Macroinvertebrate Response to Flow Changes in a Subalpine Stream: Predictions From Two‐dimensional Hydrodynamic Models

Abstract

Two-dimensional hydrodynamic models are being used increasingly as alternatives to traditional one-dimensional instream flow methodologies for assessing adequacy of flow and associated faunal habitat. Two-dimensional modelling of habitat has focused primarily on fishes, but fish-based assessments may not model benthic macroinvertebrate habitat effectively. We extend two-dimensional techniques to a macroinvertebrate assemblage in a high-elevation stream in the Sierra Nevada (Dana Fork of the Tuolumne River, Yosemite National Park, CA, USA). This stream frequently flows at less than 0.03 m 3 s À1 in late summer and is representative of a common water abstraction scenario: maximum water abstraction coinciding with seasonally low flows. We used two-dimensional modelling to predict invertebrate responses to reduced flows that might result from increased abstraction. We collected site-specific field data on the macroinvertebrate assemblage, bed topography and flow conditions and then coupled a two-dimensional hydrodynamic model with macroinvertebrate indices to evaluate habitat across a range of low flows. Macroinvertebrate indices were calculated for the wetted area at each flow. A surrogate flow record based on an adjacent watershed was used to evaluate frequency and duration of low flow events. Using surrogate historical records, we estimated that flow should fall below 0.071 m 3 s À1 at least 1 day in 82 of 95 years and below 0.028 m 3 s À1 in 48 of 95 years. Invertebrate metric means indicated minor losses in response to modelled discharge reductions, but wetted area decreased substantially. Responses of invertebrates to water abstraction will likely be a function of changing habitat quantity rather than quality. Figure 2. Study site map showing simulated 0.086 m 3 s À1 water's edge, locations of invertebrate samples (dots) and water surface measurement locations (X's)

Uncertainty in Various Habitat Suitability Models and Its Impact on Habitat Suitability Estimates for Fish

“…A 2D numerical hydrodynamic model, River 2D, was used to generate spatial maps of current velocity (V) and water depth (D) over the entire study area. The accuracy of the simulation depended strongly on the quality of bed topography data [49]. In this study, the simulated results showed strong agreement with the empirical data, supporting River 2D's applicability to small scale analysis [12].…”

Improving Hydrodynamic Modelling: an Analytical Framework for Assessment of Two‐Dimensional Hydrodynamic Models

“…Two‐dimensional hydrodynamic models have been used to evaluate aquatic habitat for a variety of fish species (Gard, ; Guay et al, ; Boavida et al, ) and invertebrates (Waddle and Holmquist, ; Alexander et al, ), as well as for planning and assessing river rehabilitation design (Wheaton et al, ; Pasternack et al, ; Brown and Pasternack, ; Gard, ). They are useful tools for describing the spatial distribution of hydraulic variables relevant to aquatic habitats, such as mean column water velocity (velocity), water depth (depth) and bed shear stress within the river channel over varying discharges.…”

Ecosystem‐based environmental flow assessment in a Greek regulated river with the use of 2D hydrodynamic habitat modelling

“…Their habitat preferences were assessed using the “benthos‐GR” dataset (https://github.com/chtheodoro/benthos-GR), consisting of 380 microhabitat observations collected from nine sampling sites in Greece (Theodoropoulos et al, ). Each microhabitat is a combination of V, D, and S corresponding to a habitat suitability value ( κ ), calculated using BM‐community metrics commonly applied to assess the quality–suitability in relevant studies (Englund & Malmqvist, ; Holmquist, Schmidt‐Gengenbach, & Roche, ; Monk et al, ; Waddle & Holmquist, ). Each metric was weighted based on a combination of expert‐judgement and previous literature to reflect its relevant contribution‐significance to the calculation of κ and the following equation was used: where κ is the calculated habitat suitability of the ith microhabitat of the jth site, ranging from 0 to 1. n ij denotes the number of BM taxa (families) found at the ith microhabitat of the jth site. H ij denotes the Shannon's diversity index for the ith microhabitat of the jth site. EPT ij is the number of EPT taxa found at the ith microhabitat of the jth site. a ij is the abundance of benthic macroinvertebrates found at the ith microhabitat of the jth site. n jmax , H jmax , EPT jmax , and a jmax denote the maximum value of the relevant variables observed at the jth site. …”

“…A three‐class hierarchy of EFA methodologies has been suggested, with HHMs being considered as “level 3,” potentially applicable in situations where a high degree of certainty is required to provide water managers with defensible eflow recommendations. But despite the excessive long‐term research, the improved predictive accuracy, and the wide recognition of their effectiveness in providing accurate EFAs, the practical application of HHMs in EFAs in Europe remains disproportionately limited compared with their hydrology‐based alternatives (Dunbar, Alfredsen, & Harby, ; Linnansaari, Monk, Baird, & Curry, ; Rivaes, Boavida, Santos, Pinheiro, & Ferreira, ) and is primarily focused on fish (Arthington, ; Leitner, Hauer, & Graf, ; Waddle & Holmquist, ). The low percentage (18%) of European HHMs‐based case studies in the relevant WFD Guidance Document is also indicative of this gap between theoretical research/knowledge and practical application of HHMs.…”