Whole-Lake Radiocarbon Experiment in an Oligotrophic Lake at the Experimental Lakes Area, Northwestern Ontario

Abstract

To gain more insight into the nature of carbon cycling in lakes and to provide a check on estimates of carbon fluxes obtained by more conventional means, 1 Ci (= 37 GBq) of C14 as NaHCO3 was added to the epilimnion of Lake 224, a dimictic, oligotrophic lake of the Canadian Shield near Kenora, Ontario. The dominant loss from the dissolved inorganic carbon (DIC) pool was via C14O2 evasion to the overlying atmosphere. The next most important loss from the DIC pool was by photosynthetic fixation of inorganic carbon by epilimnetic phytoplankton. About half of the C14 thus incorporated into the particulate organic carbon (POC) pool was converted into soluble organic molecules which became part of the epilimnetic dissolved organic carbon-14 (DOC) pool. Since the amount of C14 lost to the sediments of the epilimnion, to the hypolimnion, and to periphyton biomass was not significant to the C14 mass balance over the duration of the experiment, the rate of gas exchange can be calculated by measuring the decrease in epilimnetic C14 inventory (DIC14 + POC14 + DOC14) over a specific time period. Using the stagnant boundary model and pCO2 values calculated from pH, temperature and DIC data a range of stagnant film thicknesses of 212–316 μm was obtained. To provide a check on the film thickness calculated from C14 inventories 10 mCi if Ra226 was also added to the epilimnion of L224. Measurements of Rn222, the gaseous daughter product of Ra226, allowed an independent estimate of the film thickness. The average value of 200 μm obtained in this way is consistent with that obtained for C14O2 evasion. A simplified model was also constructed to describe the behavior of the POC and DOC pools. This model produced results in excellent agreement with the photosynthetic rate averaging 65 mg C∙m−1∙d−1 measured using C14 and the Fee incubator technique. The model also suggests that only about 10% of the POC + DOC pool is active in the photosynthetic process on the time scale of 30 d.Key words: whole-lake radiocarbon experiment, gas exchange, primary production, radium226, radon222, carbon14, carbon in lakes