Abstract:
The study was carried out to observe the vertical variation of zooplankton density
throughout 24hrs at the deepest location (18m) of Muruthawela reservoir in Southern Sri
Lanka with reference to the physico-chemical parameters (Temperature, pH, Dissolved
Oxygen, Conductivity, Nitrate, Phosphate). The results showed that, the zooplankton of the
sampling site was mainly composed of Cladocerans (Neodiaptomous sp, Phyllodiaptomous
sp, Diaphanosoma excisum, Diaphanosoma augustaensis, Bosmina sp., Moina micrura,
Ceriodaphnia), Copepods (Thermocyclops, Mesocyclops, Eucyclops), Rotifers (Keratella
Volga, Brachionous caliciflorus, Monostyla bula), Protozoans (Ephistylis sp.) and Nauplii
stages. Cladocereans showed a significant (One-way ANOVA, pco.os), vertical variation
of their density along the water column with the peak value (i23ind/L) in surface layers at
11pm. Conversely, at day hours their highest population density (94ind/L) was recorded in
the deeper layers (18 m). However, there was no significant difference of density of other
zooplankton groups in the water column. There were no significant differences of physicochemical
parameters during the 24 hours and this diurnal variation pattern of zooplankton
can be considered as a result of the significant variation of light intensity throughout the
day (One-way ANOVA, pco.os). The mean values of the nitrate and phosphate in the
water column during the day were 0.107*0.0 mg/L and 0.0037±0.0 mg/L respectively
and these values achieved at eutrophic level. The daily variation of zooplankton population
density within the water column of the reservoir can be considered as an ecological factor
for the daily variation of predatory fish population of Hypophthalmichthys nobilis,
Cyprinus carpio, Oreochromis niloticus and Oreochromis mossambicus etc. According
to the personal communication with the local fishermen in the area, the fish production
is decreasing day by day, due to the lack of food in the reservoir. Therefore, there is a
possibility to predict the distribution of predatory fish feed on zooplankton by analysing
the abundance of zooplankton in the water column of the reservoir.
