Spatial and temporal dynamics of pCO₂ and CO₂ flux in tropical Lake Malawi

Abstract

Numerous studies have documented CO 2 dynamics in temperate lakes, but few such studies have been conducted on tropical lakes. Spatial and seasonal variation of air and water pCO 2 , along with supporting limnological and meteorological variables, were measured aboard a vessel of opportunity along the north-south axis of Lake Malawi. These measurements were used to estimate annual net lake-atmosphere CO 2 flux and infer mechanisms regulating it. Surface pCO 2 and CO 2 flux varied significantly with season and location. Temporally, the lake was CO 2 -undersaturated during the rainy season and the mixing season, and supersaturated at the onset of the mixing season and during the hot, stratified season. Concurrent measurements of lake temperature, weather conditions, phytoplankton biomass, and seston δ 13 C suggest that periods of net CO 2 flux into the lake correspond with higher phytoplankton growth rates resulting from internal nutrient loading in the mixing season and allochthonous nutrient inputs in the rainy season. Unlike the rest of the lake, the southernmost region of the lake was usually CO 2 supersaturated even though phytoplankton productivity is highest in this region. While the upwelling of hypolimnetic water at the southern end of the lake is a major source of nutrients that drive phytoplankton photosynthesis and CO 2 uptake, the CO 2 introduced in upwelled water appears to overwhelm photosynthetic capacity locally, especially at the onset of the mixing season. Lake Malawi appears to be a net CO 2 sink with an annual whole-lake CO 2 flux of −2.17 AE 0.25 × 10 10 mol C yr −1 and a mean daily CO 2 flux of −2.05 AE 0.27 mmol C m −2 d −1 . A comparison of deep-water C : P ratios with epilimnetic seston C : P ratios suggests that P is recycled more efficiently than C in the lake's anoxic hypolimnion, and so P vertical mixing creates a carbon deficit that is met by flux from the atmosphere into the lake.

Satellite data is revealing long time changes in the world largest lakes

“…This was the main reason of using remote sensing products and carrying out this study. Nevertheless, we were able to find in situ data from the Laurentian Great Lakes [23][24][25][26][27][28][29] , Lake Malawi [30][31][32] , Lake Tanganyika 33 , the Caspian Sea 34 , Lake Baikal 35,36 , and Lake Victoria 37 . Comparing the OC-CCI product values with the in situ results from published literature shows that the OC-CCI products are a reliable source of information for the type of analysis undertaken in this study.…”

Dissolved greenhouse gas (CO2, CH4, and N2O) emissions from highland lakes of the Andes cordillera in Northern Ecuador

“…1 Fifteen sampled highland lakes in Northern Ecuador January 2019) as part of the sampling of the Napo river network (Chiriboga and Borges 2023); (2) Lakes Santa Lucía, Secas, and Humbolt were sampled on two occasions (24-25 September 2021 and 17-21 December 2021), along a transect across the longest length with equidistant points (n = 9, 6, 3, respectively); (3) the remaining 11 lakes (Cubilche 1 and 2, Cuicocha, Loreto, Mojanda, Negra, Papallacta, San Pablo, Sucus, Yaguarcocha, and Yanacocha) were sampled at only one station in the center of the lake on one occasion, from 22 March 2022 to 28 March 2022. Admittedly, CO 2 , CH 4 and N 2 O in surface waters of lakes show variations in time (from subdaily to seasonal) and horizontally (from shore to the deepest part frequently in the center) (Fernandez et al 2020;Ngochera and Bootsma 2020;Mandryk et al 2021;Borges et al 2022;Peacock et al 2023), but a "one station in the center" snapshot survey of lakes has frequently been used to investigate broad geographical gradients as a function, for instance, of climate or catchment land cover (Juutinen et al 2009;Finlay et al 2010;Lapierre and del Giorgio 2012;Kortelainen et al 2020;Casas-Ruiz et al 2021;Rodríguez-Cardona et al 2023). Albeit not optimal, the "one station in the center" snapshot survey approach is a tradeoff between the detail of the description of within lake horizontal/temporal variability and a broad description of geographical gradients, with typical resources allocated to sampling effort.…”

A Review of Quantifying pCO2 in Inland Waters with a Global Perspective: Challenges and Prospects of Implementing Remote Sensing Technology

“…Nevertheless, the quantification of the pCO 2 in inland waters is also important for accurately estimating CO 2 flux in the water-atmosphere interface and understanding the role of CO 2 in inland waters in the Earth's carbon budget. Some studies reported about the significant spatial and temporal variations of the pCO 2 in lakes and rivers [13][14][15][16][17] and the strong influence of ambient environment and river discharge on the pCO 2 of inland waters [18][19][20]. However, the current pCO 2 data of inland waters remain uncertain due to the large discrepancy of pCO 2 in the global inland waters.…”