INSTAR-SPECIFIC MORTALITIES OF COEXISTING DAPHNIA SPECIES IN RELATION TO FOOD AND INVERTEBRATE PREDATION

Instar-specific mortalities of Daphnia hyalina and D.cucullata were studied from May 19 to September 29, 1988 in combination with invertebrate predator and phytoplankton abundance. Simultaneous life-table experiments were conducted under semi-natural conditions in the laboratory to estimate juvenile mortality in a predator-free environment. Juvenile mortality by predation was calculated as the difference between juvenile mortality in the field and in the experiments and was the most important factor for the differences in abundance of the two species. For D.hyalina juvenile mortality was higher in early summer and probably caused by selective predation by Chaoborus flavicans. Predation by Leptodora kindtii was probably more important during the rest of the summer. Estimated mortality by predation adequately explained juvenile mortality, except for a 3-week period in August. Decreasing flagellate densities in July were accompanied by increased juvenile mortalities of D.hyalina and D.cucullata in the life-table experiments in August and coincided with a Daphnia population decrease. © 1990 Oxford University Press.