Diurnal Variation of CO2 concentration above a tropical reservoir in the central Amazonia

The tropical reservoirs of hydroeletric plants have an important role in greenhouse gas emissions into the atmosphere. Due to the large extension of the reservoirs, the lake breeze or land breeze can influence the transport and dispersion of gases. The Balbina reservoir (59° 28’ 50’’ W, 1° 53’ 25’’; S), located near Manaus-AM in central Amazonia, is the second biggest hydroeletric plant resevoir in the Amazon basin and has a flooded area of 1770 km² with an average depth of 10 m, which is enough to produce a dynamic land-lake breeze. In this reservoir, we measured wind direction and velocity from a meteorological buoy and carbon dioxide (CO₂) concentrations at 2 m above the lake surface with a high frequency sensor, from 18^th to 19^th July/2013. The CO₂ concentrations were measured at 10 Hz and data were averaged every 5 min during 32 hours. The lake breeze and land breeze from the forest around the lake showed a well-defined behavior, the lake breeze being predominantly from south (S) and the land breeze predominantly from north (N). The CO₂ concentration averages were 392 and 426 ppm for daytime and nighttime, respectively. During daytime the atmosphere above the lake was well mixed due to unstable stratification and moderately strong wind speeds. The accumulation of CO₂ concentration above the lake at night may have been affected by low wind speeds and enhanced CO2 flux from the water surface due to buoyancy-induced turbulence and physical processes that brought high-CO₂ water to the surface. Advection of CO₂ from the adjacent forest (land breeze) was also possible.