Paleolimnological records of environmental change in Lake Baringo: changes in lake level and trophic state during the 19th and 21st centuries

Abstract

Paleolimnology is the reconstruction of historical conditions of lakes using information contained in the sediment proxies. Previous studies have provided evidence for climate variability in the East African region over the last 200 years. However, the response of Lake Baringo to shifts in climate over the years remains unknown. As a consequence this thesis presents a sediment based semi-quantitative reconstruction of water level fluctuations and trophic state variations for Lake Baringo for the period between 19th century and the first decade of the 21st century (214 years). Core samples were collected in June 2014 using a Hand-Driven Fixed-Piston corer and analysed for Fossil diatoms, Biogenic silica (BSi) and Magnetic susceptibility (MS). Inference for the historic trophic state was based partly on the abundance of different trophic level indicator diatom taxa and indices based on the ratio of Centric to Pennate diatoms (C/P ratio). Lake level was inferred using indices based on the proportion of planktonic to benthic taxa (P/B ratio) at respective depths. Current limnological characteristic of the lake including physical, chemical and trophic state were studied using water sample. The number of frustules did not show significant difference along length of the core (One Way ANOVA, F (46, 625) = 0.76, p ꞊ 0.873). There was also no significant difference in the level of biogenic silica (One Way ANOVA, F (5, 41) = 1.81, p ꞊ 0.133). However, significant difference was observed in the number of frustules counted for the individual genera (One Way ANOVA, F (18, 682), p ꞊ 0.000). The sediment archive presented distinct zones dominated by planktonic and benthic diatom flora. An initial transgression phase in the early 19th century was characterized as a shallow water environment dominated by planktonic Aulacoseira spp. This was a response to extreme drought during the late 18th to early 19th century. Mid-19th century was defined by a high lake stand. Late 19th to early 20th century experienced low water level following the widely documented aridity at the time. Mid-20th century was marked by a spectacular rise in water level which coincided with remarkably wet years during the early 1960s and late 1970s. The first decade of the 21st century witnessed widespread changes in water level. Further, the records show alternating patterns in the trophic state of the lake since AD 1800. The early 19th century was characterized as a eutrophic lake environment. The highest lake productivity occurred between AD 2001 and AD 2006. It is worth noting that drawdowns are accompanied by high nutrient concentrations while high lake levels coincide with low nutrient conditions. Carlson TSI show that today the lake is eutrophic and phosphorus limited regardless of the season with light limitation occurring during the wet season.