Adapting to Change: Effects of Rising Temperatures on Bacterial Community Dynamics in New Zealand’s Alpine Lakes

<p dir="ltr">Climate change is currently one of the greatest threats to ecosystem functionality and biodiversity, particularly for alpine lake ecosystems. Rising temperatures are anticipated to induce shifts in bacterial community composition and richness, potentially favouring taxa that can negatively affect the ecosystem, such as cyanobacteria. Despite this, bacterial communities are rarely studied in alpine lakes located away from nutrient sources, limiting our understanding of the direct effects of temperature. Lake Greaney and Lake Harris are two alpine lakes in southern New Zealand, distal from nutrient sources, and provide a significant opportunity to explore the effects of temperature on bacteria. However, temperature change in New Zealand’s alpine regions is rarely recorded. Therefore, this thesis uses a combination of traditional (chironomid larvae) and molecular (metabarcoding and droplet digital Polymerase Chain Reactions) paleolimnological techniques to reconstruct temperature and bacterial community responses to anthropogenic warming from lakes Greaney and Harris.</p><p dir="ltr">Chironomids (Insecta:Diptera) were used to reconstruct temperature change over the last 1,000 years in two alpine lakes from southern New Zealand, testing their ability to detect modern warming. The temperature reconstructions revealed cool periods between ~900 – 1250 CE and ~1650 – 1850 CE, and warming intervals at ~1250 – 1650 CE and from ~1850 CE onwards. These climatic trends were observed across all models in both lakes, providing some confidence in our reconstructions. However, absolute summer mean temperature values were not accurately modelled and did not align with the instrumental record when compared to Queenstown’s meteorological station. Based on this, chironomids show some ability to quantify anthropogenic warming, but their inability to capture absolute values means caution is necessary when interpreting these records.</p><p dir="ltr">Using the chironomid-inferred temperature reconstuctions, the potential influence of global warming on bacterial and cyanobacterial community composition and richness was explored. Here I found tentative evidence for temperature-associated shifts in bacterial community composition and richness in both lakes Greaney and Harris. The most notable changes coincided with anthropogenic warming after 1850 CE, as suggested by the chironomid reconstructions and the Queenstown meteorological station. Despite this, changes could not be confidently attributed to warming temperatures, as results may have been confounded by the potential for bacteria to be actively living in the sediment core, limited taxonomic knowledge, and the potential influence from environmental variables not explored in this study.</p><p dir="ltr">Cyanobacterial communities also displayed potential temperature-driven shifts in composition, richness, and biomass peaks, all coinciding with anthropogenic warming. Toxic bloom forming cyanobacteria were also observed in Lake Harris around 1950 CE (1884 – 2019 CE) for one of the first times in New Zealand’s alpine lakes. However, cyanobacterial DNA counts were very low, indicating both lakes are likely still oligotrophic and unproductive. Although changes in cyanobacterial community dynamics may be explained by temperature, indirect evidence suggests atmospheric nutrient deposition could influence our results, thus its impacts cannot be ruled out. Despite this, bacterial community dynamics are changing, with the findings from this thesis acting as an early warning of the trajectory New Zealand’s alpine lakes may follow under a business-as-usual climate scenario.</p>