Hydrological variability and biogeochemistry of particulate organic matter in a large tropical rift lake, Lake Kivu (East Africa).

Abstract

The conservation of Great Lakes for sustainable ecosystem service benefits has become an
important research question. Lakes can be sensitive to climate variability and changes as well as
human intervention. Perturbations of a lake’s hydrology affect the limnology and
biogeochemical cycles and induce important consequences on the ecosystem functioning. This
thesis analyzed the recent hydrological records of Lake Kivu in relationship to meteorological
variation as well as the dynamics of particulate organic matter (POM) cycling with focus on
sources, vertical export and fate, and paleolimnological significance. Lake Kivu is better known
due to its enormous and hazardous dissolved gases (CO2 + CH4). The important CH4resource is
biogeochemically produced at the water-sediment interface under anoxic conditions thanks to
methanogenic bacteria by two pathways: (1) the reduction of geogenic CO2 (~2/3) in which OM
may serve as electron donor, and (2) by OM fermentation (~1/3).
The water level of Lake Kivu was estimated based on an extensive precipitation dataset using a
relatively simple water balance model for both the catchment and the lake. For the time period
with a good availability of rainfall data, the predicted water levels as well as their seasonal
variation agreed well with observations, indicating that the observed variations in the water level
were mainly driven by the variation in precipitation. The lack of rainfall data after 1991 seriously
impaired the predictive capability of the model.This highlights the importance of hydrometric
and meteorological monitoring data for reconstructing the lake water balance of East African
Great lakes. The construction of a hydropower dam at the outlet atthe end of 1950s did not show
any clear effect on the lake water level dynamics, whereas a modification of the outlet in 1977
seems to have induced some interannual variability in the lake levels.
The subaquatic groundwater system is an importanthydrological component for this Rift Lake in
terms of water, dissolved solid and heat inputs beneath the lake surface sustaining the permanent
lake stratification and nutrient uplift to the mixolimnion from the deep waters. The nutrients
made available by internal loading feed the phytoplankton growth. Prior to this work, little was
known about the decaying POM, the relative importance of its export and its fate into the
monimolimnion as well as on how the presence oforganic molecules and the elemental/stable
isotope signatures in sedimentary material can be interpreted to reconstruct the historical records
of the lake productivity, the origin and preservation of POM, the phytoplankton community
structure, nutrient cycling and mixing regime.
Organic matter (OM) in Lake Kivu is essentiallyautochthonous as evidenced by stable isotopes
and molecular biomarkers. Its production and sedimentation fluxes are highly seasonal. They
depend on the mixing status of the water column. Most of the pigment degradation occurs
between 30 and 60 m water depth. The particulate organic carbon export to the monimolimnion
is closer to 6% of the primary production indicating a high conversion rate of POM into
dissolved organic matter and thus an efficient recycling of nutrients within the mixolimnion.
Some phytoplankton pigments such as lutein, alloxanthin and zeaxanthin were preserved
throughout the water column and within the sediment archives suggesting that they are good
paleolimnological proxies for tracing the history of phytoplankton group distribution in the lake.
However peridinin, fucoxanthin and diadinoxanthin were completely degraded prior to
settlement. This study also showed that Lake Kivu underwent alternating periods of high and low
productivity caused by nutrient availability changes forced by subaquatic groundwater dynamics.
An intermittent dynamics of carbonate was also observed. Periods of organic carbon-rich
sediment accumulation characterized by low carbonate concentrations alternated with periods of
low to moderate carbon concentrations and high carbonate preservation. A high carbonate flux to
the monimolimnion occurred during periods with strong stratification. The low carbonate content
in organic-rich sediments was interpreted asa consequence of re-dissolution caused by
acidogenesis during diagenetic sedimentary OM fermentation.

Date of Award	25 Mar 2015
Original language	English
Awarding Institution	University of Namur
Supervisor	Jean-Pierre DESCY (Supervisor), Eric Depiereux (President), Martin Schmid (Supervisor), Steven Bouillon (Jury), Wim G. Vyverman (Jury) & Pascal Isumbisho Mwapu (Jury)