Utility of the MBT/CBT paleotemperature proxy in lake sediments: Spatial variation in bacteria and bacterial lipid distribution in two New Zealand lake catchments
Branched glycerol dialkyl glycerol tetraethers (GDGTs), produced by unidentified bacteria, have been found ubiquitously in terrestrial and marine environments. The methylation and cyclisation of branched GDGTs is known to change in soils due to mean annual air temperature and pH, respectively. The identification of branched GDGTs produced within the water column and sediments of lakes indicates strong potential for the development of a temperature proxy for lake environments. In order to help develop the applicability of using branched bacterial GDGT lipids as a temperature proxy in lacustrine environments, this research set out to determine the distribution and provenance of bacterial communities and their corresponding GDGT lipids present in lake sediments in two small New Zealand lakes, the Karori Upper Dam and Lake Pounui. The Karori Upper Dam is a small lake, directly fed by two tributaries. Lake Pounui, in comparison, is much larger and fed by tributaries which are buffered by swamps.
Water and sediment samples from Lake Pounui and its catchment indicate a predominantly autochthonous production of branched GDGTs. However, the lake calibrated MBT/CBT paleotemperature proxy is not applicable to sites similar to that of the Karori Upper Dam which have a strong terrestrial branched GDGT signature. This research concluded that it can be expected that a terrestrial GDGT signature of some extent will be present in all lacustrine sites; however, only some sites will display a strong authochthonous lacustrine GDGT signal suitable for the application of the MBT/CBT paleotemperature proxy. Through the use of ARISA DNA analysis this research identified bacterial species which statistically explain a significant portion of variance in branched GDGT abundances. Based on the seasonal fluctuations of measured environmental controls it can be assumed that species abundance will also fluctuate. Future work will need to be undertaken in order to further understand this relationship as the sample size for this research was too small to determine the seasonal pattern of these bacteria.