Collaborative Research: Adaptation of daphniids to digestion resistant resources

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The consumption of algae by invertebrates is a critical but poorly understood link in the food webs of all aquatic ecosystems: lakes, streams and oceans. Previous research has shown that the productivity of these invertebrate grazers, and hence the productivity of the rest of the food web including the fisheries, is often limited by the quality of the algal food. In the open water of lakes, a major determinate of algal food quality is their resistance to digestion while in the gut of the grazer. While it has been long known that some algae may pass unharmed through the guts of small invertebrates, the broad relevance of this defense mechanism to food web ecology and to the evolutionary biology of the grazers has been ignored. Major goals of this research will be to examine the physiology of grazer digestion of resistant algae, to quantify the extent to which grazer species vary in ability to overcome digestion defenses, and to experimentally test the importance of this grazer-algae interaction to the functioning of aquatic food webs. The general research approach will be to focus on an easily studied organism that is also a major grazer of algae in the open water of lakes: daphniids. Using both cultured and natural algae that differ in digestion resistance, this research will measure feeding rate, gut processing, respiration, assimilation, and growth of daphniid grazers. Several species, representing the breadth of diversity within the daphniids, will be studied at various animal ages from juvenile to adult. Finally, the ecological consequences of variation in grazer digestive physiology will be examined using a large-scale enclosure experiment in a natural lake. The experiment will manipulate grazer species and test the hypothesis that grazer digestive physiology influences population dynamics of grazers and algae in lakes.This research is innovative in exploring the diversity of feeding and gut processing in an ecologically key group of grazers, and in testing the consequences of this physiological diversity to food web function. Despite their role as dominate grazers in the open water of lakes, there is little understanding of how daphniids digest algae in nature. Similarly, while it is known that algae vary in digestion resistance, there is little understanding of how grazers adapt to such defenses. It is the general aim of this research to address these two deficiencies by studying the ecological and evolutionary physiology that constrains grazer-algal interactions in aquatic ecosystems.Training of undergraduates and graduates at the interface of physiology and ecology is emphasized in this research. The proposed work will also continue a research and training partnership between a primarily undergraduate institution and a research-intensive field station, and provide for year-round undergraduate involvement in laboratory and field studies that link organism physiology to the structure and function of food webs.