Diurnal rhythms of carbon dioxide concentration in the sea-level air layer and in the surface water of Lake Baikal in different hydrological seasons

Results of studying processes of gaseous exchange of carbon dioxide (CO₂) in the water surface-atmosphere system of Lake Baikal are considered. Measurements were carried out in the littoral of the lake near the settlement of Bol’shie Koty. The measurement point was selected for the following reasons. First, it is the place where water chemistry and biota have been studied for several tens of years. Second, by its meteorological conditions the place can be considered as background. It is shown that during all hydrological seasons the gas exchange of CO₂ in the Baikal water surface-atmosphere system has a well-pronounced trend, whose amplitude depends on the season. The data obtained are indicative of the main role that Lake Baikal biota plays in regulating the intensity of gas exchange.     

Water Exchange between Bed Depressions in Baikal

Published results of studies and calculations based on data on the salinity of river and lake waters are used to assess the resulting horizontal water exchange between bed depressions in Baikal. The role of this process in the redistribution of nutrients in the lake is demonstrated using dissolved Si as an example.     

Daily Course of CO2 Fluxes in the Atmosphere–Water System and Variable Fluorescence of Phytoplankton during the Open-Water Period for Lake Baikal according to Long-Term Measurements

The process of gas exchange of CO₂ in the atmosphere–water system and its relation to the daily course of variable fluorescence of phytoplankton is studied on the basis of long-term (2004–2014) measurements during the open water period for Lake Baikal. It is found that the decrease in photosynthetic activity of plankton is almost synchronous to the increase in the CO₂ flux from atmosphere to water. It follows from comparison of the spring and summer data with December measurements that the daily decrease in variable fluorescence of phytoplankton is caused by the internal daily rhythm of the photosynthetic activity of plankton.

     

The influence of deep convection on aeration of the bottom zone in Baikal

We investigated the influence of deep forced temperature convection on aeration of the bottom water layer in the pelagic zone of Baikal. Temperature T (± 0.002°C) and oxygen O₂ concentration (± 0.01 mg/L) were measured by using the SBE-25 probe with the SBE-43 oxygen sensor in May-July 2006?2007 and 2009?2013. Oxygen input into the bottom layer with cold intrusions was determined for 79 cases. The study revealed a quantitative correlation between the heat deficit and the rise in oxygen content in the bottom layer during cold intrusions. This correlation was used to reconstruct the values of intrusion aeration of the bottom layer during 1993?2005. It is ascertained that the intrusion-caused 2006?2013-average oxygen input into Baikal’s bottom layer is virtually close to the annual oxygen demand for oxidation processes in the water column and bottom sediments. In the southern and middle parts of Baikal, intrusion-caused input of oxygen corresponds to a higher rate of its demand (4.5?4.6x10^-4 mg/L per day). A slight increase in oxygen concentration was detected in the bottom layer of the South and Middle Baikal and a slight decrease in the bottom layer of North Baikal during 1993?2013.     

Diurnal Variations in the Rate of Gas Exchange between Lake Baikal and the Atmosphere

The diurnal course of gas exchange between Lake Baikal and the atmosphere during the summer warming up is calculated using the data of continuous 102-hour observations in August 2000 and the published materials of individual diurnal measurements of the dissolved gases concentrations made in different years. The diurnal course of the rate of gas exchange in both the coastal and open parts of the lake is found to be due to the fact that slow gas-exchange processes fail to compensate fast changes in the partial pressures of gases dissolved in the upper water layer. The changes are associated with the diurnal dynamics of the hydrometeorological conditions and the intensity of the production–destruction processes. It is found that under light winds, the contribution of biological factors to the formation of short-period variations in the CO₂ and O₂ fluxes through the lake water surface makes about 80% and that of abiotic factors, 20%. Considerable aperiodic changes in the diurnal course of hydrometeorological conditions are found to essentially distort the normal diurnal course of the gas-exchange processes.