Biogeochemical diversity, O-2-supersaturation and hot moments of GHG emissions from shallow alkaline lakes in the Pantanal of Nhecolandia, Brazil

2018

Article référencé dans le Web of Science WOS:000424144200142

Barbiéro Laurent, Neto M. S., Braz R. R., do Carmo J. B., Rezende A. T., Mazzi E., Fernandes F. A., Damatto S. R., de Camargo P. B.

Science of the Total Environment, 2018, 619, p. 1420-1430 ISSN 0048-9697

Nhecolandia is a vast sub-region of the Pantanal wetland in Brazilwith great diversity in surface water chemistry evolving in a sodic alkaline pathway under the influence of evaporation. In this region, >15,000 shallowlakes are likely to contribute an enormous quantity of greenhouse gas to the atmosphere, but the diversity of the biogeochemical scenarios and their variability in time and space is a major challenge to estimate the regional contribution. From 4 selected alkaline lakes, we compiled measurements of the physico-chemical characteristics ofwater and sediments, gas fluxes in floating chambers, and sedimentation rates to illustrate this diversity. Although these lakes have a similar chemical composition, the results confirm a difference between the black-water and green-water alkaline lakes, corresponding to distinct biogeochemical functioning. This difference does not appear to affect lake sedimentation rates, but is reflected in gas emissions. Black-water lakes are CO2 and CH4 sources, with fairly constant emissions throughout the seasons. Annual carbon dioxide and methane emissions approach 0.86 mol m(-2) y(-1) and 0.07 mol m(-2) y(-1), respectively, and no clear trend towards N2O capture or emissionwas observed. By contrast, green-water lakes are CO2 and N2O sinks but important CH4 sources with fluxes varying significantly throughout the seasons, depending on the magnitude of the phytoplankton bloom. The results highlight important daily and seasonal variations in gas fluxes, and in particular a hot moments for methane emissions, when the O-2(-) supersaturation is reached during the afternoon under extreme bloom and sunny weather conditions, provoking an abrupt O-2 purging of the lakes. Taking into account the seasonal variability, annual methane emissions are around 10.2 mol m(-2) y(-1), i. e., much higher than reported in previous studies for alkaline lakes in Nhecolandia. Carbon dioxide and nitrous oxide consumption is estimated about 1.9 mol m(-2) y(-1) and 0.73 mmol m(-2) y(-1), respectively. However, these balances must be better constrained with systematic and targeted measurements throughout the seasons.

Sciences du milieu [021] ; Limnologie physique / Océanographie physique [032]

BRESIL

Fonds IRD [F B010072359]

fdi:010072359