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Freshwater systems are dramatically transformed by human activities and are in need of effective monitoring to halt the decline of ecosystem health. Ecoacoustics presents a new cost effective way to monitor changes in ecosystems by investigating sounds and their relationships with the environment. However, while ecoacoustics has been explored as a monitoring tool in terrestrial and marine realms, it only gained traction in recent years in freshwater systems. In this thesis, I investigate ecoacoustics as a monitoring tool for freshwater systems. By investigating spatial-temporal variation in soundscapes of this realm (Chapter 2), examining the main biological drivers of these soundscapes (Chapter 3) and relating acoustic properties to environmental parameters (Chapter 4), I provide comprehensive studies on how freshwater soundscapes change and how ecoacoustics can be used as a monitoring tool. In the first study (Chapter 2), I characterize soundscapes across multiple freshwater streams, classify these streams according to their soundscape and use acoustic indices to describe spatial-temporal variation of freshwater soundscapes. The results demonstrate that soundscapes in 12 freshwater streams can be highly variable in both space and time. Even among similar streams in the same region, soundscapes differ greatly. Further all 12 streams used in this thesis have a unique soundscape with most differences between soundscapes being observed during the daytime. For the second study, I manually annotated biological sounds from the 12 freshwater streams analysed in the previous study. Results demonstrate high level of spatial and temporal variation in sound composition between streams with each stream containing a unique sound composition. The sound composition in each stream exhibits a daily cycle with site-specific sequences of sonic activity. Further, sound types are partitioned into temporal, frequency and spatial niches which aligns with the acoustic niche hypothesis. In my last study I relate acoustic properties of 12 freshwater streams to their environmental parameters. In particular, I explore the relationship between acoustic indices from Chapter 2, annotated biological sounds from Chapter 3 and environmental parameters of each stream. Further I investigate 1) the capacity of acoustic indices to detect biological sounds and 2) biological sounds and acoustic indices as potential monitoring tool for environmental parameters in freshwater streams. Results demonstrate that flow, depth and macrophyte cover are the key drivers of sound composition and that acoustic indices can be used to detect biological sounds and reflect environmental parameters in freshwater streams. Very few studies have explored soundscapes of freshwater bodies over a broad spatial and temporal scale. Overall, my thesis represents an important first step towards monitoring and analysing freshwater soundscapes. Soundscapes of freshwater streams exhibit spatial-temporal variation and are unique in their sound composition. This suggests that there is a great potential for ecoacoustics to provide a monitoring tool for freshwater systems, especially through easy and efficient use of acoustic indices. However, more research should investigate automated process to extract more acoustic information by using machine learning. My research expands our knowledge of freshwater acoustics and ecoacoustics as a cost effective long-term monitoring tool.

Background Interactions between aboveground and belowground terrestrial communities are often mediated by plants, with soil organisms interacting via the roots and aboveground organisms via the shoots and leaves. Many studies now show that plant genetics can drive changes in the structure of both above and belowground communities; however, the role of plant genetic variation in mediating aboveground-belowground interactions is still unclear. We used an earthworm-plant-aphid model system with two aphid species (Aphis fabae and Acyrthosiphon pisum) to test the effect of host-plant (Vicia faba) genetic variation on the indirect interaction between the belowground earthworms (Eisenia veneta) on the aboveground aphid populations. Results Our data shows that host-plant variety mediated an indirect ecological effect of earthworms on generalist black bean aphids (A. fabae), with earthworms increasing aphid growth rate in three plant varieties but decreasing it in another variety. We found no effect of earthworms on the second aphid species, the pea aphid (A. pisum), and no effect of competition between the aphid species. Plant biomass was increased when earthworms were present, and decreased when A. pisum was feeding on the plant (mediated by plant variety). Although A. fabae aphids were influenced by the plants and worms, they did not, in turn, alter plant biomass. Conclusions Previous work has shown inconsistent effects of earthworms on aphids, but we suggest these differences could be explained by plant genetic variation and variation among aphid species. This study demonstrates that the outcome of belowground-aboveground interactions can be mediated by genetic variation in the host-plant, but depends on the identity of the species involved.