9623148 Morin Food chain theories linking trophic structure (number and composition of trophic levels) to productivity are important to understanding the regulatory roles of these factors in ecological systems. Tests of these theories also provide insight into the mechanisms of nutrient effects in natural and enriched ecosystems. In this study, productivity and number of trophic levels in the food chain will be experimentally manipulated in microcosms containing protists and bacteria. One set of experiments will test predictions of a food chain model by assembling linear, homogeneous food chains of one, two, and three trophic levels and monitoring biomass within each trophic level over a range of productivity generated by different nutrient conditions. A second set of experiments will test the impact of added heterogeneity (multiple species or types of species per trophic level) on trophic levels one and two. These experiments will also test predictions concerning conditions under which increased productivity causes an increase in the abundance or biomass of species in the top trophic level. Adequate tests of food chain theories are lacking, largely due to the difficulty of manipulating productivity over a range sufficient to alter number of trophic levels. The experimental microcosms that will be used in this study provide a unique opportunity to test food chain theory experimentally.
