Warming and stratification changes in Lake Kivu, East Africa

Abstract

To investigate changes in the temperature and stratification structure in Lake Kivu, we have installed a string of temperature recorders and performed CTD casts. The obtained data have been compared to historical profiles and the heat budget for the lake was analyzed. Lake Kivu is a meromictic lake characterized by an anomalous temperature distribution with a temperature minimum close to the base of the seasonally mixed layer. Warming rate at the depth of the temperature inversion is consistent with the historical warming rate of the surface layer of &sim;0.14 &plusmn;0.02 &deg;C per decade. Atmospheric warming rates since the 1970's in East Africa are between 0.20 and 0.25 &deg;C per decade. Reported warming in surface waters of other East-African rift lakes is &sim;0.13 &deg;C per decade. Deep waters (greater than 350 m) in Lake Kivu exhibit variability in temperature and are currently warming at a rate of &sim0.06&plusmn;0.02 &deg;C per decade based on the increase in heat content since the 1970's and the increase in temperature seen in the deepest measurements between our 2011 and 2012 profiles. The monimolimnion of Lake Kivu cannot be considered to be in a steady state. The depth of wind-induced surface mixing during the dry season varies significantly between years. Mixing to 80 m (the present depth of the temperature inversion) requires continuous winds blowing from the south at 9&ndash;10 m s<super>-1</super>, whereas typical wind speed maxima are around 5&ndash;6 m s<super>-1</super> and capable of mixing to around 65 m depth. Occasional stronger winds cause episodic mixing closer to the inversion which removes heat, but this does not happen on a regular basis. As the temperature inversion in recent historical profiles has been as shallow as 65 m, mixing to the temperature inversion depth is possible during years with stronger than average winds. With heat diffusing towards the temperature inversion from both above and below, the temperature at the inversion depth will continue to rise, resulting in a reduced transport of heat out of the deep waters that may increase the rate at which the water column is warming.