Zooplanktonstructure and Dynamics in Two Contrasting (Debre Soda Lakes Zeit Crater Lakes: Ethiopia).

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Addis Ababa University


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.