DIEL VARIATION OF LIPID RESERVES OF ZOOPLANKTON CULTIVATED IN OUTDOOR PLANKTON TANKS

Abstract

This study was conducted at the hydrobiological station of Furnas Reservoir located in Rio Grande, Minas Gerais, Brazil. It aimed to investigate if zooplankton organisms usually found in plankton tanks have a diel cycle in their lipid storage patterns. Samples for water analysis as well as for biological analysis were taken every three hours covering the 06:00-24:00hs. period. The study was conducted in the summer of 1995 (February). The following parameters were measured: temperature, pH, conductivity, dissolved oxygen, chlorophyll-a and its degradation product, pheophitin, the composition, biomass and total lipids of zooplankton. Most parameters exhibited diel patterns including photosynthetic pigments as well as total lipids of zooplankton. Chlorophyll values were higher during the morning hours whereas its degradation product peaked at sunset or during the first night hours. This fact suggests that zooplankton is consuming pigmented food particles instead of bacteria or other microheterotrophs. There was a large diel variation on zooplankton lipid values (12.7-25.1%DW). Lipid values in zooplankton biomass doubled from midday to midnight. The observed diel cycle in the lipid storage pattern of zooplankton has important implications for the management of most tropical pisciculture plants since it suggests that the nutritional quality of zooplankton may vary significantly in a very short period of time.