Long-term dynamics of lake Kivu over the 20year period from 2002 to 2022 using phytoplankton and physico-chemical data

Abstract

This study investigated the long-term dynamics (2002-2022) of phytoplankton and physico-chemical characteristics in Lake Kivu, East Africa, analyzing the potential influence of the recent Nyiragongo eruptions (2002, 2021), and methane extraction initiation (2016 by KivuWatt). Seasonal variations significantly impacted phytoplanktons, with deeper mixing during the dry season (June-September) promoting growth up to 50-60 meters, while the rainy season limited growth to the upper 40 meters. Decadal climate variability influenced long-term phytoplankton communities, with Chlorophytes emerging as the dominant group and exhibiting rapid growth since 2012. Chrysophyte populations displayed a potential 7-year cycle of high (20092015) and low abundance (2002-2008, 2016-2022). Seasonal temperature and density variations within the mixolimnion remained consistent, except for a period of weakened thermal stratification during the rainy season (20072009). Notably, changes in salinity, conductivity, temperature, and density stratification of the monimolimnion (250-310 meters) by the end of 2021 may be linked to the degassed water reinjected around 240 meters depth after methane extraction by KivuWatt.