Regulation of CO2 emissions from temperate streams and reservoirs

[摘要] It has become more and more evident that CO₂ emission(F_CO2) from freshwater systems is an important part of theglobal carbon cycle. To date, only a few studies have addressed the differentmechanisms that regulate F_CO2 in lotic and lentic systems. Ina comparative study we investigated how different biogeochemical and physicalfactors can affect F_CO2 values in streams and reservoirs. Weexamined the seasonal variability in CO₂ concentrations and emissionsfrom four streams and two pre-dams of a large drinking water reservoirlocated in the same catchment, and compared them with environmental factorsthat were measured concurrently. All the streams were generallysupersaturated with CO₂ throughout the year, while both reservoirsfunctioned to a small degree as CO₂ sinks during summer stratificationand CO₂ sources after circulation had set in. F_CO2 fromstreams ranged from 23 to 355 mmol m⁻² d⁻¹ and exceeded thefluxes recorded for the reservoirs (−8.9 to161.1 mmol m⁻² d⁻¹). Both the generally high piston velocity(k) and the CO₂ oversaturation contributed to the higherF_CO2 from streams in comparison to lakes. In both streams andreservoirs F_CO2 was mainly governed by the CO₂concentration (r = 0.92, p < 0.001 for dams; r = 0.90, p < 0.001 forstreams), which was in turn affected by metabolic processes and nutrients inboth systems and also by lateral inflow in the streams. Besides CO₂concentration, physical factors also influence F_CO2 in lakesand streams. During stratification, F_CO2 in both pre-dams wasregulated by primary production in the epilimnion, which led to a decrease ofF_CO2. During circulation, when CO₂ from the hypolimnionwas mixed with the epilimnion, F_CO2 increased on account ofthe CO₂ input from the hypolimnion. The CO₂ from the hypolimnionoriginates from the mineralisation of organic matter. F_CO2from streams was mainly influenced by geomorphological and hydrologicalfactors affecting k, which is less relevant in low-wind lakes. Underhigh-wind conditions, however, k regulates F_CO2 from loticsystems as well. We developed a theoretical framework describing the role ofthe different regulation mechanisms for F_CO2 from streams andlakes.

In summary, the dominant factor affecting F_CO2 is theconcentration of CO₂ in the surface water. Lake stratification has avery important regulatory effect on F_CO2 from lakes on accountof its influence on CO₂ concentrations and metabolic processes.Nevertheless, F_CO2 values in heterotrophic streams aregenerally higher. The higher k values are responsible for the comparativelyhigh degree of F_CO2. On a Central European scale, CO₂emission from streams is probably of greater importance than the CO₂flux from standing waters.

[学科分类] 地球化学与岩石