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Carbon cycling of Lake Kivu (East Africa): net autotrophy in the epilimnion and emission of CO2 to the atmosphere sustained by geogenic inputs.

Borges AV, Morana C, Bouillon S, Servais P, Descy JP, Darchambeau F - PLoS ONE (2014)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Chemical Oceanography Unit, Université de Liège, Liège, Belgium.

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 11,213 ppm and 14,213 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.

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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.
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pone-0109500-g003: 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.

Mentions: Compared to the main basin, surface and deep waters of Kabuno Bay were characterized by higher salinity, DIC and TA values and by lower pH and δ13C-DIC values (Fig. 3). Comparison of DIC and TA profiles shows that the relative contribution of CO2 to DIC was more important in Kabuno Bay than in the main lake, since TA is mainly as HCO3-, and if the CO2 contribution to DIC is low, then DIC and TA should be numerically close. At 60 m depth, CO2 contributes ∼30% to DIC in Kabuno Bay, and only ∼1% in the main basin. Kabuno Bay was also characterized by a very stable chemocline (salinity, pH) and oxycline at ∼11 m irrespective of the sampling period [28]. In the main basin of Lake Kivu, the oxycline varied seasonally between ∼35 m in March and September 2007 and ∼60 m in June 2008 [28]. The deepening of the mixed layer and entrainment of deeper waters to the surface mixed layer was shown to be main driver of the seasonal variations of CH4[28]. The positive correlations between pCO2 and CH4 and between pCO2 and the MLD also show that the mixing of deep and surface waters was a major driver of the seasonal variability of pCO2 (Fig. 4). This is also consistent with the negative relation between pCO2 and δ13C-DIC (Fig. 4), as DIC in deeper waters is more 13C-depleted than that in surface waters (Fig. 3).


Carbon cycling of Lake Kivu (East Africa): net autotrophy in the epilimnion and emission of CO2 to the atmosphere sustained by geogenic inputs.

Borges AV, Morana C, Bouillon S, Servais P, Descy JP, Darchambeau F - PLoS ONE (2014)

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.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4196920&req=5

pone-0109500-g003: 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.
Mentions: Compared to the main basin, surface and deep waters of Kabuno Bay were characterized by higher salinity, DIC and TA values and by lower pH and δ13C-DIC values (Fig. 3). Comparison of DIC and TA profiles shows that the relative contribution of CO2 to DIC was more important in Kabuno Bay than in the main lake, since TA is mainly as HCO3-, and if the CO2 contribution to DIC is low, then DIC and TA should be numerically close. At 60 m depth, CO2 contributes ∼30% to DIC in Kabuno Bay, and only ∼1% in the main basin. Kabuno Bay was also characterized by a very stable chemocline (salinity, pH) and oxycline at ∼11 m irrespective of the sampling period [28]. In the main basin of Lake Kivu, the oxycline varied seasonally between ∼35 m in March and September 2007 and ∼60 m in June 2008 [28]. The deepening of the mixed layer and entrainment of deeper waters to the surface mixed layer was shown to be main driver of the seasonal variations of CH4[28]. The positive correlations between pCO2 and CH4 and between pCO2 and the MLD also show that the mixing of deep and surface waters was a major driver of the seasonal variability of pCO2 (Fig. 4). This is also consistent with the negative relation between pCO2 and δ13C-DIC (Fig. 4), as DIC in deeper waters is more 13C-depleted than that in surface waters (Fig. 3).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Chemical Oceanography Unit, Université de Liège, Liège, Belgium.

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 11,213 ppm and 14,213 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.

Show MeSH