Thermocyclops
(127.73 mg WW m-3). A total of fourteen families of macro-zoobenthos were identified
which were dominated by chironomids (15924 individuals m-2) that were attached with
the macrophyte Typha anguistifola. The above ground biomass of macrophytes
vegetation of Lake Ziway was estimated as 3011.68 tons WW km-2. The total mortality
value for Carp and Catfish was 0.74 and 1.34 year-1 respectively. The total annual catch
from Lake Ziway was estimated at 1127 tons based on the catch and effort data (2008–
2010). The estimated annual catch in tons per gear type was 683.11, 62.97 and 295.98
for beach seine, gill net and long line, respectively. The results of Ecopath with Ecosim
modeling showed the presence of six aggregated trophic levels (TLs). The fractional
trophic level assigned TLs of Lake Ziway ecosystem between 1.0 and 3.29. The biomass
flow was highly restricted to I and II trophic levels which contributed 99.76 %. The mean
transfer efficiency was only 4.4%. The fishery catch consumed 2.5% of the primary
production in the lake. Low ecotrophic efficiency (EE) of 0.47 was calculated for the
phytoplankton indicating that most of the production remains within the system
unutilized. A high value of EE was noted for carp (0.99), indicating that it is subjected to
high fishing pressure despite its low biomass. In Lake Ziway, EE values were high for all
fish groups, indicating that the fish groups were highly constrained by a combination of
fishing and predation mortality. Management scenario using Ecosim analysis indicated
that increasing catfish mortality by 1.50 times resulted in increment of tilapia to 1.6 times
the current biomass in four years period and leveling of its biomass to 1.16 times the
current value at the end of the tenth year. Doubling in mortality of catfish resulted in
increment of tilapia biomass to 1.12 times the current biomass. Decreasing the effort of
beach seine did not benefit the tilapia group as their biomass declined to 0.98 and 0.66
which is related to biomass increment of catfish. The final scenario of increasing the
effort long line by 1.50 and 2.00 times benefited tilapia and significantly decreased the
catfish community from the ecosystem. The total analysis indicated that lake Ziway
ecosystem has an open niche which can be filled with obligate herbivores to increase the
energy transfer efficiency of the lake.
Key words: Ecopath with Ecosim, Biomass, Transfer efficiency, Trophic modeling,
Species introduction
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Keywords
Ecopath with Ecosim, Biomass, Transfer efficiency, Trophic modeling, Species introduction