A Review of Sediment Gas Cycling in Lakes with Reference to Lake Victoria and Sediment Gas Measurements in Lake Tanganyika

Abstract

Little is known about gases in the sediments or the water column of African lakes. Considering the importance of lakes as life-supporting resources, these are critically neglected areas of limnologic investigation. The cycling of nutrients for food web requirements and maintenance of upper trophic levels, i.e. fish and fisheries, and a basic understanding of ecosystem processes requires a fundamental knowledge of the C and N cycles. As examples, nitrogen seems to be a limiting nutrient in these tropical lakes, likely a result of denitrification and N2 export to the atmosphere; CH4 loss from the sediments can represent a critical pathway for bacterial C production and lake deoxygenation. Sediment gases (CH4, CO2 and N2) in the central basin of Lake Tanganyika were measured, probably for the first time. Total headspace concentrations (mM ± lσ, range) were 0.47±0.18, 0.14–0.88 CH4, 0.37±0.18, 0.12–0.75 CO2 and 0.97±0.50, 0.1–1.97 N2 for 33 sediment samples, with carbon gases increasing with depth to 35 cm. The high sediment carbon (TOC) and low C gas concentrations suggest the importation of recalcitrant C, coupled with extended preservation under continual anoxia at these Kalya Horst sites. It is unknown whether the sediments are present day sources of reduced gases and CO2 to the overlying water column. High concentrations of CO2 and H2S in deep waters suggest other sources, such as the possibility of geothermal emissions coupled with their long term accumulation in this permanently stratified system.