This research project will address the question of how external nutrient inputs, such as hippo feces and wildebeest carcasses, affect secondary producers in the Mara River of East Africa. By looking into the question of where aquatic insects and fish get their nutrients from, we can learn more about how food webs work and how they are arranged. Theory suggests that a low magnitude of external inputs will stabilize food webs and increase secondary production, while high magnitude inputs will decrease stability and make trophic cascades more likely to occur. However, observation suggests that as the magnitude of input increases, so does the magnitude of the aquatic consumer population. This project seeks to further examine this phenomenon in order to better understand the relationship between the magnitude of nutrient input and the way the river ecosystem functions. Currently, we have sample specimens from the Mara River, and we will be analyzing the diets of these aquatic insects and fish. To do this, we will use stable isotope analysis to look into the carbon and nitrogen content, as well as a microscopic examination of the specimens’ stomach contents. We will perform these analyses on specimens collected at sites with a gradient of animal inputs and across a gradient of river discharge, to determine what influences secondary production. By determining whether the food source for these consumers comes from inside the aquatic ecosystem, from the algae that grows in the river, or from outside of the system, hippo feces and carcasses, we can assess the stability of the food web, potential impacts on the size of aquatic consumer population, and the potential occurrence of trophic cascades. Understanding these relationships is important if we want to better respond and adapt to changes in ecosystems, and to the decline in a great number of animal species. Furthermore, the Mara River is an important river as it provides water to nearly one million people, as well as several protected areas in the Serengeti Mara Ecosystem; thus, it is vital that we understand how it functions, and how to ensure that it remains healthy.