Seasonal variability in the abundance and stable carbon-isotopic composition of lipid biomarkers in suspended particulate matter from a stratified equatorial lake (Lake Chala, Kenya/Tanzania): Implications for the sedimentary record

We studied the distribution and stable carbon-isotopic (δ¹³C) composition of various lipid biomarkers in suspended particulate matter (SPM) from the water column of Lake Chala, a permanently stratified crater lake in equatorial East Africa, to evaluate their capacity to reflect seasonality in water-column processes and associated changes in the lake's phytoplankton community. This lake has large seasonal variation in water-column dynamics (stratified during wet seasons and mixing during dry seasons) with associated phytoplankton succession. We analyzed lipid biomarkers in SPM collected monthly at 5 depths (0–80 m) from September 2013 to January 2015. Seasonal variation in total phytoplankton biovolume is strongly reflected in the concentration of phytadienes, a derivative of the general photosynthetic pigment chlorophyll. The wax and wane of several specific biomarker lipids between June and December 2014 reflect pronounced phytoplankton succession after deep mixing, starting with a long and sustained chlorophyte bloom (reflected by C_23:1, C_25:1 and C<sub27:1</sub><i>n</i>-alkenes, and C₂₁ and C₂₃<i>n</i>-alkanes), followed by a peak in diatoms between July and October (loliolide and isololiolide), and then eustigmatophytes (C₃₀ and C₃₂ 1,15 diols) once stratification resumes in October. Peak abundance of the C_19:1<i>n</i>-alkene during shallow mixing of the water column in January–February 2014 can be tentatively linked to the seasonal distribution of cyanobacteria. The concentration, seasonal variability, and low δ¹³C values of the C₂₈ fatty acid in the SPM suggest that this biomarker is produced in the water column of Lake Chala instead of having the typically assumed vascular plant origin. The δ¹³C signature of particulate carbon and all aquatic biomarkers become increasingly more negative (by up to 16‰) during mixing-induced episodes of high productivity, whereas enrichment would be expected during such blooms. This reversed fractionation may be attributed to chemically enhanced diffusion, which generates depleted HCO₃⁻ under high pH (>9) conditions, as occur in the epilimnion of Lake Chala during periods of high productivity. The influence of this process can potentially explain previously observed ¹³C-depleted carbon signatures in the paleorecord of Lake Chala, and should be considered prior to paleorecord interpretation of organic-matter δ¹³C values derived (partially) from aquatic organisms in high-pH, i.e. alkaline, lake