Regulation of CO₂ emissions from temperate streams and reservoirs

Abstract. It has become more and more evident that CO₂ emission (F_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 F_CO2 in lotic and lentic systems. In a comparative study we investigated how different biogeochemical and physical factors can affect F_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. F_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 (k) and the CO₂ oversaturation contributed to the higher F_CO2 from streams in comparison to lakes. In both streams and reservoirs F_CO2 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 F_CO2 in lakes and streams. During stratification, F_CO2 in both pre-dams was regulated by primary production in the epilimnion, which led to a decrease of F_CO2. During circulation, when CO₂ from the hypolimnion was mixed with the epilimnion, F_CO2 increased on account of the CO₂ input from the hypolimnion. The CO₂ from the hypolimnion originates from the mineralisation of organic matter. F_CO2 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 F_CO2 from lotic systems as well. We developed a theoretical framework describing the role of the different regulation mechanisms for F_CO2 from streams and lakes.

In summary, the dominant factor affecting F_CO2 is the concentration of CO₂ in the surface water. Lake stratification has a very important regulatory effect on F_CO2 from lakes on account of its influence on CO₂ concentrations and metabolic processes. Nevertheless, F_CO2 values in heterotrophic streams are generally higher. The higher k values are responsible for the comparatively high degree of F_CO2. On a Central European scale, CO₂ emission from streams is probably of greater importance than the CO₂ flux from standing waters.