A Seasonal Study on the Species Composition and Phytoplankton Biomass in Lake Ziway. Ethiopia
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Date
1998-06
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Addis Ababa University
Abstract
Sp<:cic?s compc,sition, sf!ilsonal variilti(Jns in sp,~ci(-,s cOlapositi on and
biomass of phytoplilnktnn in Lakl· Zil>lay, ,} l>lrge (CiI.i)l12km2) hut shallow (Zm
= 2.5m), brown water lake, was studied fr0m 8-111-87 to 26-11-88. In totBl 122 phytoplankt[\n species wp.r~ identified from net and
sedimented samples. Identification of phytoplankton species fr<.lfl)
seliilllf,nted sampl"s was rinne' ttl ensure inclusion 'If smdll plankton orgdnisms
«25 m) that m~y pass though the nct(25 m mesh-size) that was used for
routine Silillpliny.There llere three InJj'lr groups of phytnplanktl)tl, CYilllophyceae (bluE'-green-:
algae), Bacillari()phyceae (diatoms) and chl,)rnphyu,ae (green algae).
The CyanophyceilP WilS reprr,sented hy 50 sp(,cies. This was followed hy
Chlorophyceae \~ith "_1 spt'ci2s <lnd the 8acillariophyceae with 31 species.
Tilt:' community biomass was dominated by blue-green algile which composed
63% of the total m~an annual biomass. Didt~ms made up ahaut 22% of the
mean annual b-iomass and the green illgac cont.ributt,d only 15% to the total
mean annual biomass. The IJhytoplanktfln sp'ccies Of qualitativ," and quantitative importance
1'12r2 !:1ng~ U}llllett,ca LQmm, ~1icrocystis, il2ruginnsil (kg) G.M. Smith and
SynechococcuJ_ el.Q~.9iltu~ Nayel; (hlue grecn algae), f.1ebsiril 9ranulatil (Ehr.) R,llfs, -{-<-.I-i-l>v-ic.. ul·j spp. ;lnd Surirella spp. (diiltoms), fmd Staurastrurn ~- ""-'---, .. ~- .-
The m"an monthly Llti!l binlMss rdng"d from 7Y.103 to 34').103 nlg. Ill-3
while chlorophyll il cnllc(!ntriltipn varied t)(!t)wen 15/) i1ncl 212 mg.m-3• There
\'len, tl'/" p,:<1ks in total phytoplankton biomass on" occurred in Apri 1
(329.103 mg.m- 3 ) ilnd on" in August (3;)').lO3 mg.m- 3 ).
P0.aks of blu8-grecm al(jil2 occurred in August (220.103 Ing rn- 3) iwd in
Nov2mber (20,1.103 mg.m- 3 ). niilt0rn maximo occurred in April (177.103 11g.m-
3) and in August (73.103 mg.m- 3), and gre"n algal: r2milin"rl at lovwr lev(~ls
in bionl,%s f·w much !)f the study period. TlIPrc wore two p2aks in grc'\'n
illgilC? which occurr"cI in Novemher and F0bruilry, at these times grl'(m alga2
biomass was higher than that of diatoms.
The chlnrophyll cnntent (r-ltio bd\lcen Chl 9.. and bimaass.100) was 10\'1.
It ranged from 0.05 t,) 0.2(:%, with highfor values in I'larch and July, at
\lh.ich tinws thp biomass \'las donrinilt.C'd by ctiat,'ms. A genQral trend nf
decrl'asing chlorophyll content \'lith increasing blue··green biomass Vias
·)bserved.
Seasonal fluctuiltions in chlorophyll a dnd biomilss and related ch2micalphysical
parameters \'Iere l"ast pronounced. Seasonal variations expressed
intenns of coefficiimt of variation (CV, standard dcviation/rm,an-100) were
higher for total binmilss (cv ~ 1f3%) and lrMer for chlorophyll ~ (CV =
10.1%). I\~lpl ituo" ,If sl.'~s"nill varii.itinn intrer!% (If Ult' ]-)910 "f seiJs"nill biomass
r'l'iXiEl~/fllinin]i, 'lilve villu,~s Ilf 0.1': "nd .l.t).· tur cllllJf")phyl L'l [; telt,]l !lLJI;ldSS
respectiv~'ly. Tlvs,: valucs Vipr" ll'sSL'f' thiJn the villues for Sllik c'Jmponcnt
specie's 2<). PerJiastruffi (2 0 67), navicula (2.05) and Surirell" (2.0H),
Fur the majo" groups, the amplitud0. of s(]ason,ll variiltilln \'las the
highest for green alga2 (102fi) folloViI,rl by diiltoms (1,2li) and blue green
algae (O.81).
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Biology