Regulation of CO₂ emissions from temperate streams and reservoirs

• Main Authors: S. Halbedel, M. Koschorreck
• Format: Article
• Language: English
• Published: Copernicus Publications 2013-11-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/10/7539/2013/bg-10-7539-2013.pdf
• ISSN: 1726-4170; 1726-4189

It has become more and more evident that CO₂ emission (<i>F</i>_CO₂) 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 <i>F</i>_CO₂ in lotic and lentic systems. In a comparative study we investigated how different biogeochemical and physical factors can affect <i>F</i>_CO₂ 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. <i>F</i>_CO₂ 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 (<i>k</i>) and the CO₂ oversaturation contributed to the higher <i>F</i>_CO₂ from streams in comparison to lakes. In both streams and reservoirs <i>F</i>_CO₂ was mainly governed by the CO₂ concentration (<i>r</i> = 0.92, <i>p</i> < 0.001 for dams; <i>r</i> = 0.90, <i>p</i> < 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 <i>F</i>_CO₂ in lakes and streams. During stratification, <i>F</i>_CO₂ in both pre-dams was regulated by primary production in the epilimnion, which led to a decrease of <i>F</i>_CO₂. During circulation, when CO₂ from the hypolimnion was mixed with the epilimnion, <i>F</i>_CO₂ increased on account of the CO₂ input from the hypolimnion. The CO₂ from the hypolimnion originates from the mineralisation of organic matter. <i>F</i>_CO₂ from streams was mainly influenced by geomorphological and hydrological factors affecting <i>k</i>, which is less relevant in low-wind lakes. Under high-wind conditions, however, <i>k</i> regulates <i>F</i>_CO₂ from lotic systems as well. We developed a theoretical framework describing the role of the different regulation mechanisms for <i>F</i>_CO₂ from streams and lakes. <br><br> In summary, the dominant factor affecting <i>F</i>_CO₂ is the concentration of CO₂ in the surface water. Lake stratification has a very important regulatory effect on <i>F</i>_CO₂ from lakes on account of its influence on CO₂ concentrations and metabolic processes. Nevertheless, <i>F</i>_CO₂ values in heterotrophic streams are generally higher. The higher <i>k</i> values are responsible for the comparatively high degree of <i>F</i>_CO₂. On a Central European scale, CO₂ emission from streams is probably of greater importance than the CO₂ flux from standing waters.