Figure 1.
Map of Lake Kivu, showing bathymetry (isobaths at 100 m intervals), catchment area (shaded in grey), rivers, and sampling stations (small circles indicate the rivers).
Primary production and bacterial production measurements were made at the stations identified with a square (I = Ishungu; K = Kibuye), adapted from [29].
Figure 2.
Spatial distribution of the partial pressure of CO2 (pCO2, ppm) in the surface waters of Lake Kivu (1 m depth) in March 2007, September 2007, June 2008 and April 2009.
Table 1.
Average wind speed (m s−1), air-water gradient of the partial pressure of CO2 (ΔpCO2, ppm), and air-water CO2 flux (FCO2, mmol m−2 d−1) in the main basin of Lake Kivu and Kabuno Bay in March 2007, September 2007, June 2008, and April 2009.
Figure 3.
Vertical profiles in March 2007 of pH, oxygen saturation level (%O2, %), total alkalinity (TA, mmol L−1), salinity, dissolved inorganic carbon (DIC, mmol L−1), δ13C signature of DIC (δ13C-DIC, ‰) in Kabuno Bay and in the three northernmost stations in the main basin of Lake Kivu.
Figure 4.
Average partial pressure of CO2 (pCO2, ppm) in the surface waters of the main basin of Lake Kivu (1 m depth) versus mixed layer depth (MLD, m), δ13C signature of dissolved inorganic carbon (DIC) (δ13C-DIC, ‰), and methane concentration (CH4, nmol L−1) in March 2007, September 2007, June 2008, and April 2009.
Vertical and horizontal bars represent standard deviations.
Figure 5.
Relation between δ13C signature of dissolved inorganic carbon (DIC) (δ13C-DIC, ‰) and DIC concentration (mmol L−1), in the mixed layer of the main basin of Lake Kivu, Kabuno Bay, and various inflowing rivers, in March 2007, September 2007, June 2008, and April 2009.
Figure 6.
Relation between δ13C signature of particulate organic carbon (POC) (δ13C-POC, ‰) and δ13C signature of dissolved inorganic carbon (DIC) (δ13C-DIC, ‰), in the mixed layer of the main basin of Lake Kivu, Kabuno Bay and various inflowing rivers, in June 2008, April 2009, and October 2010.
Figure 7.
Relation between δ13C signature of dissolved organic carbon (DOC) (δ13C-DOC, ‰) and δ13C signature of particulate organic carbon (POC) (δ13C-POC, ‰), in the mixed layer of the main basin of Lake Kivu, Kabuno Bay and various inflowing rivers, in June 2008, April 2009, and October 2010.
Solid line is the 1∶1 line.
Figure 8.
Vertical profiles of δ13C signature of dissolved organic carbon (DOC) (δ13C-DOC, ‰) and δ13C signature of particulate organic carbon (POC) (δ13C-POC, ‰) in the main basin of Lake Kivu in October 2010.
Figure 9.
Observed data (circles and squares) and predicted values from a mixing model (diamonds) from March 2007 to September 2007 of total alkalinity (TA, mmol L−1), dissolved inorganic carbon (DIC, mmol L−1), and the partial pressure of CO2 (pCO2, ppm) as a function of mixed layer depth (MLD, m) in the main basin of Lake Kivu.
FCO2 = air-water CO2 flux; FPOC = export of particulate organic carbon to depth; FCaCO3 = export of CaCO3 to depth.
Figure 10.
Average mass balance of total alkalinity (TA) and dissolved inorganic carbon (DIC) in the mixed layer of the main basin of Lake Kivu based on data collected in March 2007, September 2007, June 2008 and April 2009.
FPOC = export of particulate organic carbon to depth; FCaCO3 = export of CaCO3 to depth. Numbers in black correspond to the mass balance based on bulk concentrations, and numbers in red correspond to the mass balance based on DIC stable isotopes. All fluxes are expressed in mmol m−2 d−1.
Table 2.
Photic depth (Ze, m), chlorophyll-a concentration in the mixed layer (Chl-a, mg m-2), particulate net primary production (PNPP, mmol m−2 d−1), bacterial production integrated over Ze (BP, mmol m−2 d−1), bacterial respiration (BR, mmol m−2 d−1), and percent of extracellular release (PER, %), at two stations in the main basin of Lake Kivu (Kibuye, Ishungu) in March 2007, September 2007, June 2008, and April 2009.