DURATION OF DEVELOPMENT, BIOMASS AND RATE OF PRODUCTION OF THE DOMINANT COPEPODS (CALANOIDA AND CYCLOPOIDA) IN LAKE TANA, ETHIOPIA

Abstract

Data on zooplankton development and production rates are relatively scarce for
copepods in large turbid tropical lakes like Lake Tana, Ethiopia. We determined the duration times of
instars of the dominant planktonic copepods Thermodiaptomus galebtlacustris (Calanoida), Thermocylops
ethiopiensis and MesoLyclops aequatorialis similis (Cyclopoida) in laboratory algal cultures enhanced with
growth media and maintained at 20 and 25°C. Development times were temperature and fooddependent.
These la.boratory results together with field abundance data collected monthly for two years
were used to estimate biomass and production rates with the growth summation (Winberg) method.
'Ihe calanoid T. galebi was 50% more abundant than each cyclopoid species (average 20.4/L) with a
mean biomass of 20.92 mg OW m-3 and daily and annual production rates of 1.04 mg ow m-3 day-1 and
380 mg ow m-3 yrl, respectively. T. ethiopiensis and M. aequatorialis had lower abundances (9.59/L and
7.22/L) and biomass (7.33 mg ow m-3 and 6.18 mg ow m-3) but similar daily (- 0.42 mg DW m-3 day-I)
production rates. All the three copepods have close daily (P / B -0.05 - 0.08 d -1) biomass turnover rates.
Production was continuous for all cope pods with peaks during the post and pre-rainy seasons, and was
regulated predominantly by food availability. Copepod production and biomass turnover rates are low
for Lake Tana when compared to other tropical lakes. Turbidity showed depressed effect on biomass
and production rates only during'the late-rainy season. The contribution of c1adocerans and rotifers to
secondary production and energy flow in the Lake Tana ecosystem will be reported elsewhere