Abstract
Lake Kivu in East Africa has gained notoriety for its prodigious amounts of dissolved methane and dangers of limnic eruption. Being meromictic, it is also expected to accumulate heat due to rising regional air temperatures. To investigate the warming trend and distinguish between atmospheric and geothermal heating sources, we compiled historical temperature data, performed measurements with logging instruments, and simulated heat propagation. We also performed isotopic analyses of water from the lake's main basin and isolated Kabuno Bay. The results reveal that the lake surface is warming at the rate of 0.12°C per decade, which matches the warming rates in other East African lakes. Temperatures increase throughout the entire water column. Though warming is strongest near the surface, warming rates in the deep waters cannot be accounted for solely by propagation of atmospheric heat at presently assumed rates of vertical mixing. Unless the transport rates are significantly higher than presently believed, this indicates significant contributions from subterranean heat sources. Temperature time series in the deep monimolimnion suggest evidence of convection. The progressive deepening of the depth of temperature minimum in the water column is expected to accelerate the warming in deeper waters. The warming trend, however, is unlikely to strongly affect the physical stability of the lake, which depends primarily on salinity gradient.
Highlights
Deep meromictic lakes are good climate monitors, as changes in heat fluxes across the lake surface become reflected in heat content of the deeper waters [1]
Temperature records in the epilimnion (Fig. 2) indicated that surface mixing during the dry season in 2011 and 2012 reached the depth of approximately 55 m, about 20 m shallower than the depth of the temperature minimum
Lake Kivu has experienced significant warming over the course of the 20th century, with temperatures rising over the entire water column
Summary
Deep meromictic lakes are good climate monitors, as changes in heat fluxes across the lake surface become reflected in heat content of the deeper waters [1]. The sublacustrine heat comes from a complex geological system that includes two active volcanoes, Nyiragongo and Nyamuragira [3,4,5], which constitute the lake’s northern watersheds These deep heat sources cause the lake temperature to increase with depth below the surface mixed layer, a feature not observed in other lakes. The resultant reverse temperature gradient in deep waters implies an upward transport of heat towards the depth of the temperature minimum This heat may be removed only through epilimnetic mixing, e.g. if weaker temperature gradients during a dry season [7] allow mixing to that depth, or if a cold inflow enters the lake at that depth
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
