Landscape legacies in rivers: retrospective analyses of changes in ecosystem function across two centuries

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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.