"Modelling approach for interstitial sediment dynamics and reproduction of gravel-spawning fish"The complexity and dynamic nature of ecosystem processes impose high requirements on the approaches, methods and modelling techniques applied to support ecological assessments of rivers. Particularly the interactions of abiotic and biotic variables, the high spatial and temporal variability of parameters and processes and the interdisciplinary research field present a special challenge on the development of appropriate tools. Given the naturally dynamic creation, destruction and maintenance of habitat templates in rivers (habitat dynamics) the habitat can be regarded as a basic element of fluvial ecosystems. Accordingly, high demands are placed on aquatic habitat modelling techniques emphasizing the need for the improvement and further development of existing approaches.
The present study predominantly addresses three research fields encompassing the hydromorphology, the fluvial ecology and the hyporheic interstitial of rivers. All disciplines are involved by interacting processes defining the quality of reproduction habitats for gravel-spawning fish. This work is focused on implementing the hydromorphological and hyporheic variability in physical habitat modelling considering all variables that describe the habitat in their spatial and temporal variability to allow a dynamic representation of habitat suitability. The reproduction period of gravel-spawning fish works as an excellent indicator for interstitial habitats, as the life-stages during reproduction are characterised by high requirements on the habitat. Based on the abiotic description of the environment a multi-step habitat modelling framework is developed that addresses each life-stage during the reproduction by an appropriate selection of key habitat variables that are linked via a multivariate fuzzy-logic model to simulate habitat suitability indices of each life stage during the reproduction period. The last step of the modelling framework includes the aggregation of the dynamic habitat values to a temporally integrated parameter and the final result of the modelling framework, the reproduction habitat suitability.
The proposed multi-step habitat modelling framework is applied in a mountainous river reach downstream of a dam and produced reliable results. The simulated habitat suitability indices for each life-stage during reproduction allow for a representation of physical habitats in the form of spatial distribution maps for different time-steps, time-series for different locations and an integrated habitat supply over the entire reproduction period. This provides highly valuable information about habitat dynamics as all spatially and temporally varying input variables are considered in the multi-step habitat modelling framework. Consequently a direct identification of occurring bottlenecks during the reproduction of brown trout is feasible and can be referred back to responsible habitat variables. In the case study it is found that the spawning and emergence stages are not limiting the reproduction success and the most restricting conditions occurred during hatching. These limitations are predominantly caused by critical temperatures during the winter season and critical permeability conditions due to sediment infiltration processes during the regulated flow period. The aggregated reproduction habitat suitability contains the summarized effects of all varying abiotic conditions during the reproduction period of gravel-spawning fish and allows for a quick identification of the availability and quality of reproductive habitats.
Although the obtained results provide valuable results it is worth noting that models in general are never able to fully reflect the dynamic behaviour of rivers and its ecological relations given their numerous and complex interactions. The simplification of the physical and ecological processes requires a well-founded verification of obtained simulation results against field observations and reference sites. The highest benefit of the proposed modelling framework comprises the spatial and temporal consideration of conventional and new habitat variables resulting in a detailed representation of habitat dynamic processes occurring in river reaches. Further, the presented work is the first attempt to simulate the quality of reproduction habitats for gravel-spawning fish using physical habitat modelling. Possible future applications predominantly include the support of ecological impact assessments but also the applicability as an instrument supporting the management and planning processes of restoration measures (e.g. for re-establishing reproducing fish population in rivers) as the simulation of reproduction habitats presents one fundamental process for the development of stable fish populations.