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

Abstract

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.

Biogeochemistry of a large and deep tropical lake (Lake Kivu, East Africa: insights from a stable isotope study covering an annual cycle

“…As the depth profile of δ 13 C-DIC revealed that the DIC pool was isotopically lighter in the bottom of the mixolimnion, Overall, the data suggest that the input of DIC originating from the monimolimnion during the dry season had a strong influence on δ 13 C-DIC in the mixolimnion, but the seasonal variability of δ 13 C-DIC observed in the mixed layer holds information on biological processes. The gradual increase with time of the δ 13 C-DIC in the mixed layer supports the conclusions of other studies carried out in Lake Kivu Borges et al, 2014) which showed, based on a detailed DIC and DI 13 C mass balance approach and several microbial processes measurements, that photosynthetic CO 2 fixation should exceed the respiration of OM. Indeed, in Lake Kivu, riverine inputs of allochthonous OM from the catchment (0.7-3. the Ruzizi River is also relatively low and was computed to be 0.6 mmol m −2 d −1 (this study) based on the long-term discharge average of Ruzizi (83.2 m 3 s −1 , Borges et al, 2014), the average POC and DOC in surface waters (0.052 and 0.142 mmol L −1 , this study).…”

“…The gradual increase with time of the δ 13 C-DIC in the mixed layer supports the conclusions of other studies carried out in Lake Kivu Borges et al, 2014) which showed, based on a detailed DIC and DI 13 C mass balance approach and several microbial processes measurements, that photosynthetic CO 2 fixation should exceed the respiration of OM. Indeed, in Lake Kivu, riverine inputs of allochthonous OM from the catchment (0.7-3. the Ruzizi River is also relatively low and was computed to be 0.6 mmol m −2 d −1 (this study) based on the long-term discharge average of Ruzizi (83.2 m 3 s −1 , Borges et al, 2014), the average POC and DOC in surface waters (0.052 and 0.142 mmol L −1 , this study). It implies that the outputs of OM (9.4 + 0.7 = 10.1 mmol m −2 d −1 ) are higher than the inputs of OM from the catchment (0.7-3.3 mmol m −2 d −1 ) suggesting a net autotrophic status of Lake Kivu.…”

“…However, these results contradict the commonly held view that oligotrophic lacustrine and marine systems tend to be net heterotrophic (Del Giorgio et al, 1997;Cole, 1999). Net heterotrophy implies that heterotrophic prokaryotes rely on a substantial amount of allochthonous OM; however, in Lake Kivu, riverine inputs of allochthonous OM from the catchment (0.7-3.3 mmol m −2 d −1 , Borges et al, 2014) are minimal. Indeed, the magnitude of allochthonous OM inputs relative to phytoplankton production depends strongly on the catchment to surface area ratio (Urban et al, 2005), that is particularly low (2.2) in Lake Kivu.…”

“…Contradicting the common observation that oligotrophic aquatic ecosystems tend to be net heterotrophic, the seasonality of δ 13 C-DIC supports the view that the mixed layer of Lake Kivu is net autotrophic, as demonstrated by Borges et al (2014) based on DIC and DI 13 C mass balance considerations. The δ 13 C-POC showed an important variation with depth due to the abundance of methanotrophic bacteria in the oxycline that fixed the lighter CH 4 -derived C into their biomass.…”

“…This concept seems to hold especially true for oligotrophic unproductive ecosystems (Del Giorgio et al, 1997), that are subsidised by substantial inputs of allochthonous OM of terrestrial origin, which support the production of heterotrophic organisms. Net heterotrophy has been recognised as one of the main cause for the net emission of carbon dioxide (CO 2 ) from freshwater ecosystems to the atmosphere (Prairie et al, 2002), although there is growing evidence of the contribution from external hydrological CO 2 inputs from the catchment (Stets et al, 2009;Finlay et al, 2010;Borges et al, 2014;Marcé et al, 2015). However, the current understanding of the role of inland waters on CO 2 emissions could be biased because most observations were obtained in temperate and boreal systems, and mostly in medium-to-small lakes, during open-water (ice-free) periods, but tropical and temperate lakes differed in some fundamental characteristics.…”