Regulation of CO 2 emissions from temperate streams and reservoirs

Abstract. It has become more and more evident that CO2 emission (FCO2) from freshwater systems is an important part of the global carbon cycle. To date, only a few studies have addressed the different mechanisms that regulate FCO2 in lotic and lentic systems. In a comparative study we investigated how different biogeochemical and physical factors can affect FCO2 values in streams and reservoirs. We examined the seasonal variability in CO2 concentrations and emissions from four streams and two pre-dams of a large drinking water reservoir located in the same catchment, and compared them with environmental factors that were measured concurrently. All the streams were generally supersaturated with CO2 throughout the year, while both reservoirs functioned to a small degree as CO2 sinks during summer stratification and CO2 sources after circulation had set in. FCO2 from streams ranged from 23 to 355 mmol m−2 d−1 and exceeded the fluxes recorded for the reservoirs (−8.9 to 161.1 mmol m−2 d−1). Both the generally high piston velocity (k) and the CO2 oversaturation contributed to the higher FCO2 from streams in comparison to lakes. In both streams and reservoirs FCO2 was mainly governed by the CO2 concentration (r = 0.92, p < 0.001 for dams; r = 0.90, p < 0.001 for streams), which was in turn affected by metabolic processes and nutrients in both systems and also by lateral inflow in the streams. Besides CO2 concentration, physical factors also influence FCO2 in lakes and streams. During stratification, FCO2 in both pre-dams was regulated by primary production in the epilimnion, which led to a decrease of FCO2. During circulation, when CO2 from the hypolimnion was mixed with the epilimnion, FCO2 increased on account of the CO2 input from the hypolimnion. The CO2 from the hypolimnion originates from the mineralisation of organic matter. FCO2 from streams was mainly influenced by geomorphological and hydrological factors affecting k, which is less relevant in low-wind lakes. Under high-wind conditions, however, k regulates FCO2 from lotic systems as well. We developed a theoretical framework describing the role of the different regulation mechanisms for FCO2 from streams and lakes. In summary, the dominant factor affecting FCO2 is the concentration of CO2 in the surface water. Lake stratification has a very important regulatory effect on FCO2 from lakes on account of its influence on CO2 concentrations and metabolic processes. Nevertheless, FCO2 values in heterotrophic streams are generally higher. The higher k values are responsible for the comparatively high degree of FCO2. On a Central European scale, CO2 emission from streams is probably of greater importance than the CO2 flux from standing waters.