Assessment of factors influencing the composition, body size and turnover rate of zooplankton in Parakrama Samudra, an irrigation reservoir in Sri Lanka

Abstract

This paper considers which of the following factors influenced the taxonomic composition, body size, fecundity and birth rates of the zooplankton inhabiting a tropical irrigation reservoir: (a) wash-out and/or dilution of the population which arose from the water regime to which the reservoir was subjected by management; (b) the size-selective or species-selective predation; (c) the effect of food limitation upon the body size, egg size, post-embryonic duration and fecundities of the planktonic species and (d) the metabolic consequences of high tropical temperatures. Many of these impinge upon the characteristics of tropical zooplankton and it is appropriate that this synthesis of published results on the zooplankton of Parakrama Samudra, Sri Lanka, was prepared for the Symposium on Tropical Zooplankton held at Sao Carlos, Brazil, in December 1982.

Daily losses of planktonic rotifers due to wash-out or by dilution were low and not significant compared with their capacity for recruitment but this may be a serious source of loss for the planktonic crustaceans which were virtually absent from this reservoir. Daily death rates (estimated by subtraction) were much higher and were mainly due to predation by a planktonic fish, Ehirava fluviatilis, and by Asplanchnella brightwelli. The main loss occurred during the day and on larger individuals of the brachionid species, Trichocerca spp. and Filinia longiseta, thus resulting in a small-sized rotifer community. This appeared to be due mainly to predation by the fish which was responsible also for the further reduction of rotifer body size in 1980 compared with 1979. Small rotifer body size was not caused by conditions of food deficiency because fecundity and birth rates were relatively high though not optimal. High tropical temperatures aggravate the cost of cumulative respiration resulting from food-limited prolongation of the juvenile phase and will raise the food threshold levels for growth and for reproduction above those adequate for temperate situations.