Zooplanktonstructure and Dynamics in Two Contrasting (Debre Soda Lakes Zeit Crater Lakes: Ethiopia).
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Date
1992-06
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Addis Ababa University
Abstract
Studies on . the species composition, age struc.ture, sex
ratio, pattern of abundance and measu~ement of~body sizean4
brood s'ize was performed on the zooplankton community in Lakes
,.?:;:/j'/:~~-<-<' '-' -,; .. ';--'-~'" . -, - - : .-.;"---~ . '-----:..,
Bishoftli'andArenguadefrom January,'i991 to DeCember, 199'l;
The former is inhabited byplanktivorous fish, APlocheilichth;s
~ and Oreochromis niloticus while the latter is devoid. of .. fish.
The species composition of the zooplankton community in both
lakes is poorly diverse and share a dominant calartoid, Lovenula
(pardiaptomus) africana (Daday, 1908) and one dominant rotifer
genus Brachionus . Additionally Lake Bishoftu is inhabited by
the cylopoid Mesocyclops aequatorialis.
Body size and brood size of ovigerous females of L.africana
from these lakes were significantly different both in body size
and mean brood size (P <0.001). The frequency distribution of
the adult size classes is negatively skewed in Lake Bishoftu as
opposed to Lake Arenguade.
Body size' and brood size were positively related in both
lakes and strongly positive {R = 0.447)in the population under
predation pressure than with no predation (R = 0.297). Most
size classes of Lake Bishoftu's copepods bear significantly
different mean brood size (GT2 test).
Sex ratio were also significantly different from the
expected one to one ratio in both lakes (P < 0.001) and the
ratio is strongly skewed in favour of females in Lake Arenguade. Also a higher proportion of adults to juveniles was
observed in the latter. Seasonal variation :in body and ,brood
size were also observed in relation to the mixing pel;'iod
(November - February) and fis. h kil.1. The differ-e.'n ce.s in. body ,'-" - .:, -" '. "'.' .
. ' <lnd brood sizes were <lttributed to size selective pred<lttonby
- . _ . : _ .! _. _.--'., ,_. '._ '-: ','---,_::> _,:' ~ .~;~;,.c~~'~)~~h::'~?':Jh--; .
fiSh in Laj{e Bishoftu, in the <lbsence of intense competit;iQfi'?
food .limi tation <lnd temper<lture differences"p.eJween'tiii~}iJ;wo • """--'-'_7"::,:-, ,-,,_,' , , ''''-;:-:;'',.:''''_,~:~:-/t'o,-;_.-.-.
• , ~-- <
One f<lctor ANOVA reve<lled th<lt zooplankton <lbund<lnce
diff~f.'ed signific<lntly between months (P < o:()O~):
Except for rotifers, zoopl<lnkton biom<lss showed' cle<lr
se<lson<llity with popul<ltion coll<lpse during the mixing period
coinciding with upwelling of anoxi'c water and H.S. Recovery and
peak abundance were observed during the stratification period
(July - September). Rate of re.covery was variable for different'
species and stages. Phytoplankton and major nutrients showed
less pronounced seasonality in both lakes, with nitrogen r!lther
than phosphorous being more limiting.
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Biology