Distribution of dissolved green-house gases (CO2, CH4, N2O) in Lakes Edward and George: Results from the first field cruise of the HIPE project

Inland waters (streams, rivers, lakes, reservoirs) are quantitatively important components of the global budgets of atmospheric emissions of long-lived greenhouse gases (GHGs) (CO2, CH4, N2O). Available data indicate that a very large fraction of CO2 and CH4 emissions from rivers and reservoirs occurs at tropical latitudes. Data on GHGs at tropical latitudes from lakes however are much more scarse, and the relative importance of emissions, in particular in Africa, remains to be determined. Large tropical lakes are net autotrophic (hence potentially sinks for atmospheric CO2) due generally low dissolved organic carbon concentrations, seasonally near constant light and temperature conditions, and generally deep water columns favourable for export of organic matter to depth. This sharply contrasts with their much better documented temperate and boreal counterparts, usually considered as CO2 sources to the atmosphere sustained by net heterotrophy. Here, we report a data-set of issolved CO2, CH4, N2O obtained in October 2016 in Lakes Edward and George and adjacent streams and cater lakes in he frame of Belgian Science Policy (BELSPO) HIPE (Human impacts on ecosystem health and resources of ake Edward, http://www.co2.ulg.ac.be/hipe/) project. Lake George and part of Lake Edward were sinks for tmospheric CO2 and N2O due to high primary production and denitrification in sediments, respectively, and modest ources of CH4 to the atmosphere. Sampled rivers and streams were oversaturated in CO2 and CH4 and close to tmospheric equilibrium with regards to N2O. Spatial variations within rivers and streams were related to elevation and vegetation characteristics on the catchments (savannah versus forest). Levels of CO2, CH4, and N2O were within the range of those we reported in other African rivers. Crater lakes acted as sinks for atmospheric CO2 and N2O but were extremely over-saturated in CH4, due to intense primary production sustained by cyanobacteria. These CH4 levels were much higher than what we have reported in other lakes and reservoirs elsewhere in Sub- Saharan Africa

Carbon Cycling of Lake Kivu (East Africa): Net Autotrophy in the Epilimnion and Emission of CO2 to the Atmosphere Sustained by Geogenic Inputs

We report organic and inorganic carbon distributions and fluxes in a large (>2000 km2) oligotrophic, tropical lake (Lake Kivu, East Africa), acquired during four field surveys, that captured the seasonal variations (March 2007–mid rainy season, September 2007–late dry season, June 2008–early dry season, and April 2009–late rainy season). The partial pressure of CO2 (pCO2) in surface waters of the main basin of Lake Kivu showed modest spatial (coefficient of variation between 3% and 6%), and seasonal variations with an amplitude of 163 ppm (between 579±23 ppm on average in March 2007 and 742±28 ppm on average in September 2007). The most prominent spatial feature of the pCO2 distribution was the very high pCO2 values in Kabuno Bay (a small sub-basin with little connection to the main lake) ranging between 11213 ppm and 14213 ppm (between 18 and 26 times higher than in the main basin). Surface waters of the main basin of Lake Kivu were a net source of CO2 to the atmosphere at an average rate of 10.8 mmol m−2 d−1, which is lower than the global average reported for freshwater, saline, and volcanic lakes. In Kabuno Bay, the CO2 emission to the atmosphere was on average 500.7 mmol m−2 d−1 (~46 times higher than in the main basin). Based on whole-lake mass balance of dissolved inorganic carbon (DIC) bulk concentrations and of its stable carbon isotope composition, we show that the epilimnion of Lake Kivu was net autotrophic. This is due to the modest river inputs of organic carbon owing to the small ratio of catchment area to lake surface area (2.15). The carbon budget implies that the CO2 emission to the atmosphere must be sustained by DIC inputs of geogenic origin from deep geothermal springs.AFRIVA

Data-set of CO2, CH4, N2O dissolved concentrations and ancillary data in surface waters of 24 African lakes

Geo-referenced and timestamped data-set of water temperature, Specific conductivity (SpCond), oxygen saturation level (%O2), dissolved methane (CH4) concentration, dissolved nitrous oxide (N2O) concentration, partial pressure of carbon dioxide (pCO2), carbon stable isotope composition of dissolved inorganic carbon (δ13C-DIC), dissolved organic carbon (DOC) concentration, chlorophyll-a (Chl-a) concentration, cyanobacteria abundance (CHEMTAX), nitrate (NO3-) and ammonia concentration (NH4+), coloured dissolved organic matter slope ratio (CDOM SR) in surface waters of African 24 lakes (Victoria, Tanganyika, Albert, Kivu, Edward, Mai Ndombe, Tumba, George, Kamohonjo, Alaotra, Ndalaga, Nyamusingere, Kyamwinga, Mbita, Lukulu, Yandja, Mbalukira, Nkugute, Nyamunuka, Kitagata, Mrambi, Kyashanduka, Katinda, Lac Vert).Geo-referenced and timestamped data-set of water temperature, Specific conductivity (SpCond), oxygen saturation level (%O2), dissolved methane (CH4) concentration, dissolved nitrous oxide (N2O) concentration, partial pressure of carbon dioxide (pCO2), carbon stable isotope composition of dissolved inorganic carbon (δ13C-DIC), dissolved organic carbon (DOC) concentration, chlorophyll-a (Chl-a) concentration, cyanobacteria abundance (CHEMTAX), nitrate (NO3-) and ammonia concentration (NH4+), coloured dissolved organic matter slope ratio (CDOM SR) in surface waters of African 24 lakes (Victoria, Tanganyika, Albert, Kivu, Edward, Mai Ndombe, Tumba, George, Kamohonjo, Alaotra, Ndalaga, Nyamusingere, Kyamwinga, Mbita, Lukulu, Yandja, Mbalukira, Nkugute, Nyamunuka, Kitagata, Mrambi, Kyashanduka, Katinda, Lac Vert)

Dissolved organic matter composition and reactivity in Lake Victoria, the World’s largest tropical lake

peer reviewedWe report a data set of dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition (stable carbon isotope signatures, absorption and fluorescence properties) obtained from samples collected in Lake Victoria, a large lake in East Africa. Samples were collected in 2018-2019 along a bathymetric gradient (bays to open waters), during three contrasting seasons: long rainy, short rainy and dry, which corresponded to distinctly water column mixing regimes, respectively, stratified, semi-stratified and mixed regimes. Eight DOM components from parallel factor analysis (PARAFAC) were identified based on three-dimensional excitation–emission matrices (EEMs), which were aggregated into three main groups of components (microbial humic-like, terrestrial humic-like, protein-like). Spatially, the more productive bays were characterized by higher DOM concentration than deeper more offshore waters (fluorescence intensity and DOC were ~80% and ~30% higher in bays, respectively). Seasonally, the DOM pool shifted from protein-like components during the mixed regime to microbial humic-like components during the semi-stratified regime and to terrestrial humic-like components during the stratified regime. This indicates that pulses of autochthonous DOM derived from phytoplankton occurred when the lake was mixing, which increased the availability of dissolved inorganic nutrients. Subsequently, this freshly produced autochthonous DOM was microbially processed during the following semi-stratified regime. In the open waters, during the stratified regime, only terrestrial refractory DOM components remained because the labile and fresh stock of DOM created during the preceding mixed season was consumed. In the bays, the high terrestrial refractory DOM during the stratified regime may be additionally due to the allochthonous DOM input from the runoff. At the scale of the whole lake, the background refractory DOM probably comes mainly from precipitation and followed by river inputs.LAVIGA

Anaerobic methane oxidation and aerobic methane production in Lake Kivu

peer reviewe

Prevalence of Autotrophy in Non-humic African Lakes

peer reviewedHeterotrophic respiration of organic matter (OM) is thought to dominate over aquatic primary production (PP) in most freshwater lake ecosystems. This paradigm implies that lateral transport of OM from the terrestrial biosphere subsidize the major fraction of aquatic respiration and that many lakes are a net source of carbon dioxide (CO2) to atmosphere. Nevertheless, African lakes were absent of the datasets upon which this paradigm was built. Here, we report a comprehensive and methodologically consistent data set of pelagic PP and community respiration (CR) obtained over the last decade in contrasting non-humic African lakes including 5 of the East African Great lakes (Tanganyika, Kivu, Edward, Albert, Victoria) and smaller shallow lakes located in Eastern Africa. Also, we determined the partial pressure of CO2 in surface waters and examined the sources and dynamics of organic and inorganic carbon by means of stable isotope tools across a wide range of physical and chemical conditions and productivity status. Our observations revealed that the threshold value at which the equivalence between PP and CR is met is substantially lower in Africa (10 mmol C m−3 d−1) than at higher latitude (25 mmol C m−3 d−1), suggesting that non-humic African lakes tend to be more autotrophic than expected from empirical relationships derived from data collected in boreal and temperate regions. Integrated at the regional scale, we estimate that PP is about 20 times higher than the organic carbon burial in sediments. It implies that a large fraction (< 90%) of PP is effectively recycled in the warm water column of non-humic African lakes

Spatial and temporal variations of dissolved CO2, CH4 and N2O in Lakes Edward and George (East Africa)

peer reviewedWe report dissolved CO2, CH4 and N2O concentrations in two large East African Rift lakes, Edward (surface area 2,325 km2, average depth of 37 m) and George (surface area 273 km2, average depth of 2 m). Lake George showed modest seasonal and spatial variations, and lower partial pressure of CO2 (pCO2) (26±16 ppm, mean±standard deviation), CH4 (234±208 nmol L-1) and N2O saturation levels (%N2O) (80±9%) than Lake Edward (404±145 ppm, 357±483 nmol L-1, 139±222%). Surface waters in both lakes were over-saturated in CH4, and Lake George was under-saturated in CO2 while Lake Edward was slightly over-saturated in CO2. This difference was related to higher phytoplankton biomass in Lake George than Lake Edward, with average chlorophyll-a concentrations of 177±125 and 18±25 µg L-1, respectively. Permanent high cyanobacterial biomass in Lake George led to uniform dissolved CO2, CH4 and N2O concentrations. In surface waters of Lake Edward, spatial variations of pCO2, CH4 and N2O were related to bottom depth, and locally (in particular in Katwe Bay) also related to the inputs of water from Lake George via the Kazinga Channel, a 40-km natural channel connecting the lakes. Short-term mixing events related to storms increased CO2, CH4 and N2O content in surface waters, in particular for CH4 and N2O. This indicates that mixing events in response to storms can create ‘hot moments’ for CH4 and N2O emissions to the atmosphere in tropical lakes, given the weaker vertical density gradients compared to higher latitude systems

Greenhouse gas emissions from African lakes are no longer a blind spot

peer reviewedNatural lakes are thought to be globally important sources of greenhouse gases (CO2, CH4, and N2O) to the atmo-sphere although nearly no data have been previously reported from Africa. We collected CO2, CH4, and N2O data in 24 African lakes that accounted for 49% of total lacustrine surface area of the African continent and covered a wide range of morphology and productivity. The surface water concentrations of dissolved CO2 were much lower than values attributed in current literature to tropical lakes and lower than in boreal systems because of a higher productivity. In contrast, surface water–dissolved CH4 concentrations were generally higher than in boreal sys-tems. The lowest CO2 and the highest CH4 concentrations were observed in the more shallow and productive lakes. Emissions of CO2 may likely have been substantially overestimated by a factor between 9 and 18 in African lakes and between 6 and 26 in pan-tropical lakes.AFRIVA