Hydrogeomorphology creates heterogeneous landscapes—But do fish care?

Freshwater habitats display a high degree of heterogeneity in both space and time. This heterogeneity is a result of the hierarchical interplay of hydrology and geomorphology, producing the physical habitat template to which biological communities respond and interact. Thus, it follows that any effort to fully understand the complexity of these systems requires a spatial and temporal perspective of heterogeneity at multiple scales within a hierarchical framework. In this study we 1) test if fish assemblage structure differs between hydrogeomorphically distinct, off-channel patches of the Upper Mississippi River, USA, 2) identify which species are driving differences in assemblages between patch types and, 3) assess which hydrogeomorphic variables are associated with fish assemblages and if these are the same suite of variables responsible for the physical differentiation of patches. Different fish assemblages were associated with hydrogeomorphically distinct patches, importantly, sub-groups of backwaters. Species including the Bluegill Lepomis macrochirus Rafinesque, 1819, the Black Crappie Pomoxis nigromaculatus (Lesueur in Cuvier and Valenciennes, 1829), and the Gizzard Shad Dorosoma cepedianum (Lesueur, 1818) were influential in driving differences in assemblages. Fish assemblages did not respond to the same suite of hydrogeomorphic variables responsible for the physical character of patches, although there was some overlap. Instead, they responded to patch-scale characteristics and long-term hydrology and, in the case of backwaters, which navigation pool they were located in. We suggest that classification of hydrogeomorphic character across multiple spatio-temporal scales is critical. This classification should be done prior to sampling so patches can be targeted based on their physical character to draw out key habitat-assemblage relationships. We also contend that broad classification of habitats (e.g., floodplain lake vs backwater) may be too simplistic. Instead, individual characteristics defining the physical nature of a habitat need to be linked to biological responses across scales. A more nuanced understanding of these relationships is needed to improve restoration and management outcomes with respect to increasing or maintaining heterogeneity in the landscape.