Seasonal variability and sources of in situ brGDGT production in a permanently stratified African crater lake
Authors: van Bree Loes G. J. , Peterse Francien , Baxter Allix J. , De Crop Wannes , van Grinsven Sigrid , Villanueva Laura , Verschuren Dirk , Sinninghe Damsté Jaap S. 
Published: November 2020

Abstract. Lake sediments are important archives of continental climate history, and
their lipid biomarker content can be exploited to reconstruct
paleoenvironmental conditions. Branched glycerol dialkyl glycerol
tetraethers (brGDGTs) are bacterial membrane lipids widely used in
paleoclimate studies to reconstruct past temperature. However, major gaps
still exist in our understanding of the environmental controls on in situ (i.e.
aquatic) production in lake systems. In Lake Chala, a permanently stratified
tropical crater lake in East Africa, we determined the concentrations and
fractional abundances of individual brGDGTs along depth profiles of
suspended particulate matter collected monthly from September 2013 to
January 2015 and in settling particles collected monthly at 35 m water
depth from August 2010 to January 2015 and compared these brGDGT
distributions with those in surficial lake bottom sediments and catchment
soils. We find that brGDGTs are primarily produced within the water column
and that their concentrations and distributions vary greatly with depth and
over time. Comparison with concentration–depth profiles of the monthly
distribution and abundance of bacterial taxa, based on 16S rRNA gene
amplicon sequencing and quantification, indicates that Acidobacteria are
likely not the main producers of brGDGTs in Lake Chala. Shallowing of the
oxic–anoxic boundary during seasonal episodes of strong water column
stratification promoted production of specific brGDGTs in the anoxic zone.
BrGDGT distributions in the water column do not consistently relate with
temperature, pH, or dissolved-oxygen concentration but do respond to
transitions between episodes of strong stratification and deep (but partial)
lake mixing, as does the aquatic bacterial community. Hence, the general
link between brGDGT distributions and temperature in brGDGT-based
paleothermometry is more likely driven by a change in bacterial community
composition than by membrane adaptation of specific members of the bacterial
community to changing environmental conditions. Although temperature is not
the principal driver of distributional changes in aquatic brGDGTs in this
system, at least not during the 17-month study period, abundance-weighted
and time-integrated averages of brGDGT fractional abundance in the 53-month
time series of settling particles reveal systematic variability over longer
timescales that indirectly relates to temperature. Thus, although we do not
as yet fully understand the drivers of modern-day brGDGT fluxes and
distributions in Lake Chala, our data do support the application of brGDGT
paleothermometry to time-integrated archives such as sediments.
Highlights.
BrGDGTs in the tropical African lake Chala are produced in situ. Acidobacteria are not the dominant source of aquatic brGDGTs. Stratification and mixing drive aquatic brGDGT production and their
signature.