SUPERVISOR: Thomas HEIN

PROJECT ASSIGNED TO: Anna-Lisa DITTRICH

Waterways used for freight and recreational navigation are exposed to various stressors. Besides morphological modifications such as river straightening (Gregory, 2006), ships have direct impacts on the adjacent shoreline zones through an increase in flow velocity and the production of waves (Thunnissen et al., 2019). This affects not only bank erosion and increases turbidity (Göransson et al., 2014), but influences all aquatic organism groups and can therefore alter aquatic food web interactions (Gabel et al., 2017).

The concept of food webs is useful to describe the feeding interactions and energy flow among species within an ecological community (Memmott, 2009). A differentiation can be made between green food webs, primarily driven by photosynthesis-based primary production (Heath et al., 2014), and brown food webs, primarily driven by non-living organic matter, so called detritus (Moore et al., 2004). Among other factors, their distribution is influenced by flow velocity (Yang et al., 2022) and substrate size (Cordone et al., 2020). A linkage between both food webs occurs through mobile consumers such as fish (Sitvarin et al., 2016) or via nutrient cycling by microorganisms (Zou et al., 2016). Human activities can alter food webs structures by affecting resource availabilities, energy flows and interactions. Therefore, food webs can be useful to describe consequences of human interactions and figure out how resilient an ecological system is against perturbations (Cross et al., 2013). Modelling trophic networks is difficult, as comparable data on a high taxonomic level is often not available (Patonai and Jordán, 2021). Therefore, this dissertation aims to simplify this attempt by modeling the distribution of green and brown food webs using simple predictive variables such as shear stress, flow velocity or substrate size distribution with data from a free-flowing stretch of the Danube River east of Vienna.

Figure 1: A schematic representation of brown food webs (based on detritus) and green food webs (based on primary production) with their coupling through consumers and nutrient loops. Shown in red are the impacts by wave action stressors that cascade throughout the food web (red arrows).

Navigation can alter these food webs by changing flow velocities or increasing turbidity. Additionally, ship-induced wave action and increased flow velocities at littoral zones show negative impacts particularly on the critical early life stages of fish, which inhabit those shallow river sections (Hirzinger et al., 2002). While direct effects through stranding and drifting of juvenile fish have received increased attention in the last years (Ratschan et al., 2012; Schiemer et al., 2001; Schludermann et al., 2014), less is known about indirect effects on juveniles via bottom-up and top-down effects in the food webs. For instance, this can occur via decreased food availability through increased turbidity (Zauner and Schiemer, 1994) or by alteration of the composition of benthic invertebrates or algae by increased flow velocities (Finlay et al., 1999). Both top-down and bottom-up cascading effects in aquatic food webs are going to be investigated in the framework of this dissertation. Therefore, enclosure experiments with juvenile fish are going to be conducted at shoreline areas of the Austrian Danube at sites with different hydro-morphological properties and exposures to wave action.

To protect aquatic ecosystems from ship-induced waves, mitigation measures are implemented in some rivers, lakes and sea shorelines (Preuß et al., 2023). These include the creation of wave-protected shore channels by so-called longitudinal training dams (Collas et al., 2018), modified groynes, (re-)connected side channels (Ramler and Keckeis, 2019) or vegetated shorelines (Roo and Troch, 2015). To date, there is no comprehensive overview which wave-protecting measures exist and how they affect the different organism groups. Therefore, this dissertation aims to review the currently existing protective structures against ship-induced waves with their effects on benthic algae, benthic invertebrates, and juvenile fish and to synthesize the implications for aquatic food webs.

Overall, this PhD aims to achieve a better understanding of aquatic food webs, how they are affected by navigation-induced waves and currents and which measures can help to reduce wave exposure for all groups of organisms with a special emphasis on young fish whose survival is crucial for the preservation of the entire population. As human uses of rivers often represent an area of conflict between the ecological demands of a river and human interventions, this PhD aims to generate knowledge for designing river restorations in a way that they can meet both needs, the ecological functioning of rivers as well as their ecosystem services for human recreation or transportation. These societal aspects are an important part of the dissertation, along with the ecological aspects, and engineering aspects are also addressed by measuring hydraulic and morphological parameters such as wave action or river bank slope. Therefore, the coupling and interaction of natural and social systems is investigated with the aim of finding the best balance regarding navigation in human rivers.

References

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