| dc.description.abstract | The first quantitative study of the limnetic
zooplankton community of Lake Naivasha, Kenya, was
undertaken from July 1978 to July 1980. Using a 105
um mesh net, vertical haul zooplankton samples were
taken from 3 stations in the limnetic zone of the
main lake, at weekly intervals during the first year
and at fortnightly intervals during the second year
of the study. The vertical net haBl numerical
estimates of zooplankton abundance were adjusted by
employing net correction factors determined by using
a newly developed volume sampler, similar ln principle
to the conventional Van Dorn sampler.
The pelagic zooplankton community of Lake Naivasha
was composed of 2 cyclopoid and 1 calanoid copepod
species;11 cladoceran and 13 rotiferan species.
During the two year study period the limnetic zooplankton
was ,numerically dominated by the following species, in
order of their numerical dominance: Thermocyclops oblongatl
Sars; (Copepoda), 54-61%); Diaphanpsoma excisum_Sars
(Cladocera)(18-29%)) Brachionus caudatus Barrois and
Daday (Rotifera),(8-11%); ~. calyciflorus Pallas
(Rotifera) (7-10%); arre-Simocephalus vetulus King
(Cladocera) (1-3%); the values in parentheses are mean
ranges recorded during the two.year study period.
Other occasionally important liMnetic zooplankters
were Mesocyclops eguatorialis Kiefer (Copepoda),
Daphnia laevis Sars (Cladocera), and,Tropodiaptomus
banforanus Kiefer (Copepoda). Throughout ..the study
period the species cbmposition of the zooplankton
community in Lake Naivasha was relatively stable In
proportion and variety when compared with the
zooplankton of temperate lakes.
Significant temporal chan&es in abundance of the
zooplankton populations occurred on a seasonal basis
associated with the dry and wet periods in the lake
basin. High densities of the zooplankton are positively
correlated with h\ph 'concentrations of chlorophylla_ ,
as well as rlslng lake level.
Annual mep.ntotal zooplan~ton biomass was 237 and
3 •
425 mg dw/m for the first and second years of study,
I
respectively. Zooplankton biomas peaks coincided
with the rainy seasons in the lake basin.
Thermocyclops
oblongatus and Diaphdnosoma excisum (which have been
studied in detail) dominated the zooplankton biomass
contributing 50-60% and 29-35% respectiyely.
Rotiferan contribution to biomasswas usually low
(Less than 7%) and rnainly from the two Bramionus speci.es .
The dissolved oxygen and thermal regimes, which are
mainly seasonal In temperate lakes, were found to be
confined to diet events in this tropical lake. There
was a pronounced (though temporary) oxygen and thermal
stratification in the lake during calm sunny weather,
usually between 11.00 hrs and 14.00 hrs. The
stratification was initially broken down by the
afternoon winds and subsequently by further night
time cooling.
Diel vertical distrjbution of the major
zooplankton species was found to be homogen.eous
throughout the water column; but with an apparent
tendency for most zooplankters to aggregate above
.the 4 metre zone, usually at noon, and most probably
ln association with feeding and breeding activities
No evidence of diel vertical migration by any of
the zooplankters was 'recorded in the present study.
Light does not seem to play a major role as a
synchroniser to the diel activities and vertical
distribution of zooplankton ln Lake Naivasha.
The durations of development of the embryonic
(egg) and post-embryonic stages of Thermocyclops
oblongatus and Diaphanosoma excisum were determined
ln the laboratory. The embryonic development times
- 0
of both species determined at 19, 22, 25 and 28 C
were found to be inversely and linearly correlated
with temperature. The durations of embryonic and
post-embryonic development determined at the lake's
mean annual temperature of 220C were used to provide
the time element in the calculation of production by
the two species.
Instantaneous birth rates and rates of change
of oblongatus and D. excisum population densities
were estimated. The calculated birth rates gave mean
turnover times of numbers of 5.3 days for !. oblongatus
and 13.5 days for D. excisurn. Turnover times 0 f
biomass were 11. 4and 13.7 days for !. oblongatus and
Q. excisu~respectively. The daily losses of the
population biomass of 9% for T. oblongatus and 7% for
D. excisum In Lake Naivasha are extremely low when
compared with zooplankton ,, populations In lakes where
predation pressure on the zooplankton lS intense.
Estimates of production,by the two dominant
limnetic zooplankters Thermocyclops oblongatus and
Diaphanosoma excisum were calcuated using Winberg's
(1971) weight increment method. The annual mean
:' 3
production rates were 11.0 m~ dw/m /d. for !. oblongatus
and 6.0 mg dw/m3/d. for D. exclsum. Eggs, nauplii
and copepodites of T. oblongatus contributed 5%, 48%
and 47%, respectively to the total annual net
production, in dry weight, by the copepod.
Since there are virtually no zooplanktivorous
predators in the open lake, predation of the limnetic
zooplankton in Lake Naivasha is virtually zero; in the
littoral area, however, fish fry of Sarotherodon
leucostictus and the adults of Gambusiaaffinis
exert substantial predation pressure on the
zooplankton. The bulk of all the zooplankton
productionpasses to the decomposer food chai n and is
not utilised by higher trophic levels. | en |
