Plankton Communitiy Structure and Interactions in a Cyanobacteria-Dominated Tropical Reservoir (Koka, Ethiopia)
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Date
2016-06
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Addis Ababa University
Abstract
Koka is a large multi-purpose reservoir impacted by the ever-increasing human activities
that resulted in its enrichment with algal nutrients and contamination with chemicals of
industrial origin. With a view to come up with scientific information usable in the
protection of aquatic resources and public health, temporal dynamics of the community
structure and interactions of the major plankton components (phytoplankton and
zooplankton) in relation to selected physico-chemical parameters were investigated at
monthly intervals from May, 2013, to April, 2014. The observed exceedingly low ZSD,
which may have partly resulted from particles resuspended in the water column by windgenerated
turbulence to which shallow water bodies like Koka Reservoir are especially
susceptible, indicates the remarkably high turbidity of the reservoir. The levels of
inorganic nutrients recorded for Koka, which are quite high in comparison to those of
Ethiopian rift valley and highland lakes, are typical of reservoirs, which, unlike natural
lakes, have large phosphorus and nitrogen loads. The mean values of total phosphorus
(0.27 mg L,I), chlorophyll-a (85.79 J.lg L'I), and transparency (0.15 m) recorded for
Koka Reservoir WaJTant its classification as a hypereutrophic water body characterized by
high biomass production and elevated concentrations of nutrients.
The phytoplankton community in the reservoir was constituted by a total of 89 species
with the larger number of species belonging to the Chlorophyceae (34) and
Cyanophyceae (28). Cyanobacteria were the most important taxa both in terms of
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abundance and biomass, with Microcystis and Cylindrospermopsis species altemately
dominating the phytoplankton community and accounting for up to 72.29% and 62.87 %
and 21.45. % and 20.23% of the total phytoplankton abundance and biomass as
biovolume, respectively. The mean concentrations of chl-a (~g L-1) of the nano- (2-20
,um, 39) and net- (>20 ,um, 40.4) phytoplankton were much higher than those of the
picophytoplankton (10.48) as would be expected considering the level of eutrophication
of the reservoir.
The composition and abundance of cyanobacteria and their toxins, microcystins (MCs)
and cylindrospermopsins (CYN), were investigated using samples collected at monthly
intervals from the Amudde side of Koka Reservoir from May, 2013, to April, 2014.
Analyses of cyanotoxins in filtered samples by HPLC-DAD and LC-IMSIMS identified
and quantified 5 variants ofMCs (MC-RR, MC-YR, MC-dmLR, MC-LA and MC-LR) in
all samples, with their total concentrations ranging from 1.86 to 28.3 ~g L -I and from
1.71 to 33 ~g L -I, respectively and greatly surpassing WHO's drinking water guideline
value of 1 ~g L-1
• The maximum level of MCs occurred in December, 2013, when the
phytoplankton community of the reservoir was constituted almost entirely by Microcystis
aeruginosa. The total concentrations of MCs measured in freeze-dried plankton samples
by HPLC-DAD and LC-MSIMS varied between 312 and 641 ~g (g dwtrl and 351 and
709 ~g (g dwtrl, respectively. Thus to prevent intoxications continuous monitoring of
these reservoirs is strongly recommended. This has to be taken into consideration in the
management of the reservoir when it used for various domestic purposes. Studies
regarding the prediction of the occurrences of toxic blooms and their toxins must be
implemented and strengthened in the future in order to avoid or reduce the potential risks
associated with human and animals' exposure to the toxins. Despite the presence and
occasional abundance of Cylindrospermopsis spp., cylindrospermopsin was not detected
by HPLC-DAD and LC-MSIMS in any of the samples collected tlu'oughout the study
period.
Analysis of the zooplankton community in the reservoir revealed a total of 52 species, 25
of which are new records for Koka Reservoir. With 40 species, rotifers were the most
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species-rich group followed by cladocerans (8 species), and copepods (4 species). The
rotifer Keratella tropica was the species recorded with the highest abundance among the
zooplankton taxa contributing 42.11 % of total rotifers abundance. The cladocerans were
dominated by small species, Ceriodaphnia corn uta, Diaphanosoma excisum and Moina
m/crura, with rare occurrence of the large cladoceran D. barbata during a few months of
the sampling period. Total abundance of copepods was constituted largely by
Thermocyclops decipiens, which is the most commonly dominant species in tropical
inland waters. Calanoid copepods were represented by a single species; Tropodiaptomus
sp. Zooplankton abundance was generally higher during the dry season than the rainy
season.
Eutrophication of Koka Reservoir has resulted in the dominance of cyanobacteria, which
are generally regarded as unsuitable food source for zooplankton. It was, therefore,
hypothesized that seston may playa role in the diet of the zooplankton community of this
reservoir. The contribution of seston fractions (<20 11m, ::::20<IOOllm) to zooplankton
diets was thus assessed using dual isotope (olsN and ol3C) SIAR and MixSIAR models.
Posterior area estimates of both consumer species and potential food sources were also
modeled. The SIAR and MIXSIAR results have demonstrated that the general diet of the
zooplankton of the reservoir was composed primarily of small seston particles «20Ilm)
whose importance varied over time and, which was constituted by green algae,
flagellates, diatoms, small colonies of Microcystis and fragments of Cylindrospermopsis
filaments. Analysis of isotopic niche breadth of the two major clUstacean zooplankton
spp., Ceriodaphnia corn uta and Thermocyclops decipiens gave niche width, which was
generally higher for the former than for the latter indicating consumption of a different
prey and/or change in feeding strategy of both species. The slight l3C-depletion of
zooplankton species in relation to seston fractions and the variations in their isotopic
niche area suggests that these zooplankton species may have carbon sources other than
phytoplankton.
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Keywords
Plankton Communitiy Structure, and Interactions, in a Cyanobacteria-Dominated