INFLUENCE OF PHYTOPLANKTON COMPOSITION AND STRATIFICATION DEGREE ON GUT PIGMENT CONTENT OF CERIODAPHNIA SP AT DAWN

Very short-term feeding activity of the cladoceran Ceriodaphnia sp. wa s investigated in situ in a eutrophic reservoir in the south of Spain, using fluorimetric analysis of the gut pigment content in periods whe n the water column was relatively mixed or strongly stratified. The re sults obtained in the mixed water column showed a clear increase in gu t pigment content at dawn, a period sampled with high frequency. The a ccumulation of the cladoceran at the depth of maximum concentration of phytoplankton, and the high gut pigment concentration in cladocerans at that depth just after dawn, suggested active feeding of Ceriodaphni a on phytoplankton at that time. During stratification, the abundance of Ceriodaphnia was higher, but the gut pigment contents were very low and they did not reflect any clear feeding patterns, with either time or depth. Changes in phytoplankton concentration and composition betw een the relatively mixed and the stratified water column suggest a shi ft in feeding activity from herbivorous to detritivorous.