Unveiling Undisturbed Alpine Lake Ecosystems: A Multiproxy Approach to Reconstruct past Ecological Response to Climate Variability in the New Zealand Southern Alps

Apart from natural variations in environmental and climatic conditions, human activities (e.g. land-use changes) can have major impacts on freshwater ecosystems. In the context of New Zealand, the arrival of humans approximately 750 years ago has left a permanent mark on landscapes, posing challenges in discerning undisturbed natural conditions for paleoclimatic reconstructions. Yet, remote alpine lakes in the Southwest Pacific can serve as pristine archives of environmental and climate changes, hardly influenced by human activities. This study focuses on sediment cores obtained from two neighbouring catchments of Lake Bright and Lake Laffy in Fiordland National Park, situated in an understudied remote area of New Zealand’s Southern Alps. Employing a multiproxy approach, we utilized bulk C and N stable isotopes, lipid biomarker analysis, and high-resolution X-ray fluorescence and hyperspectral imaging to reconstruct Holocene changes in catchment dynamics and climate. Here, we present a new precipitation record based on the compound-specific composition of hydrogen isotopes of plant wax n-alkanes as well as a branched glycerol dialkyl glycerol tetraethers (brGDGTs) derived temperature reconstruction. Examining a sequence spanning the late Holocene, we discern events of evolving environmental conditions impacting these catchments. Our biomarker paleorecord from Lake Bright revealed a relatively stable temperature range during the last 4 ka, indicating that the changes in the sedimentary record might be dominantly shaped by the local hydroclimate. The low compound δ2H values observed reflect precipitation that has been orographically uplifted over the Southern Alps. Downcore δ2H variations are likely influenced by changes in the isotopic composition of rainfall driven by regional temperature and latitudinal sources of westerly precipitation. In this context, we present these episodes of hydroclimatic variability and derive the ecosystem response to prevalent conditions by correlating the respective changes in lake productivity, water quality and shifts in vegetation composition. Finally, we extend this record through the Holocene to the last local glacial termination about 10 ka ago using the Lake Laffy stratigraphy and engage in a discussion regarding the potential implications of enhanced climate variability on the future integrity of New Zealand's pristine alpine ecosystems.