9320303 Thorp The Mississippi River and some of its tributaries are currently experiencing catastrophic floods which have significantly modified their typical hydrological patterns. The ecological impacts of an extreme flood occurring outside of the normal flood period should have substantial effects on this important lotic ecosystem, such as altering normal patterns of input, storage, and transport of organic matter and nutrients. In addition, sources of organic matter which are not normally available may be introduced onto the river at this time. These changes in nutrient supply and timing should act as a major disturbance to the trophic dynamics of the Mississippi River ecosystem. We hypothesize that the catastrophic flood presently occurring in the Mississippi River ecosystem will produce a shift in normal trophic processes by altering the role of different sources of organic matter in the riverine food web. This hypothesis will be tested by evaluating the stable-isotope ratios in the food webs of four rivers (two flooded and two unflooded, flood plain rivers). During three dates over a year period, we will collect 5 replicates of 21 sample units per river, each representing a nutrient source of organic processor. We will then determine the composition of 13C and 15N and C/N ratio in the sample units. To ascertain whether changes in the Mississippi River Ecosystem over time reflect the effects of the flood or merely represent normal seasonal variations, we will obtain samples from four rivers: the Missouri and upper Mississippi Rivers, which are currently experiencing the worst floods of the century, and the Ohio and Wabash Rivers, which are flood plain rivers that have not been inundated by flood waters. Information from this study will contribute to a data base useful for making management decisions involving the role of flood plains in river ecosystems.
