Young-of-the-year fish assemblages as an alternative to adult fish monitoring for ecological quality evaluation of running waters

Studies on the Upper Mississippi River, particularly over the last 15 years, have contributed to our understanding of trophic processes in large rivers. The framework established by earlier population-specific studies, however, cannot be overlooked. Examination of the feeding habits of fish ranging from planktivores to piscivores gave the first indication that trophic processes were influenced by the spatial complexity and annual hydrological patterns of river-floodplain ecosystems. Experimental studies, which have often been considered impossible or impractical in large rivers, demonstrated the potential for biotic controls of system dynamics through predator–prey and competitive interactions. Such studies have been particularly helpful in understanding the potential impact of non-native species, including zebra mussels and Asian carp, to biodiversity and secondary production. Our understanding of riverine ecosystem function expanded greatly as food web studies began the application of a new tool—natural stable isotopes. Studies employing stable isotopes illustrated how food webs in a number of large rivers throughout the world are supported by the autochthonous production of microalgae. This study, coupled with other studies testing the prevailing models of riverine ecosystem function, has brought us to a point of better understanding the nature of river ecosystem functions. It is through looking back at the earlier studies of fish diet that we should realize that the temporal and spatial complexities of river ecosystem function must still be addressed more fully. This and a better grasp of the significance of the arrangement of patches within the riverine landscape will prove beneficial, as we assess the appropriate scale of river rehabilitation with an eye on how rehabilitation promotes productivity within complex ecosystems, including the Upper Mississippi River.