The ecology of Lake Nakuru (Kenya) : V. Production and consumption of consumer organisms.

Abstract

Consumer production and consumption were studied in the equatorial alkaline-saline Lake Nakuru from 1972 to 1976. Together with earlier reports (including a study of the dominant consumer, the Lesser Flamingo Phoeniconaias minor), the data provide the basis for estimating the major pathways of energy flow. Detritus food chains were not included in this project.Production and consumption rates were estimated from the distribution of numbers and size classes in the lake and laboratory experiments on growth and filtration rates. Rotifers (Brachionus dimidiatus and B. plicatilis), though not especially significant in biomass, had the highest production rates (1.7 KJ m-3 d-1) due to a very short juvenile phase (ca. 2 days) and fast production of very large eggs (about 1 per day). Consumption rates were correspondingly high (11.3 KJ m-3 d-1), comparable only to those of the Lesser Flamingo (12.6 KJ m-3 d-1; in this species, production was negligible because the birds do not breed at L. Nakuru). Copepods almost matched rotifers in 1972/73 (production 1.5, consumption 6.5 KJ m-3 d-1) but vanished from the lake in the following years. Chironomid larvae (mainly Leptochironomus deribae) and fish (Sarotherodon alcalicus grahami) had similar ranges of production (0.7 and 0.4 KJ m-3 d-1) and consumption (3.6 and 3.4 KJ m-3 d-1) although the fish had about twice the biomass (20 KJ m-3) of the insects.Most primary consumer organisms fed on the dominant primary producer, the cyanophyte Spirulina platensis, but rotifers and Leptochironomus met an unknown fraction of their energy requirements by consuming bacteria and detritus. Of the secondary consumers only fisheating birds (≈90% adult Pelecanus onocrotalus) and the Greater Flamingo (Phoenicopterus ruber, mainly full-grown individuals) contributed significantly to the energy flow. Neither pelicans nor Greater Flamingos breed at L. Nakuru, therefore their production rates were negligible. The total fish yield taken by birds was 2,700-9,500 metric tons (wet weight) per year (≈3.8-13.4·107 KJ m-3 d-1) which is among the highest fish yields taken from natural lakes including commercial fishery. Greater Flamingos consumed only 0.05-0.7 KJ m-3 d-1. Some additional data are given on aquatic heteroptera (corixids and a notonectid) and hippopotamuses.