Abstract
Climate and land use changes put New Zealand’s freshwater ecosystems at risk and threaten biodiversity and human health. Recent research has largely focused on directly impaired freshwater ecosystems. However, little is known about the implications of environmental and climate change for New Zealand’s pristine alpine lakes and their catchments.
To identify current trajectories and improve the understanding of environment–climate dynamics, I reconstruct the climatic and environmental evolution of two remote alpine lakes in central Fiordland. Fiordland’s location within the influence zone of two global climate modes (ENSO and SAM), as well as the orographically pronounced steep local precipitation and temperature gradients, provides ideal conditions to study past climate variability and its impacts on primary productivity and catchment evolution.
The two sediment records obtained from adjacent Lake Bright and Lake Laffy cover the last 17,000 years since the last deglaciation. Applying sedimentary lipid biomarker proxies, I reconstruct average air temperatures from bacterial membrane lipids (i.e., branched glycerol dialkyl glycerol tetraethers, brGDGTs) and identify changes in moisture source by analysing compound-specific stable hydrogen isotopes in leaf wax n-alkanes. In combination with XRF-core scanning and bulk sediment stable isotope analysis, this multi-proxy analysis provides a novel approach for paleoenvironmental research in New Zealand.
The combined record of the two lakes provides important constraints on temperature and hydroclimate development over the Late Glacial and Holocene. My interpretations suggest that a northward shift of the Southern Hemisphere Westerly Winds (SHWW) during the Antarctic Cold Reversal (∼14,500 years BP) shaped cold and wet conditions in Fiordland, modulating glacial dynamics and influencing lake water level fluctuations. Over the Holocene, temperatures remained relatively stable, but hydroclimate variability intensified as SHWW influence over Fiordland decreased and ENSO-driven northerly airflow increased. Over the last millennium, declining runoff, lowering water tables, and enhanced in situ productivity in the lakes suggest
a strong ecosystem response to changes in the dominant moisture source and an increased seasonality. Together, these findings show that while the SHWW historically dictated Fiordland’s moisture balance, future warming and an altered ENSO/SAM dynamic may promote more pronounced climate extremes, influencing lake productivity and biodiversity in Fiordland’s sensitive alpine ecosystems.