FOOD LIMITATION IN A WILD CYCLOPOID COPEPOD POPULATION - DIRECT AND INDIRECT LIFE-HISTORY RESPONSES

We studied the effects of food limitation on the population dynamics o f the freshwater cyclopoid copt pod Diacyclops thomasi in Oneida Lake, New York. In the field population. maximum juvenile abundance coincid ed seasonally with high phytoflagellate concentration. During the clea r-water phase (a seasonal period of low algal density), D. thomasi dis appeared from the water column, but fourth-instar copepodids (CIV) wer e found encysted in developmental arrest in the sediment. Laboratory a ssays of the effect of the density of two types of food on copepod lif e history parameters showed that temporal variation in the concentrati on of relatively small phytoflagellates significantly affected stage-s pecific development times. This food limitation was most pronounced du ring the clear-water phase, and supplementation of the diet with a lab oratory-cultured phytoflagellate, Chlamydomonas, prevented food limita tion. Although developmental arrest appears to be controlled primarily by photoperiod, availability of the larger, more mobile food, Euglena , also influenced the percentage of individuals entering developmental arrest in the laboratory. An investigation of the spatial and tempora l emergence pattern in the field revealed that CIV copepodids started to emerge in late autumn and that emergence rates were significantly g reater at deepwater locations (9-12 m water depth) compared with shall ow-water locations (5-7 m). The clear-water phase in Oneida Lake is an annual event. probably produced by intense grazing by Daphnia pulicar ia and Daphnia galeata. Food limitation is thus very likely a recurren t phenomenon for D, thomasi. This apparent seasonal competitive impact of Daphnia on Diacyclops affects both nauplii and immature copepodids . Diacyclops shows two types of responses to the food limitation: (1) the physiological response of slowed active development, and (2) the a daptive response of developmental arrest.