CHARM (CHAllenges of Reservoir Management) WP4

(A) Motivation. Reservoirs potentially act as an important storage of terrestrial carbon and have been identified as an important source of atmospheric greenhouse gases (GHG) in the global budgets. However, the quantity and main pathways of emissions from reservoirs (mid-latitude systems) are still under debate and the influence of reservoir operation on GHG emissions has not been investigated in detail yet.

(B) Aims. The overall objectives of WP4 are to quantify the increase of CH4 and CO2 emissions in reservoirs as compared to natural river sections and to investigate how operational and environmental conditions (e.g. stratification, seasonal temperature changes, wind forcing, trophic conditions) influence CH4 and CO2 emissions as well as the carbon budget of mid-latitude reservoirs. A major focus will be on highly resolved spatial and temporal distribution of ebullition and diffusive CH4 fluxes, because such data, in particular on ebullition, are scarcely available for reservoirs. Based on a data set of CH4 concentrations and fluxes collected with a unique spatial and temporal resolution we will

(1) estimate the relative contribution of ebullition to the annual emission of methane from the reservoir,

(2) compare ebullition and downstream emission of methane during different seasons,

(3) test whether ebullition is correlated to sediment thickness and sediment properties within the reservoir (together with WP1),

(4) assess the relative contribution of stored methane released during overturn in comparison to the annual lake-wide emissions

(5) test in cooperation with WP1 and WP2 whether littoral zones and regions with increased resuspension potential are hot spots of CH4 concentrations as well as subsequent emissions and (6) investigate the consequences of water-level changes induced by reservoir operation on ebullition and the overall flux of methane. Further, in cooperation with WP1 we will estimate the amount of methane released during flushing events.

This pathway of GHG emissions has not been included yet in the estimations of life-time carbon emissions from reservoirs, which is considered as relevant measure when judging the GHG saving potential of hydro-power (Hertwich 2013). Apart from CH4, we will assess the concentrations and fluxes of CO2 continuously at high temporal resolution to address the questions on

(1) how do annual CO2 emissions in the tributaries compare to the carbon release from the reservoir on an annual time scale,

(2) during which seasons are reservoirs a net-source or netsink of CO2 and

(3) how much inorganic carbon is fixed by primary production.

Further, we will collect data on relevant abiotic and biotic factors affecting the fate of GHG and the carbon budget to enable a comparison between the balance of carbon in in- and outflow with the carbon release to the atmosphere and to learn more about the processes affecting the vertical distribution of CO2 and CH4, such as production and respiration, and their implications for the development of hypoxia.