The ecological impacts of climate warming and pollution on freshwater microbial communities

Our planet is home to a rich diversity of species, communities and ecosystems which all contribute to providing a multitude of important functions and services. However, this diversity is changing at an unprecedented rate in response to human-induced changes. Microorganisms make up the base of food webs and drive ecosystem processes, functions and services, yet there is minimal research devoted to understanding microbial compositional and abundance responses to global change, such as temperature increases, within urbanised contexts. This is particularly pertinent for freshwater systems which are comparatively more sensitive to perturbations than terrestrial and marine ecosystems. This research aims to bridge this knowledge gap by studying how prolonged and acute warmings impact freshwater sediment bacterial communities originating from streams spanning an urbanisation gradient using experimental freshwater mesocosms. Sediment was collected from eight streams around Oslo, Norway which form part of a larger European project (CROSSLINK) studying the impacts of urbanisation on freshwater streams. Sixteen mesocosms were used to emulate streams wherein sediment samples from each of the eight sourcestreams were randomly split across two treatments and exposed to ambient (13 °C) or warmed +4 °C (17 °C) water temperatures. A two-tiered experimental design meant that bacterial community responses across the urbanised gradient exposed to both sustained periods of warming (chapter one) and an acute heatwave event (chapter two). Our results suggest that microbial community responses to warming are predominantly driven by pre-exposure to urbanised contexts. Warmingwas found to only be a significant driver of community responses when it interacted with catchment impact. We also show that communities can recover from stress regardless of impactsassociatedwithurbanisation.Thisnovel research highlights the importance of considering streams on a case-by-case basis due to the inherent complexity associated with multi-stressor responses.