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 (delta C-13) 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-28:1 and C-27:1 n-alkenes, and C-21 and C-23 n-alkanes), followed by a peak in diatoms between July and October (loliolide and isololiolide), and then eustigmatophytes (C-30 and C-32 1,15 diols) once stratification resumes in October. Peak abundance of the C-19:1 n-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 delta C-13 values of the C-28 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 delta C-13 signature of particulate carbon and all aquatic biomarkers become increasingly more negative (by up to 16 parts per thousand) 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 C-13-depleted carbon signatures in the paleo-record of lake Chala, and should be considered prior to paleorecord interpretation of organic-matter delta C-13 values derived (partially) from aquatic organisms in high-pH, i.e. alkaline, lakes. (C) 2018 The Authors. Published by Elsevier Ltd.