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.
Highlights
Freshwater ecosystems are frequently considered to be net heterotrophic, whereby the consumption of organic carbon (C) is higher than the autochthonous production of organic C, and excess organic C consumption is maintained by inputs of allochthonous organic C [1]
In surface waters (1 m depth) of the main basin of Lake Kivu, pressure of CO2 (pCO2) values were systematically above atmospheric equilibrium (,372 ppm to,376 ppm depending on the cruise), and varied within narrow ranges of 534–605 ppm in March 2007, 701–781 ppm in September 2007, 597–640 ppm in June 2008, and 583–711 ppm in April 2009 (Fig. 2)
Wind speed showed little seasonal variability, the seasonal variations of the CO2 emission rates followed those of DpCO2 with higher FCO2 values in September 2007 (14.2 mmol m22 d21) and lowest FCO2 in March 2007 (8.0 mmol m22 d21) in the main basin (Table 1)
Summary
Freshwater ecosystems are frequently considered to be net heterotrophic, whereby the consumption of organic carbon (C) is higher than the autochthonous production of organic C, and excess organic C consumption is maintained by inputs of allochthonous organic C [1]. Net heterotrophy in freshwater ecosystems promotes the emission of carbon dioxide (CO2) to the atmosphere [2], [3], [4], [5], [6], [7], [9], [10], with the global emission from continental waters estimated at ,0.75 PgC yr21 [4] (0.11 PgC yr from lakes, 0.28 PgC yr from reservoirs, 0.23 PgC yr from rivers, 0.12 PgC yr from estuaries, and 0.01 PgC yr from ground waters). Tranvik et al [7] revised the CO2 emission from lakes to 0.53 PgC yr, while Battin et al [6] estimated CO2 emission from streams at 0.32 PgC yr. Raymond et al [10] estimated an emission of 1.8 PgC yr for streams and rivers and 0.32 PgC yr for lakes and reservoirs
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