Lake Chala, a permanently stratified crater lake in tropical eastern Africa, provides a unique sedimentary record deposited continuously over the last 250,000 years. Biomarkers preserved in these sediments allow reconstruction of regional palaeoclimate history in unprecedented detail. However, for sound interpretation of the biomarker record a proper knowledge on their biological sources and factors influencing their production are a prerequisite. Previous studies on biomarkers derived from phytoplankton and vascular (terrestrial) plants in suspended particulate matter sampled monthly over almost 1.5 years revealed large seasonal variation in their distribution in the water column. Here we extend this time series to 4.5 years (September 2010 to January 2015) by analyzing biomarker fluxes in settling particles collected monthly by a sediment trap deployed at 35 m water depth. The flux of vascular plant lipids was overall low and irregular. Variation in phytoplankton biomarker fluxes indicate that aquatic microbial production and community composition in Lake Chala are strongly influenced by the seasonal succession of water-column stratification and deep but partial mixing. Fluxes of biomarkers representing Chlorophyte algae (long-chain n -alk-1-enes, C 21 and C 23 n -alkanes) and diatoms (loliolide and isololiolide) were highest during the deep mixing season, but not every deep mixing period elicited a strong phytoplankton response. Temporal variation in the flux of the n -C 28 fatty acid, often used as a plant wax biomarker, resembles that of Chlorophyte biovolume and biomarkers, indicating an aquatic source with the Chlorophyte Tetraedron minimum as most likely producer. Large fluctuation in δ 13 C values of the n -C 28 fatty acid indicate regular occurrence of chemically enhanced fractionation at high pH. Fluxes of Eustigmatophyceae biomarkers (C 30 and C 32 long-chain diols) were highest during the period of stratification. Fluxes of the C 19 n -alkene, which increase during periods of water-column stratification and/or shallow mixing, can be linked to cyanobacteria. Taking seasonal and interannual variability together, microbial production and productivity in the upper water column is primarily determined by the depth of seasonal water column mixing, as this controls the amount of nutrients being recycled from the hypolimnion. Large inter-annual variability of aquatic lipid biomarkers in the settling particles of Lake Chala indicates that small differences in ambient water-column conditions can cause strong turnover in the microbial communities, preventing systematic recurrence of a stable phytoplankton succession pattern. This implies that monitoring programs organized to capture the natural variability of lake systems must run for many years to fully reveal the relationship between biomarkers in the modern lake system and biomarker proxies in the sedimentary record used for paleoenvironmental reconstruction.
Damsté et al. (Fri,) studied this question.