Population dynamics of Daphnia rosea in a small eutrophic pond

The population dynamics of Daphnia rosea were studied in a small, eutrophic fish‐free pond during November 1986–November 1987. D. rosea showed two density peaks in 1987, a high peak in spring and a slightly lower one in autumn, but they disappeared in mid‐ and late summer when a smaller cladoceran, Ceriodaphnia reticulata , dominated. The rapid decline in density of D. rosea in mid‐May and the low density in June/July were preceded by or coincided with a period of increased male density, the appearance of ephippial females, smaller body length of adult females, decreased brood size, and increased frequency of empty brood chambers, all of which indicate a worsening food situation. The rapid density decline of D. rosea in mid‐May was considered attributable neither to increasing water temperature, nor predation by the dipteran Chaoborus flavicans , but to insufficient food conditions for the maintenance of stable reproduction. Although edible phytoplankton was abundant in June/July, the general food situation was not suitable for D. rosea due to blooms of the large‐celled dinoflagellate, Ceratium hirundinella , which probably inhibited the feeding activity of D. rosea . The C. hirundinella blooms might also be important for the shift of the dominant herbivore from the larger D. rosea to the smaller Ceriodaphnia reticulata in August/September.