ALBERT

Description: 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.

Description: Settling particles in the water column of Lake Chala, a small (4.2 km2), deep (~90 m), and permanently stratified tropical crater lake at ~880 m above sea level in East Africa (3°19'S, 37°42'E) were collected monthly at 35 m water depth from August 2010 to January 2015 from mid-lake position. The collected material was allowed to settle and stored frozen after decantation of excess water. Prior to analysis, the samples were thawed, filtered over pre-combusted glass fibre filters, and the frozen and freeze dried. Bulk mass flux is expressed as mg m-2 d-1. The concentration of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in the settling particles was analyzed using ultra high performance liquid chromatography-mass spectrometry, separating 5-methyl and 6-methyl brGDGT isomers.

Description: The fractional abundance of branched glycerol dialkyl glycerol tetraethers (brGDGTs) including 5-methyl and 6-methyl isomers was analyzed in a selection of seven soils collected by Buckles et al. (2014) from various locations within and around the crater catchment of Lake Chala, a small (4.2 km2), deep (~90 m), and permanently stratified tropical crater lake at ~880 m above sea level in East Africa (3°19'S, 37°42'E). The soils were collected between 25 January and 5 February 2010. The selection covers the four different soil types that have been identified based on cluster analysis using the organic carbon content and the concentration and distribution of brGDGTs in a total of 32 soils (Buckles et al., 2014).

Description: The fractional abundance of branched glycerol dialkyl glycerol tetraethers (brGDGTs) including 5-methyl and 6-methyl isomers was analyzed in surficial lake sediments from Lake Chala, a small (4.2 km2), deep (~90 m), and permanently stratified tropical crater lake at ~880 m above sea level in East Africa (3°19'S, 37°42'E). The lake sediments were collected in January-February 2010 by gravity coring at three different water depths along a transect from close to the creek inlet (at 64.7 m water depth) towards the depocenter of the lake (at 61.6 m water depth).

Description: Vertical profiles of temperature, dissolved oxygen (DO), conductivity and pH in the water column of Lake Chala, a small (4.2 km2), deep (~90 m), and permanently stratified tropical crater lake at ~880 m above sea level in East Africa (3°19'S, 37°42'E), were measured at 2 m intervals through the upper 50 m of the water column at monthly intervals between September 2013 and January 2015. In the same time interval, suspended particulate matter (SPM) was collected monthly at 13 discrete depths by filtering 5-10L of water over pre-combusted glass fibre filters. A 1/32 section of the same SPM filters as used for brGDGT analysis was used for DNA extraction, after which 16S rRNA gene amplicon sequencing and quantification was performed to generate concentration-depth profiles of the monthly distribution and abundance of bacterial taxa.

Description: Vertical profiles of temperature, dissolved oxygen (DO), conductivity and pH in the water column of Lake Chala, a small (4.2 km2), deep (~90 m), and permanently stratified tropical crater lake at ~880 m above sea level in East Africa (3°19'S, 37°42'E), were measured at 2 m intervals through the upper 50 m of the water column at monthly intervals between September 2013 and January 2015. In the same time interval, suspended particulate matter (SPM) was collected monthly at 13 discrete depths by filtering 5-10L of water over pre-combusted glass fibre filters. The concentration and fractional abundance of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in SPM was analysed for all depths for the months November 2013 and August 2014, and at 0, 10, 25, 35, 50, 60, 70, and 80 m depth for all other months (total n= 146). The brGDGTs were analyzed using ultra high performance liquid chromatography-mass spectrometry, separating 5-methyl and 6-methyl brGDGT isomers.