Using organic geochemical biomarkers to understand water column processes occurring in Lake Challa

Abstract

Lake Challa a, tropical crater lake, south east of Mt. Kilimanjaro, has been the site of previous biomarker studies as it lies east of the Congo Air Boundary, thus only recording climate variations related to the Indian Ocean climate dynamics, and because it offers excellent preservation of biomarkers as it remains anoxic below 45m. Past studies have however, predominantly focused on biomarker variations over the past 25ka and thus little is known about the present relationship between biomarkers, biota and climate, which would aid in the understanding of the past. The aim of this study is thus to ameliorate the understanding of this relationship by studying the temporal and spatial distribution of biomarkers in suspended particulate matter. Biomarker distributions through depth, in two contrasting months, and through time, at a depth of 25m, over a 17-month period, were studied. The results indicate that major productivity occurred between July and October, which coincided with the breakdown of thermal stratification and the upwelling of nutrients. Increased diatom presence during this period is indicated by increased loliolide and isololiolide concentrations, while C23:1, C25:1 and C27:1 n-alkene production suggests elevated green algae productivity. Depth distributions indicate that diatoms inhabited the deeper part of the water column, while the n-alkene producers inhabited the surface of the water column. The C28 fatty acid, which has been used to produce a terrestrial precipitation record of east Africa is detected in SPM. Aquatic production of the C28 fatty acid could have implications for this record if it is significant and thus requires further study. Indications for organism succession is also present as the C19:1 n-alkene peaked during the onset of the rainy season, December-February, while tetrahymanol, commonly used as a biomarker for stratification, peaked over the period of March-May, which coincides with increased stratification. The source organisms of many compounds could not be identified, highlighting the need for more biomarker studies focusing on the present system and culturing studies.