Copepods live in a nutritionally dilute environment, experiencing temporal and spatial variations in food supply which differ in magnitude and predictability. Understanding the mechanisms by which organisms deal with changes in their food is a primary concern in elucidating the nutritional ecology of zooplankton and the role of food in structuring zooplankton communities. In this thesis, I examine changes in behavior, morphology, and physiology of two species of calanoid copepods (Diaptomus kenai and D. leptopus) in response to variation in food composition and density. In Chapter Two, I present a study of population-level responses to variation in food composition and quality, using fertilization techniques to generate a range of phytoplankton communities in field enclosures. The phytoplankton assemblages studied supported copepod populations which differed in such attributes as population size, reproduction, and body size. The most striking finding of this study was the occurrence of two co-existing size classes of D. kenai, the abundance and clutch size of which varied among the enclosures, suggesting differences between the two classes in their ability to utilize the different phytoplankton communities. Chapter Three investigates patterns of lipid storage in response to changes in food supply. I concluded that lipid stores were affected by species composition of the phytoplankton food and the two copepod species differed in their sensitivity to differences in cell chemistry. Chapter Four investigates behavioral responses of the two size classes of D. kenai to changes in food composition and abundance. Subtle differences in feeding behavior suggest that the two sizes differ in their utilization of available food. These results demonstrate that D. kenai and D. leptopus are capable of responding to changes in their food supply through modifications of their behavior, morphology, and physiology over a range of magnitudes and time scales. There can be very subtle changes in feeding behavior or very pronounced changes in size structure. Responses occur over time scales ranging from hours to days to seasons. These results also bring into question the utility of models generated under laboratory conditions in predicting behaviors or dynamics of copepod populations and communities in nature. / Science, Faculty of / Zoology, Department of / Graduate