Turbulence, instream wood and fish: Ecohydraulic interactions under field conditions

Previous work suggests that ecohydraulics should consider the broad range of parameters that characterize turbulent flow combining intensity, periodicity, orientation and scale (IPOS; Lacey, Neary, Liao, Enders, & Tritico, 2012), but ecohydraulics research under field conditions in natural river systems remains rare, largely due to practical constraints. A novel combination of turbulence properties, computed from high-frequency velocity time series, and underwater video of fish habitat use are presented here for two submerged large wood patches on a side channel of the Tagliamento River, Italy, providing insights into ecohydraulic interactions and the first known field application of the IPOS framework. Two adjacent wood patches of similar size reveal distinct differences in turbulence properties and fish habitat use, emphasizing the importance of considering the diverse properties of turbulent flow and reflecting the role of wood structural properties and position in determining the exact nature of hydraulic habitat. Key gradients in turbulence properties derived from multivariate analysis broadly align with IPOS categories, providing statistical validation for the IPOS framework. The application of IPOS to a habitat-focused study demonstrates its utility in understanding and deriving key trends from large and complex turbulence data sets. The results also provide a preliminary indication that IPOS-derived gradients may be helpful in explaining fish habitat selection, but these findings need further validation through high spatial resolution studies with different species and bioenergetics models. These insights support previous calls for inclusion of diverse turbulence parameters in ecohydraulics research and, where possible, more explicit consideration of turbulence properties in river assessment, conservation and restoration.