Regulation of CO 2 emissions from temperate streams and reservoirs

Halbedel, S.; Koschorreck, M.

It has become more and more evident that CO 2 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 CO 2 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 CO 2 throughout the year, while both reservoirs functioned to a small degree as CO 2 sinks during summer stratification and CO 2 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 CO 2 oversaturation contributed to the higher FCO2 from streams in comparison to lakes. In both streams and reservoirs FCO2 was mainly governed by the CO 2 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 CO 2 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 CO 2 from the hypolimnion was mixed with the epilimnion, FCO2 increased on account of the CO 2 input from the hypolimnion. The CO 2 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 CO 2 in the surface water. Lake stratification has a very important regulatory effect on FCO2 from lakes on account of its influence on CO 2 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, CO 2 emission from streams is probably of greater importance than the CO 2 flux from standing waters.

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Halbedel, S. / Koschorreck, M.: Regulation of CO2 emissions from temperate streams and reservoirs. 2013. Copernicus Publications.

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