Dynamics of Phytoplankton in Relation to Physico-Chemical Factors in Lake Bishoftu, Ethiopia
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Date
2007-07
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Addis Ababa University
Abstract
The seasonal dynamics of phytoplankton in relation to some physico-chemical
and biological parameters were studied at a near-shore and central stations in
Lake Bishoftu from August, 2006 to May, 2007. Water transparency (ZSD) ranged
from 0.50 to 1,05 and from 0.46 to 1.0 m at the Central and Near Shore stations
respectively, with high and low values corresponding to dry and rainy periods
respectively. Mean vertical extinction coefficient varied temporally with higher
values coincident with high algal biomass and abiogenic turbidity following rainy
periods. The lake exhibited both superficial and deep-seated thermal
stratifications, with ill-defined thermoclines on some occasions and never
underwent complete mixing during the study period. Algal nutrients except
nitrate-nitrogen showed temporal variations whose changes were inversely
correlated with Chl a.The phytoplankton community of the lake, which consisted
of five algal classess was dominated by cyanobacteria with Microcystis spp. as
the most abundant and persistent taxa and with their contributions to the total
abundance of the phytoplankton assemblage of the lake ranging from 58 to 99%.
The zooplankton community was dominated by Calanoids and Rotifers with
Lovenula africana and Brachionus spp.as the most numerically important taxa.
Phytoplankton biomass measured as Chl a showed seasonal variations with
maximum values during the rainy and post rainy months at both stations. Depth
profile of photosynthesis showed a region of photoinhibition and condensed
photosynthetic zone, a feature of some east African lakes. Light-saturated rates
of gross photosynthesis (Amax) at the central station varied from a minimum of
410 to a maximum of 1630 mg O2 m-3 h- and with fairly strong and positive
correlations with Chl a (R2=0.38, p=0.08), Soluble Reactive Phosphate, SRP
(R2=0.46, p=0.07) and Light utilization efficiency, Φ (R2=0.61, p=0.02). Biomass
specific rate at light-saturation exhibited unusually high values (29-91) and was
positively and significantly correlated with SRP (R2=0.59, p=0.02), Areal rates
varied from 0.89 to 3.88 g O2 m-2 h- 1 in association with biomass and Amax. The
possible causes for the observed saptio-temporal variations in physico-chemical
and biological features of the lake are discussed and suitable conservation
strategies recommended.phytoplankton was based on short-term observations, which were not systematic.
Wood and Talling (1988) have compiled the sporadic information on the
dominant or most common species of phytoplankton reported in various
limnological papers. The systematic and detailed taxonomic studies conducted
on phytoplankton to date are those of Tsegaye Mihrete-Ab (1988), Elizabeth
Kebede and Amha Belay (1994), Elizabeth Kebede (1996) and Elizabeth Kebede
and Willén (1998). Also several studies (Talling et al., 1973; Amha Belay and
Wood, 1984; Girma Tilahun, 1988; Brook Lemma, 1994; Taylor and Zinabu
Gebre-Mariam 1989; Demeke Kifle and Amha Belay, 1990 and Eyasu Shumbulo,
2004; Girma Tilahun, 2006) have investigated the dynamics of phytoplankton in
relation to some limnological aspects in Ethiopian lakes.
Although the Bishoftu crater lakes have been subjects of many limnological
investigations, some dating as far back to the early 1930's (Prosser et al., 1968),
information on the temporal dynamics of the community structure and
photosynthetic production of phytoplankton in relation to physico-chemical factors
in Lake Bishoftu is non-existent. The early short-term comparative studies made
on the Bishoftu lakes focused mainly on the assessment of general trends in
physico-chemical limnological features. Even the few published reports on the
phytoplankton biomass (Zinabu Gebre-Mariam, 1994; Wood and Talling,1988)
and photosynthetic production (Talling et al., 1973) in the Bishoftu crater lakes
were based on short-term observations although it was indicated that
phytoplankton of tropical waters exhibit temporal changes in their species
composition, biomass and primary production (Talling,1986).
In Lake Bishoftu, some-short term studies have been reported on bathymetry,
stratification, some chemical and morphometric aspects by expeditionary
limnologists such as Baxter et al. (1965), Baxter and Wood (1965), and Wood
and Talling (1988). These short-term studies were based on occasional sampling
for the purpose of comparison among crater and tropical Rift Valley lakeComparative study with another soda lake, Lake Aranguade, on zooplankton
structure and dynamics has been conducted by Afeworki Ghebrai (1992).
Therefore, in comparison with other Debre-Zeit and Rift Valley lakes, someone
can confidently talk of that Lake Bishoftu is not studied in phytoplankton aspect
over extended periods.
Zinabu Gebre-Mariam (1994), Brook Lemma (2002) and Zinabu Gebre-Mariam
et al. (2002) have noted that the Ethiopian Rift Valley and crater lakes have been
undergoing changes in their limnological features during the last two decades or
so because of increased human interventions. The present study lake is no
exception. Lake Bishoftu is surrounded by a fast growing town, Debre Zeit, and
its shores are used for washing clothes, watering livestock and recreation and
the like. Shoreline modifications made on almost all sides of the lake for various
purposes (e.g. for the construction of resort hotels and residences in the
catchments) introduce enormous amounts of particulate wastes, which form
suspensions in the water column thereby reducing light penetration at least in the
near-shore regions of the lake. The lake has also been serving as a major
dumping site for domestic wastes (solid and liquid wastes) originating from
households, hotels and small industrial operations, including a privately-owned
tannery. These activities could lead to changes in the physico-chemical
limnological features of the lake resulting in the disruption of the lake ecosystem,
with consequent changes in the species composition and photosynthetic
production of phytoplankton, which determine the pelagic food web structure and
fish production in the lake. It is also possible that degradation of this aquatic
ecosystem may go on unnoticed for a long time and eventually result in
unexpected disastrous effects. The assessment of qualitative and/or quantitative
changes taking place in this lake, therefore, necessitates the generation of basic
information on the physical, chemical and biological aspects of this water body
over an extended period of time.Thus, the purpose of this research project was to look into the temporal variation
in the species composition, seasonal abundance and photosynthetic production
of phytoplankton in relation to some physico-chemical variables in Lake Bishoftu
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Biology