Natural variations in physical processes alter ecosystem structure and function, with the greatest impacts caused by natural and human disturbances that alter the control mechanisms defining how an ecosystem operates. The primary ecosystem driver in rivers relates to the timing, volume of water, and duration of high- and low-water periods as part of the natural flow regime. Ecological responses to altered control mechanisms can be linear or nonlinear, with the latter including somewhat unpredictable threshold responses where a community shifts abruptly to an alternative state of ecosystem function. Research on alternative states may determine whether some shifts to undesired alternative states are both predictable and reversible. Unfortunately, most studies of alternative stable states have been theoretical and very limited in time due to funding constraints. Our project circumvents these problems by using stable isotope analysis of museum specimens to determine how changes in the natural flow regime over a period of 100-150 years have altered food web complexity in medium to large rivers. Our initial project will focus on differences in food web complexity in the main channel, side channels, and backwaters of three geomorphically diverse areas of the Upper Mississippi River in response to altered flow regimes. Food webs will be constructed from real-time and museum samples of fish in multiple feeding groups.
This project will also help alleviate the general dearth of knowledge about the functioning of rivers and will thus focus on ecosystems that have experienced relative declines in biodiversity far greater than those in the most affected terrestrial ecosystems of North America. This historical knowledge of effects of changing ecosystem drivers on functioning of river ecosystems is vital because we need to predict effects of future disturbances if, as expected, global climate changes and the need for more so-called green energy increases socioeconomic pressures to build more dams. The initial project takes the form of a proof-of-concept study and is thus limited in personnel and opportunities for the extensive broader impacts proposed for our full study. However, the project will include undergraduate assistants from two universities who will learn both field and lab techniques as well as gain a broader appreciation of the ecology of complex ecosystems.
Understanding effects of human impacts on aquatic ecosystems is often very difficult because ecological responses may stretch over years, decades, or even longer periods and scientists are constrained by research time and grant funds. Therefore, techniques that enable scientists to conduct retrospective analyses can not only save time and money but also provide insights to potential effects of future anthropogenic impacts. One such technique relies on analysis of environmental impacts on the length and complexity of river food webs using changes to the chemical composition (ratio of heavy to light stable isotopes of carbon and nitrogen) of living and preserved museum specimens of fish and mussels. The current NSF project focused on changes occurring in food webs from construction of low-head dams (less than 15 m tall) navigation dams on the Upper Mississippi River. The project examined several sections of the river because the physical character (number of channels, size of floodplain) may result in different environmental responses to river modification. Samples were also collected in main channels, side channels, and backwaters, when present, because these areas present the physical diversity of a river. The results of this NSF study combined with previous analyses of six rivers in the Mississippi River drainage revealed that: (a) modification of river height by low-head dams significantly changed food webs to a new steady equilibrial state by shifting the primary base of the food web from bottom-dwelling to water-column algae, with resulting effects on consumers; (b) food chain length varies significantly with the physical characteristics (hydrogeomorphic structure) of the river section; and (c) environmental impacts can be detected using samples from both main channel and side channel systems. In addition to the scientific results of this research, this project had a number of broader impacts. A total of 5 female and 8 male undergraduate students were trained in field sampling techniques and laboratory analyses using stable isotopes. This number included 9 students who based their senior capstone projects on this NSF funded study. Moreover, of the 18 scientific presentations at conferences and other universities, 9 were senior-authored by undergraduate students. Results of this study contributed to 3 manuscripts "in review" within scientific journals and 1 "in press" chapter in a book to be published by Elsevier. At least 2 more scientific journal articles are expected to result from this study. The project demonstrated fruitful collaborations among a Research 1 university (University of Kansas), a predominately undergraduate institution (Winona State University), and a natural history museum at the University of Illinois. Finally, the results were incorporated into lectures for undergraduates and graduate students at the University of Kansas and Winona State University.
